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  • Structure, hydration, and chloride ingress in C-S-H: Insight from DFT calculations
    Cem. Concr. Res. (IF 5.618) Pub Date : 2020-01-02
    Ingeborg-Helene Svenum; Inga G. Ringdalen; Francesca L. Bleken; Jesper Friis; Daniel Höche; Ole Swang

    The structure of Calcium-Silicate-Hydrate (C-S-H) and the effect of variations in its water content have been investigated using density functional theory (DFT) calculations. Trends for calculated densities as a function of hydration are in good agreement with experimental values, and in line with what is found using molecular mechanics in the literature. While we observe very little variation in SiO and CaO bond lengths between different structures, structural diversity is otherwise great, in accordance with experimental observations, as we see no obvious correlation between structural features and material system stability. A mapping of energetics of hydroxyl substitution with chloride reveals, unsurprisingly, that chloride preferentially coordinates to calcium. More specifically, it was found that the most stable sites for chlorine substitution involves at least two adjacent calcium atoms. Computed chloride substitution energies indicate that the C-S-H phase may bind chloride from aqueous solution, potentially influencing chloride diffusion in concrete.

    更新日期:2020-01-02
  • Effect of a novel starch-based temperature rise inhibitor on cement hydration and microstructure development
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-12-31
    Yu Yan; Alexandre Ouzia; Cheng Yu; Jiaping Liu; Karen L. Scrivener

    This study investigates the mechanism by which a novel starch-based admixture named Temperature Rise Inhibitor (TRI) modifying cement hydration and limiting heat evolution. This admixture is designed to avoid thermal cracking of concrete. Depending on the initial state, TRI has two distinct effects: Solid TRI has a “depressing effect” that modifies the maximum heat flow and reduces the 1d cumulative heat by up to 89.4% with less than 0.15% addition. Addition of TRI pre-dissolved in the mixing water has a “retarding effect” with a delay in heat release, but little impact on the shape of main hydration peak. The nucleation and growth of C-S-H was semi-quantified by SEM. It was found that, irrespective of adding mode, TRI has a negligible influence on the growth of C-S-H “needles”. Simulations based on the C-S-H “needle model” support the hypothesis that the modification of hydration kinetics can be caused by affecting C-S-H nucleation.

    更新日期:2019-12-31
  • Monitoring early-age acoustic emission of cement paste and fly ash paste
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-12-31
    Evin Dildar Dzaye; Geert De Schutter; Dimitrios G. Aggelis

    In this study, a combined approach of several monitoring techniques was applied to allow correlations between the AE activity and related processes such as shrinkage and settlement evolution, capillary pressure and temperature development in fresh cementitious media. AE parameters related to frequency, energy, and cumulative activity which exhibit sensitivity to the particle size distribution of cement paste are compared with inert fly ash (FA) leading to isolation of the mechanical sources from the chemical ones. Characterization of the origin of different processes occurring in cement paste during hydration is complex. Although acoustic emission (AE) monitoring has been used before, a qualitative relation between the microstructural formation or other early-age processes and the number or parameters of AE signals has not been established. The high sensitivity of AE enables the recording of elastic waves within the cementitious material, allowing the detection of even low-intensity activities.

    更新日期:2019-12-31
  • The evolution of interfacial transition zone in alkali-activated fly ash-slag concrete
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-12-28
    Guohao Fang; Mingzhong Zhang

    This paper investigates the mechanisms of microstructure evolution of interfacial transition zone (ITZ) in alkali-activated fly ash-slag (AAFS) concrete using scanning electron microscope. Results indicate that the formation of original ITZ depends on the so-called “wall effect”, leading to a deficit of large grains and a higher effective alkaline activator/precursor ratio compared to paste matrix. The alkaline reaction process is correspondingly accelerated, which promotes the formation of low Ca C-(N)-A-S-H gels and reduces the porosity in the ITZ. Afterwards, the high Ca C-(N)-A-S-H gels are generated due to the release of more Ca from slag, resulting in the continuous refinement of pores. The C-(N)-A-S-H gels with rich Si and Al are then produced at 7 d, attributing to the species diffusion from paste matrix to ITZ. Consequently, a compact and dense microstructure is formed in the ITZ at 28 d, which would be beneficial to the long-term performance of concrete.

    更新日期:2019-12-29
  • Investigation on the microstructure-related characteristics to elucidate performance of composite cement with limestone-calcined clay combination
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-12-27
    Yuvaraj Dhandapani; Manu Santhanam

    This paper discusses the role of physical structure alterations on three binder types: plain portland cement, fly ash-based binder and calcined clay-limestone binder. The kinetics of physical structure development and the relevance in transport properties were distinguished using an interlinked parameter in concrete and paste. A systematic experimental investigation was carried out on a range of critical parameters such as strength development, resistivity development, transport characteristics and the time-dependent change in transport parameter. The role of microstructure in terms of the chemical composition of C-A-S-H and its physical states in the different systems is identified as the critical factor governing the development of microstructure. Chloride resistance was assessed by chloride migration experiments for a period of 4 years. The durability behaviours of the concrete with various binder were generalised using pore network parameters such as formation factor and tortuosity. A sensitivity analysis was used to dissect the contribution of the pore solution dilution and pore connectivity to the change in the pore network parameter. Based on the rise in macroscopic physical characteristics (i.e., formation factor here), a two-fold structure development mechanism to conceptualise microstructural evolution in cement composites is presented. Initially, capillary pore space reduces to a critical size range (i.e., 10–30 nm), which is followed by the densification of the physical state of the microstructure. At the point of densification, the pores become largely disconnected which leads to a dramatic increase in the formation factor. The binding matrix in calcined clay concretes reaches the critical pore size at an early age which leads to early densification of capillary pore space region in comparison to fly ash concretes.

    更新日期:2019-12-27
  • Atomistic structure of alkali-silica reaction products refined from X-ray diffraction and micro X-ray absorption data
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-12-27
    Guoqing Geng; Zhenguo Shi; Andreas Leemann; Camelia Borca; Thomas Huthwelker; Konstantin Glazyrin; Igor V. Pekov; Sergey Churakov; Barbara Lothenbach; Rainer Dähn; Erich Wieland

    The alkali-silica reaction (ASR) causes internal expansion that leads to severe concrete degradation. The structure of the ASR products remains largely unclear, due to the limitation of laboratory probes in micro-chemical-crystallographic study. We hereby performed synchrotron-radiation-based X-ray absorption spectroscopy and diffraction investigations to both ASR product samples collected from the field, and reference samples with known structure. Our results suggest that the field ASR crystals from distinct sources are nearly identical. They share a layer-silicate structure similar to the mineral shlykovite, whereas the stacking of the layers in field ASR crystals is altered in several ways, such as the variable basal spacing and significant glide of adjacent layers along the b-axis. We also demonstrate that the amorphous ASR product highly resembles C-S-H. Our study adds new insights to the atomistic structure of ASR products.

    更新日期:2019-12-27
  • Validated hydration model for slag-blended cement based on calorimetry measurements
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-12-23
    Markus Königsberger; Jérôme Carette

    Modeling of the complex interlinked chemical reactions involved in the hydration process of slag-blended cement is rather challenging, in particular since accurate prediction of the hydration kinetics of clinker and slag hydration, respectively, is still out of reach. To overcome this challenge, we propose a hybrid modeling approach based on calorimetry measurements combined with state-of-the-art hydration models. The model features intrinsic C-(A)-S-H nanoparticles related to clinker and slag hydration, respectively, with precipitation space-dependent densities of the corresponding gel phases. Successful validation against published experimental data, obtained on 54 different mixes in 7 different laboratories, proves the model's applicability and corroborates that C-(A)-S-H gel densifies progressively during hydration, that portlandite consumption upon slag reaction is caused by the precipitation of calcium-aluminate hydrates and that the ultimate heat of slag depends significantly on its chemical composition.

    更新日期:2019-12-25
  • Application of thermodynamic modeling to predict the stable hydrate phase assemblages in ternary CSA-OPC-anhydrite systems and quantitative verification by QXRD
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-12-23
    J.J. Wolf; D. Jansen; F. Goetz-Neunhoeffer; J. Neubauer

    Thermodynamic modeling was used to predict the stable hydrate phase assemblages in a ternary CSA-OPC-anhydrite system. On the basis of this modeling, five systems with diverging expected hydrate phases were chosen for experimental examination via QXRD and TGA. The aim of this study was to examine whether and to which extent the predicted equilibrium states are reached after 28 days of hydration. Qualitatively the predicted hydrate phase assemblages were reached for all five systems after 28 d of hydration. When the reaction degrees of the anhydrous phases are taken into account the experimental data fits remarkably well to the thermodynamic model. Additionally, C2S dissolution was found to be the most rapid in straetlingite forming environments.

    更新日期:2019-12-25
  • A 3D petrographic analysis for concrete freeze-thaw protection
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-12-13
    Yu Song; Robbie M. Damiani; Chuanyue Shen; Daniel I. Castaneda; David A. Lange

    Petrographic analysis is routinely implemented to measure air void parameters for predicting freeze-thaw durability of concrete. The 1D or 2D measurements obtained from polished concrete sections are used to infer 3D protection of the paste. Since an arbitrary point in a section view may be partially protected by air voids that are not intersected, a stereological model is proposed to consider both intersected and un-intersected voids. The model is tested with a group of concrete mixtures with varying air entrainment, mix proportion, and aggregate type. As compared with the 2D analysis, this 3D analysis indicates a higher percent of cement paste being protected. When equating the protection range to Powers' spacing factor, it is found that roughly 98% of paste is protected in 3D. Findings in this study also align with results obtained by other authors using numerical simulation and computed tomography. This model offers a realistic concrete freeze-thaw assessment.

    更新日期:2019-12-13
  • Effect and mechanism analysis of functionalized multi-walled carbon nanotubes (MWCNTs) on C-S-H gel
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-12-13
    Yue Li; Hongwen Li; Zigeng Wang; Caiyun Jin

    The effect and the interaction mechanism of functionalized multi-walled carbon nanotubes (MWCNTs) on C-S-H gel were studied in this paper. Fourier infrared spectroscopy, acid-base titration, Raman spectroscopy and transmission electron microscopy were utilized to analyze the pristine MWCNTs and functionalized MWCNTs, showing that the degree of graphitization of the functionalized MWCNTs reduced. And the surface of the functionalized MWCNTs was grafted with acidic functions which the content was 2.5 × 10−3 mol/g. Afterward, the MWCNTs/C-S-H composite was prepared by well-dispersing the functionalized MWCNTs and the C-S-H. The experimental results showed that for the C-S-H produced with different designed molar ratios of CaO to nanosilica (C/SD), the measured molar ratio of Ca to Si (C/SM) of the C-S-H decreased and the interlayer spacing of the C-S-H increased with the increase of the amount of the functionalized MWCNTs. When C/SD = 0.8, the polymerization degree of the C-S-H decreased and the proportion of the capillary pores increased gradually with increasing the amount of the functionalized MWCNTs. When the values of C/SD were 1.5 and 2.0, the trends the polymerization degree and the proportion of the capillary pores of C-S-H were opposite to those of C/SD = 0.8. The reasons were further analyzed based upon the above experimental results.

    更新日期:2019-12-13
  • Radiological and leaching assessment of an ettringite-based mortar from ladle slag and phosphogypsum
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-12-03
    Katrijn Gijbels, Hoang Nguyen, Paivo Kinnunen, Pieter Samyn, Wouter Schroeyers, Yiannis Pontikes, Sonja Schreurs, Mirja Illikainen

    In this investigation, ettringite-based mortars were synthesized from ladle slag (LS) and phosphogypsum (PG), promoting the concept of a circular economy. However, the reuse of naturally occurring radioactive materials (NORM), such as PG, requires radiological investigation. Also, the immobilization degree for contaminants contained in PG should be evaluated. The former was investigated using gamma spectroscopy and radon exhalation/emanation tests, while the latter was assessed using an up-flow percolation column test according to the CEN/TS 16637-3. The produced mortars comply with current legislation on naturally occurring radionuclides (NOR) in building materials, proving that they can be safely used for building purposes. The radon emanation decreased upon increasing the Polish PG content, which was mainly determined by the microporosity. The specific surface areas were 20–30 times lower than conventional cement, and the immobilization degree for contaminants was generally high (>90%). This investigation demonstrates high potential for PG reuse in ettringite-based mortars.

    更新日期:2019-12-03
  • Influence of low curing temperatures on the tensile response of low clinker strain hardening UHPFRC under full restraint
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-12-02
    Mohamed Abdul Hafiz, Jørgen Skibsted, Emmanuel Denarié

    The tensile response of strain hardening UHPFRC under full restraint, subjected to curing temperatures of 20 °C, 10 °C, 5 °C, was investigated for two types of mixes with silica fume; Mix I with pure type I cement and Mix II with 50% replacement of cement with limestone filler, both having a similar steel fibrous mix. The development of the elastic modulus, tensile strength, autogenous shrinkage and eigenstresses were put into perspective with the hydration kinetics. Two phases of pozzolanic reaction with different rates of consumption of silica fume were identified. A systematic increase of the autogenous shrinkage and eigenstresses with the curing temperatures was observed. The eigenstresses development was much slower in the case of Mix II, owing to its larger relaxation potential. The eigenstresses in Mix I reached the strain hardening domain after one month, whereas the same in Mix II were only approaching the strain hardening domain after three months.

    更新日期:2019-12-02
  • Influence of mixing solution on characteristics of calcium aluminate cement modified with sodium polyphosphate
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-11-30
    Keita Irisawa, Inés Garcia-Lodeiro, Hajime Kinoshita

    This study investigated characteristics of a calcium aluminate cement modified with a phosphate (CAP) by changing an amount and concentration of mixing solution with sodium polyphosphate. When the amount of mixing solution was increased with a constant amount of sodium polyphosphate, an enhanced consumption of monocalcium aluminate was observed compared with gehlenite in calcium aluminate cement. Formation of gibbsite, Al(OH)3, was also increased as a hydration product in the CAP and a reduction of water in the amorphous gel phase. When the amount of mixing solution was increased with a constant concentration of sodium polyphosphate, the enhanced consumption of monocalcium aluminate was not observed. Neither gibbsite nor any other crystalline hydration products were identified in this series. In addition, unreacted sodium polyphosphate remained in the system. The increased formation of gibbsite and the possible reduction of water from the amorphous gel phase appears to contribute to the improvement of the microstructure in the products.

    更新日期:2019-11-30
  • Effect of alkalis content on calcium sulfoaluminate (CSA) cement hydration
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-11-30
    L.U.D. Tambara, M. Cheriaf, J.C. Rocha, A. Palomo, A. Fernández-Jiménez

    The behavior of sulfoaluminate (CSA) cements hydrated in neutral or alkaline solutions (0 M, 0.1 M, 1 M, 4 M and 8 M NaOH) was studied. 2-d, 28-d and 90-d mechanical strengths were determined, as well as the reaction rate by isothermal conduction calorimetry. The reaction products were characterised with XRD, DTA and SEM. The findings revealed differences in performance depending on whether hydration transpired at pH ≤ 14 (0.1 and 1 M) or pH > 14 (4 M and 8 M). To 0.1 M and 1 M, there is an improvement of mechanical strength than in water-hydrated pastes. The majority reaction products in these conditions were ettringite and microcrystalline aluminium hydroxide. Mechanical strength was lower in the high alkalinity (4 M and 8 M) than in the reference pastes, mainly due to the formation of different reaction products. Ettringite was unstable at 4 M and failed to form at 8 M, whilst thenardite and U-phase were detected in the 28d and later materials hydrated at both concentrations.

    更新日期:2019-11-30
  • A discussion of the paper “Effect of design parameters on microstructure of steel-concrete interface in reinforced concrete”
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-11-26
    Zhidong Zhang, Ueli Angst

    The steel-concrete interface is fundamental to many aspects related to the performance of reinforced concrete, particularly for corrosion behavior. A recently published paper by Chen et al. [Effect of design parameters on microstructure of steel-concrete interface in reinforced concrete, Cem. Concr. Res. 119 (2019) 1–10] quantitatively characterized the thickness of the gap observed between the steel and concrete. Considering the low number of available publications, studies documenting the microstructure at the steel-concrete interface are important. However, in our discussion, we think that aspects related to the terminology, the specimen preparation and possible experimental artefacts in the discussed paper can be improved.

    更新日期:2019-11-27
  • Multi-scale prediction of chemo-mechanical properties of concrete materials through asymptotic homogenization
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-11-25
    E. Bosco, R.J.M.A. Claessens, A.S.J. Suiker

    In the present contribution, the effective mechanical, diffusive, and chemo-expansive properties of concrete are computed from a multi-scale and multi-physics approach. The distinctive features of the approach are that i) the mechanical and diffusive responses are modelled in a coupled fashion (instead of separately, as is usually done), and that ii) the multi-scale model considers three different scales of observation, which allows for including heterogeneous effects from both the micro- and meso-scales in the effective macro-scale properties of concrete. At the macro-scale, the concrete material is considered as homogeneous, whereas at the meso-scale it consists of particle aggregates embedded in a porous cement paste. At the micro-scale the porous cement paste is described as a two-phase material, composed of a solid cement phase and saturated capillary pores. Adopting a two-level asymptotic homogenization procedure, the effective meso-scale properties of the porous cement paste are computed first, using a unit cell that includes the cement paste and pore characteristics. Subsequently, the obtained meso-scale response of the porous cement paste, together with the aggregate characteristics, defines the material properties of a second unit cell, which is used for calculating the effective macro-scale response of concrete. The distributions of the pores and the aggregates within the unit cells are determined from a uniform, random distribution of points, and their radii are defined from a probability distribution function. The efficacy of the proposed framework is illustrated by studying the effective mesoscopic response of a porous cement paste, which demonstrates the influence of the micro-scale porosity and pore percolation. Next, the effective macroscopic response of concrete is analysed, by considering the influence of the aggregate volume fraction, the mismatches in elastic stiffnesses and diffusivity between the aggregate and the cement paste, and the porosity. The computed effective properties are compared with experimental data from the literature, showing a good agreement.

    更新日期:2019-11-26
  • Aluminum incorporation into magnesium silicate hydrate (M-S-H)
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-11-25
    E. Bernard, B. Lothenbach, C. Cau-Dit-Coumes, I. Pochard, D. Rentsch

    The incorporation of aluminum in magnesium silicate hydrate (M-S-H) phases was investigated. Magnesium (alumino) silicate hydrate (M-(A-)S-H) with Mg/Si ratios equal to 1.1 or 1.7 and Al/Si ranging from 0 to 0.2 were synthetized in batch experiments and equilibrated at 20, 50 and 70 °C. pH values between 9 and 10.5 were observed and aluminum up to Al/Si ~0.15–0.18 was incorporated in M-(A-)S-H. Thermogravimetric analysis (TGA), X-ray diffraction (XRD), X-ray pair distribution function (PDF) analysis, transmission electron microscopy (TEM), 29Si and 27Al MAS NMR data showed that the M-(A-)S-H phases formed were similar to M-S-H with limited coherent size and a comparable polymerization degree of the tetrahedral silicates. Aluminum was incorporated in both tetrahedral and octahedral sites of M-S-H, while no aluminum was present as exchangeable cation on the surface sites.

    更新日期:2019-11-26
  • Multiscale X-ray tomography of cementitious materials: A review
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-11-22
    Sébastien Brisard, Marijana Serdar, Paulo J.M. Monteiro

    X-ray computed tomography (CT) is a non-destructive technique that offers a 3D insight into the microstructure of thick (opaque) samples with virtually no preliminary sample preparation. Since its first medical application in 1973, the technique has improved dramatically in terms of acquisition times and resolution. The best resolution achievable for both parallel- and cone-beam setups is presently sub-micron. The macroscopic properties (stiffness, resistance, permeability, durability, …) of cementitious materials are known to be significantly affected by their multiscale microstructure. This calls for x-ray CT investigations of cementitious materials down to the smallest length-scales. The present review first provides background information on the technique (including image-processing). It then covers various applications of x-ray CT to cementitious materials: imaging of the porous network, durability experiments, damage experiments, …, cutting-edge nanotomography experiments. Current challenges such as time- and chemically-resolved experiments are also discussed.

    更新日期:2019-11-26
  • A discussion of the paper “Dynamic microstructural evolution of hardened cement paste during first drying monitored by 1H NMR relaxometry” by I. Maruyama, T. Ohkubo, T. Haji et al.
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-11-20
    Chunsheng Zhou, Xiaoyu Zhang, Zhendi Wang

    The recent paper “Dynamic microstructural evolution of hardened cement paste during first drying monitored by 1H NMR relaxometry” authored by I. Maruyama et al. is discussed. Besides the observed transformation of relatively thin gel pores into interlayer pores, it is worth noting that the coarse gel pores and inter-hydrate pores become coarser during drying. Additionally, interlayer pores simultaneously contract during drying at beyond 40% RH. The drying shrinkage of cement-based material may be physically rooted in the remarkable contraction of C-S-H gel, which is also responsible to the considerably lower specific surface area accessible to nitrogen than that to water.

    更新日期:2019-11-20
  • The hydration of nearly pure ye'elimite with a sulfate carrier in a stoichiometric ettringite binder system. Implications for the hydration process based on in-situ XRD, 1H-TD-NMR, pore solution analysis, and thermodynamic modeling
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-11-12
    D. Jansen, J.J. Wolf, N. Fobbe

    The hydration of synthetic ye'elimite was studied with sulfate carriers in a stoichiometric ettringite binder system. Heat flow calorimetry, in-situ XRD, 1H-TD-NMR and pore solution analysis were applied in order to gain a deeper understanding of the hydration kinetics of the system. Additionally, thermodynamic modeling was used in order to show the evolution of the pore solution in the system Ca-Al-S-H2O. The experiments showed that a first ettringite generation might be formed in a regime where CAH10, monosulfate, ettringite and AHamorphous are stable while at later points in time the ettringite is formed in a regime where AHmicrocrystalline and ettringite is stable. The different shoulders in the heat flow curve seem to arise from the recrystallization of AHmicrocrystalline and the final adjustment of the water content of the aluminum hydroxide towards AH3.

    更新日期:2019-11-13
  • A load induced thermal strain (LITS) semi-empirical model for plain and steel fiber reinforced concrete subjected to uniaxial compressive load
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-11-08
    Thomaz Eduardo Teixeira Buttignol

    A Load Induced Thermal Strain (LITS) semi-empirical model for plain and steel fiber reinforced concrete is proposed, assuming concrete as a heterogeneous 2-phase (matrix + aggregates) material. The effects of dehydration creep and the geomechanical properties decay of the coarse aggregates are taken into account separately in the model by independent parameters calibrated on the basis of the main results on plain concrete available in the scientific literature. The role of the coarse aggregates and the cement content are investigated. It was noticed that the amount of coarse aggregates is directly proportional to LITS, with a logarithmic variation. A comparison between the results of the model with experimental tests in literature, considering different types of concretes (conventional, high strength and ultra-high performance), demonstrated the reliability of the proposed model. Moreover, the model was compared with experimental tests in uniaxial compression on Steel Fiber Reinforced Concrete (SFRC) cylinders with 11-years-old, showing a reasonable approximation. The effect of concrete aging to LITS is also investigated. It is demonstrated that concrete aging is responsible for the reduction of LITS. A reduction factor β(t0) was defined on the basis of a set of experimental values ranging between 28 days and 11 years.

    更新日期:2019-11-11
  • Influence of coal fly ash on the early performance enhancement and formation mechanisms of silico-aluminophosphate geopolymer
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-11-08
    Yan-Shuai Wang, Yazan Alrefaei, Jian-Guo Dai

    This study proposes the use of two types of coal fly-ash (CFA) (high- and low-calcium, HCFA and LCFA respectively) as additives in silico-aluminophosphate (SAP) geopolymers for early performance improvement. The chemistry of CFA in phosphate environment was clarified using CFA/orthophosphoric acid suspensions through XRD, SEM and NMR analyses. The results revealed that HCFA could seed the active calcium sources which induced the acid-base or/and hydrolysis reaction between the alkaline calcium species within HCFAs and the acidic phosphate species in the activator. The produced calcium phosphate compounds, e.g., brushite and monetite, hurried up the setting (initial and final setting were 18 and 26 min, respectively) and early strength development (1-d and 3-d strengths increased to 15 and 29 MPa, respectively) of the HCFA-SAP geopolymer relative to CFA-free SAP geopolymer. However, the incorporation of LCFA was found to improve the workability but weakened the setting and strength properties of the LCFA-SAP geopolymers.

    更新日期:2019-11-11
  • Processing and characterisation of standard and doped alite-belite-ye'elimite ecocement pastes and mortars
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-11-09
    Jesus D. Zea-Garcia, Angeles G. De la Torre, Miguel A.G. Aranda, Isabel Santacruz

    Here, we report the processing optimisation of two laboratory-prepared alite-belite-ye'elimite ecocements (standard and doped) that release to the atmosphere ~13% less CO2 than Portland Cement during fabrication. The processing was optimised through rheological measurements, where homogeneous pastes and mortars were finally prepared through the study and optimisation of both the superplasticiser content and the water-to-cement ratio. Both parameters were correlated with the phase assembly of selected pastes and compressive strength of the corresponding mortars. After optimisation, mortars with high compressive strengths (~72 and ~77 MPa for the standard mortar, and ~41 and ~75 MPa for the doped one, at 7 and 28 days, respectively) were prepared. Furthermore, the important increase in compressive strength from 7 to 28 days of the optimised mortar prepared from the doped ecocement is due to its composition (the higher content of belite jointly with the reaction of its active polymorph (α'H-belite)).

    更新日期:2019-11-11
  • Yield stress of aerated cement paste
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-11-09
    Blandine Feneuil, Nicolas Roussel, Olivier Pitois

    Yield stress of aerated cement paste is studied. Samples are prepared by mixing aqueous foam with cement paste, which allows controlling bubble size, gas volume fraction and yield stress of the cement paste. Two distinct behaviors are observed depending on the surfactant used to prepare the precursor aqueous foam: (i) For a surfactant with low adsorption ability with respect to cement grains, bubbles tend to decrease the yield stress of the paste with magnitude governed by the Bingham capillary number, which accounts for bubble deformability. (ii) For a surfactant with high adsorption ability, bubbles increase significantly the yield stress. This behavior is shown to result from the surfactant-induced hydrophobization of the cement grains, which adsorb at the surface of the bubbles and tend to rigidify them. Within this regime, the effect of air incorporation is comparable to the effect of added solid particles.

    更新日期:2019-11-11
  • Time-resolved porosity changes at cement-clay interfaces derived from neutron imaging
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-11-10
    A. Shafizadeh, T. Gimmi, L.R. Van Loon, A.P. Kaestner, U.K. Mäder, S.V. Churakov
    更新日期:2019-11-11
  • Composition design and pilot study of an advanced energy-saving and low-carbon rankinite clinker
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-11-07
    Guihua Hou, Jianan Chen, Bao Lu, Sijia Chen, Entian Cui, Hamdy M. Naguib, Ming-Zhi Guo, Qinfang Zhang

    In order to save energy and reduce greenhouse gas emission, a new type of clinker with a self-pulverization ability was successfully designed, which mainly consisted of rankinite (3CaO·2SiO2 or C3S2) and auxiliary γ-dicalcium silicate (γ-2CaO·SiO2 or γ-C2S). Furthermore, a pilot study was also carried out in a Φ 0.8 × 10 m rotary kiln. The cement was prepared by grinding the clinker to control the target particle size, and then it was hardened by carbonation. The suitable composition range of the clinker was 53–55% CaO, 38–41% SiO2 and 4–9% Al2O3. The optimum clinkerization temperature range was 1260–1320 °C. The powder clinker had a BET surface area of 365.3 m2/kg. After being carbonated with CO2 for 7 d, the flexural and compressive strengths of cement mortar reached 13.5 MPa and 57.4 MPa, respectively. The weight percentage of CO2 in the 3-day carbonated specimen could be as high as 18.9%. The overall results well demonstrate that the developed new-type clinker is both energy-saving and greenhouse gas reducing, thereby holding great promise for applications in construction industries.

    更新日期:2019-11-08
  • Dependence of unsaturated chloride diffusion on the pore structure in cementitious materials
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-11-06
    Yong Zhang, Zhengxian Yang, Guang Ye

    Conceptual analysis is performed to examine the effects of pore features on the water continuity in unsaturated porous systems. The roles of pore features in relative chloride diffusion coefficient (Drc) of mortar specimens at various degrees of water saturation (Sw) were studied based on mercury intrusion porosimetry and resistivity tests. It is found that the role of pore structure in the Drc-Sw relationship is a result of its effect on the water continuity. Porosity and tortuosity are not relevant to the Drc-Sw relationship. A finer pore size distribution or lower pore connectivity tends to result in a lower Drc. The pore size effect on the Drc is pronounced primarily at high Sw, while the Drc is dominated by the pore connectivity at low Sw. Cement mortar with a higher water-to-binder ratio shows larger chloride diffusion at high relative humidity levels but smaller chloride diffusion at low relative humidity levels.

    更新日期:2019-11-07
  • Study of nucleation and growth processes of ettringite in diluted conditions
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-11-06
    Estelle Poupelloz, Sandrine Gauffinet, André Nonat

    Ettringite is a key compound of cementitious materials, formed during hydration that influences the rheological behavior of the cement paste. Despite its importance, ettringite formation has been poorly studied. In this work, nucleation and growth processes of ettringite were separately investigated. Ettringite nucleation was described using the Classical Nucleation Theory and an interfacial crystal-solution energy was determined. After the initial primary homogeneous nucleation, when the surface of already formed crystals was still low, the precipitation rate was shown to depend almost exclusively on the solution supersaturation. When the surface of crystals in suspension was getting bigger, a secondary nucleation responsible for an increase of ettringite needle diameter was observed. The precipitation rate became surface dependent and increased significantly. Results of this work show that nucleation of ettringite is easy as it requires few energy and that after primary nucleation precipitation occurs rapidly via secondary nucleation and growth.

    更新日期:2019-11-06
  • The combined effect of potassium, sodium and calcium on the formation of alkali-silica reaction products
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-11-06
    Zhenguo Shi, Barbara Lothenbach

    Both alkalis and calcium play essential roles in the formation of alkali-silica reaction (ASR) products. Investigation of their combined effect helps to better understand the conditions of ASR. In this study, samples with a constant Ca/Si ratio of 0.3 but different K(or Na)/Si and K/Na ratios have been synthesized at 80 °C. Experimental studies and thermodynamic modelling show that a sufficient amount of K or Na is essential to initiate ASR; at low alkali concentrations C-S-H is stabilized instead. However, too high alkaline concentrations (≥900 mM at K(or Na)/Si ≥ 1) also favor C-S-H formation and suppress ASR product formation. The results reveal a strong effect of the alkalis (K and/or Na) on calcium concentrations and on the formation of ASR products; a maximum ASR product formation is observed at Na or K concentrations between 200 and 500 mM and at initial Ca/Si ratio between 0.1 and 0.4.

    更新日期:2019-11-06
  • Concrete mixing truck as a rheometer
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-11-05
    Jon Elvar Wallevik, Olafur Haralds Wallevik

    An increasing interest has emerged in correlating the output of the concrete mixing truck to values obtained by rotational rheometers. The output of the former has usually been the hydraulic pressure needed to turn the drum. In such research, experimental errors can be higher than usual, which makes it harder to obtain confident relationships. To better understand the physical characteristics of the truck's rheological values, the above analysis is made by a series of computer simulations (i.e. with CFD). From this, it is evident that the slope H of the truck's pressure values depends both on the plastic viscosity μ as well as on the yield stress τ0. However, for the intercept G of the truck's values, it is mostly dependent on the yield stress τ0. In addition to this, both values H and G depend on volume of concrete in the truck as well as on density.

    更新日期:2019-11-06
  • The influence of the filler effect on the sulfate requirement of blended cements
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-11-03
    Franco Zunino, Karen Scrivener

    In some blended cements, the optimum amount of sulfate addition differs from that observed in OPC. This study aims to understand the mechanism behind the impact of two SCMs, namely metakaolin and limestone, on the sulfate balance of blended cements. No significant impact of the aluminum content of these SCMs was observed. Instead, it is observed that the filler effect of the SCM, which accelerates the reaction of alite, is the main factor impacting the sulfate balance. As the rate of precipitation of C-S-H is increased, more sulfate is adsorbed by the C-S-H and consequently, the depletion of gypsum is reached earlier in time during the hydration process. A relationship between heat release at the onset of the aluminate peak and the gypsum content of the system was established.

    更新日期:2019-11-04
  • Experimental and micromechanical investigation on the mechanical and durability properties of recycled aggregates concrete
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-11-01
    Ayodele Adessina, Amor Ben Fraj, Jean-François Barthélémy, Camille Chateau, Denis Garnier

    In this work, we conducted an experimental and micromechanical investigation on the mechanical and chloride diffusion properties of recycled aggregates concrete (RAC). Due to the composite aspect of the recycled concrete aggregates (RCA), their impact on the properties of concretes is first evaluated by conducting an experimental campaign and further numerically modeled using micromechanical tools. Local mechanical properties of phases such as the attached mortar, the interfacial transition zones (ITZ) and the aggregates are also investigated by means of indentation tests at nano and microscale. Finally, experimentally informed multi scale model is established and the overall elastic modulus and chloride diffusion coefficient of concretes (with different rates of RCA) are computed and compared to experimental results at macroscopic scale for discussion. The established model provides a predictive tool for the mechanical and chloride diffusion properties of RAC.

    更新日期:2019-11-04
  • Retarder effect on hydrating oil well cements investigated using in situ neutron/X-ray pair distribution function analysis
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-11-01
    Kunal Kupwade-Patil, Peter J. Boul, Diana K. Rasner, S. Michelle Everett, Thomas Proffen, Katharine Page, Dong Ma, Daniel Olds, Carl J. Thaemlitz, Oral Büyüköztürk

    Understanding the role of retarder on the chemical nature and molecular architecture of hydrating cement paste is essential for engineering oil well cements with additives. Here, synchrotron X-ray and total neutron scattering with pair distribution function (PDF) analysis were performed in combination with calorimetry and nuclear magnetic resonance (NMR) to examine the retarder effect in hydrating tri-calcium silicate (C3S) and Class G oil well cement paste. Primarily, the retarder, Diethylenetriamine pentamethylene phosphonic acid (DTPMP) influenced the hydration by affecting the Ca-O and Ca-Si pair correlation providing evidence of calcium playing a predominant role in the retardation process. Secondary effects related to Calcium-Silicate-Hydrate (C-S-H) nuclei poisoning influencing the suppression of calcium hydroxide precipitation were observed. These findings provide insights into the retardation mechanism of hydrating cement paste influenced by calcium depletion when subjected to phosphonate retarders.

    更新日期:2019-11-01
  • Direct Measurements of 3D Structure, Chemistry and Mass Density During the Induction Period of C3S Hydration.
    Cem. Concr. Res. (IF 5.618) Pub Date : 2017-09-19
    Qinang Hu,Mohammed Aboustait,Taehwan Kim,M Tyler Ley,Jeffrey W Bullard,George Scherer,Jay C Hanan,Volker Rose,Robert Winarski,Jeffrey Gelb

    The reasons for the start and end of the induction period of cement hydration remain topic of controversy. One long-standing hypothesis is that a thin metastable hydrate forming on the surface of cement grains significantly reduces the particle dissolution rate; the eventual disappearance of this layer re-establishes higher dissolution rates at the beginning of the acceleration period. However, the importance, or even the existence, of this metastable layer has been questioned because it cannot be directly detected in most experiments. In this work, a combined analysis using nano-tomography and nano-X-ray fluorescence makes the direct imaging of early hydration products possible. These novel X-ray imaging techniques provide quantitative measurements of 3D structure, chemical composition, and mass density of the hydration products during the induction period. This work does not observe a low density product on the surface of the particle, but does provide insights into the formation of etch pits and the subsequent hydration products that fill them.

    更新日期:2019-11-01
  • Direct three-dimensional observation of the microstructure and chemistry of C3S hydration.
    Cem. Concr. Res. (IF 5.618) Pub Date : 2016-10-01
    Qinang Hu,Mohammed Aboustait,Taehwan Kim,M Tyler Ley,Jay C Hanan,Jeffrey Bullard,Robert Winarski,Volker Rose

    Disagreements about the mechanisms of cement hydration remain despite the fact that portland cement has been studied extensively for over 100 years. One reason for this is that direct observation of the change in microstructure and chemistry are challenging for many experimental techniques. This paper presents results from synchrotron nano X-ray tomography and fluorescence imaging. The data show unprecedented direct observations of small collections of C3S particles before and after different periods of hydration in 15 mmol/L lime solution. X-ray absorption contrast is used to make three dimensional maps of the changes of these materials with time. The chemical compositions of hydration products are then identified with X-ray fluorescence mapping and scanning electron microscopy. These experiments are used to provide insight into the rate and morphology of the microstructure formation.

    更新日期:2019-11-01
  • Anion Capture and Exchange by Functional Coatings: New Routes to Mitigate Steel Corrosion in Concrete Infrastructure.
    Cem. Concr. Res. (IF 5.618) Pub Date : 2017-11-07
    Gabriel Falzone,Magdalena Balonis,Dale Bentz,Scott Jones,Gaurav Sant

    Chloride-induced corrosion is a major cause of degradation of reinforced concrete infrastructure. While the binding of chloride ions (Cl-) by cementitious phases is known to delay corrosion, this approach has not been systematically exploited as a mechanism to increase structural service life. Recently, Falzone et al. [Cement and Concrete Research72, 54-68 (2015)] proposed calcium aluminate cement (CAC) formulations containing NO3-AFm to serve as anion exchange coatings that are capable of binding large quantities of Cl- ions, while simultaneously releasing corrosion-inhibiting NO3- species. To examine the viability of this concept, Cl- binding isotherms and ion-diffusion coefficients of a series of hydrated CAC formulations containing admixed Ca(NO3)2 (CN) are quantified. This data is input into a multi-species Nernst-Planck (NP) formulation, which is solved for a typical bridge-deck geometry using the finite element method (FEM). For exposure conditions corresponding to seawater, the results indicate that Cl- scavenging CAC coatings (i.e., top-layers) can significantly delay the time to corrosion (e.g., 5 ≤ df ≤ 10, where df is the steel corrosion initiation delay factor [unitless]) as compared to traditional OPC-based systems for the same cover thickness; as identified by thresholds of Cl-/OH- or Cl-/NO3- (molar) ratios in solution. The roles of hindered ionic diffusion, and the passivation of the reinforcing steel rendered by NO3- are also discussed.

    更新日期:2019-11-01
  • Prediction of autogenous shrinkage of cement pastes as poro-visco-elastic deformation
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-10-26
    Zhangli Hu, Mateusz Wyrzykowski, Karen Scrivener, Pietro Lura

    Precise and robust predictions of autogenous shrinkage of high performance concrete are essential to limit self-induced stresses and cracking in modern concrete structures. This paper presents predictions of autogenous shrinkage for three cement pastes of different compositions by considering first only the poro-elastic deformation and then including also the poro-visco-elastic response. This study used experimentally-determined parameters for the material properties of both the elastic and the visco-elastic components, which ensured the reliability of the calculations. The prediction of the poro-visco-elastic response of the material to the internal stress was performed using a generalized Kelvin-Voigt model that also considered aging. It is found that, while the poro-elastic calculations systematically underestimates the measured shrinkage, predictions with the inclusion of the viscous response overestimates the shrinkage.

    更新日期:2019-10-27
  • The role of calcium on the formation of alkali-silica reaction products
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-10-25
    Zhenguo Shi, Barbara Lothenbach

    Predicting the conditions for alkali-silica reaction (ASR) had been difficult for several decades due to the lack of in-depth knowledge of the ASR products. In this study, thermodynamic data for the synthesized ASR products (i.e., K-shlykovite, Na-shlykovite and ASR-P1) at 80 °C are determined. The effect of the initial Ca/Si ratio, from 0 to 0.5, on the formation of ASR products at 80 °C is investigated for the samples prepared with an initially fixed K/Si or Na/Si ratio of 0.5. The results show that the amount of ASR products formed first increases and then decreases with increasing the initial Ca/Si ratio. The reduced amount of ASR products at higher Ca/Si ratio is accompanied by formation of C-S-H, suggesting that a conversion of ASR products to C-S-H can occur at high Ca/Si ratio. The solid phases and aqueous chemistry predicted by thermodynamic modelling agrees very well with the experimental results.

    更新日期:2019-10-25
  • Understanding why Alite is responsible of the main mechanical characteristics in Portland cement
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-10-25
    M. Laanaiya, A. Bouibes, A. Zaoui

    We provide here a full analysis of the ground state properties of Tricalcium silicate Ca3SiO5 (C3S) by means of density functional theory. Elastic behaviour of C3S was characterized by considering the stable phase and anisotropy of this material. Moreover, the electronic structure is determined in order to get an insight into the reaction mechanism of C3S. Local density of states (LDOS) of valence band maximum (VBM) and conduction band minimum (CBM) are analysed to characterize reactive sites of C3S. Two oxygen atom-types were distinguished from the charge density of the system. LDOS of VBM is highly localized explaining the observed high reactivity of C3S under electrophilic attacks (e.g. H+). In addition, reactive sites under electrophilic attacks are noticed to be only around some specific oxygen atoms. In contrast, the analysis of LDOS for CBM suggests that some calcium atoms are responsible for undergoing reaction with anions such as hydroxide ion.

    更新日期:2019-10-25
  • Combined experimental and numerical study of uniaxial compression failure of hardened cement paste at micrometre length scale
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-10-25
    Hongzhi Zhang, Yading Xu, Yidong Gan, Ze Chang, Erik Schlangen, Branko Šavija

    The aim of this work is to investigate the mechanical performance of hardened cement paste (HCP) under compression at the micrometre length scale. In order to achieve this, both experimental and numerical approaches were applied. In the experimental part, micrometre sized HCP specimens were fabricated and subjected to uniaxial compression by a flat end tip using nanoindenter. During the test, the load-displacement curves can be obtained. In the modelling part, virtual micrometre sized specimens were created from digital material structures obtained by X-ray computed tomography. A computational compression test was then performed on these virtual specimens by a discrete lattice fracture model using the local mechanical properties calibrated in the authors' previous work. A good agreement is found between the experimental and numerical results. The approach proposed in this work forms a general framework for testing and modelling the compression behaviour of cementitious material at the micrometre length scale.

    更新日期:2019-10-25
  • Accelerating the reaction kinetics and improving the performance of Na2CO3-activated GGBS mixes
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-10-24
    N.T. Dung, T.J.N. Hooper, C. Unluer

    This study aimed to accelerate the reaction kinetics and improve the performance of Na2CO3-activated ground granulated blast-furnace slag (GGBS) via the use of reactive MgO and hydromagnesite seeds. The effect of 3–10% MgO and 0.5–1% seeds on the reaction mechanisms of Na2CO3-activated pastes was evaluated by pH, isothermal calorimetry and setting time measurements. Compressive strength results were used to assess the mechanical performance of the Na2CO3-activated GGBS concrete. A thorough investigation involving XRD, TG-DTG, and 29Si and 27Al solid-state NMR was performed to investigate the evolution of different phases. The inclusion of MgO and seeds promoted the removal of CO32−, increasing the pH and accelerating the dissolution of GGBS. The improved reaction kinetics via the use of MgO and seeds led to samples with shortened setting times, increased hydration and enhanced performance (43 vs. 14 MPa at 28 days) when compared to the control sample.

    更新日期:2019-10-25
  • Using graphene oxide to strengthen the bond between PE fiber and matrix to improve the strain hardening behavior of SHCC
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-10-24
    Zeyu Lu, Jie Yao, Christopher K.Y. Leung

    This study develops a novel graphene oxide (GO) coated polyethylene (PE) fiber (GO/PE fiber) by simply mixing PE fibers in GO aqueous solution at a certain temperature. The experimental results indicate that due to the different thermal expansion behavior, the shrinkage of GO at a higher temperature facilitates the formation of a 3D cover around the surface of PE fiber. This would increase the surface wettability, roughness and chemical reactivity of PE fiber, making it much easier for GO/PE fiber to physically and chemically interact with cement hydrates. Compared with the control strain-hardening cementitious composites (SHCC) with pristine PE fiber (2.0 vol%), the use of GO/PE fiber can improve the tensile strength and strain capacity of SHCC by 46.3% and 70.4%, without compromising the compressive strength, and the average crack opening width can be reduced from 138 μm to 58 μm. The remarkable enhancement in the mechanical properties of SHCC is due to the strengthened PE fiber/matrix bond by adding GO, which is further confirmed by results from the aligned single fiber pull out test, showing the increase of interfacial friction from 2.33 MPa to 3.99 MPa. Finally, a micromechanical model is adopted to explain the mechanism behind the improvement of the strain hardening behavior. In conclusion, the research findings provide an effective strategy to functionalize the surface properties of PE fiber by GO coating and to achieve a stronger bond at the fiber/matrix interface, leading to the development of a novel high strength SHCC with tensile strain capacity up to 6%.

    更新日期:2019-10-25
  • Optimized design of ultra-high performance concrete (UHPC) with a high wet packing density
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-10-24
    Xinpeng Wang, Rui Yu, Qiulei Song, Zhonghe Shui, Zhen Liu, Shuo Wu, Dongshuai Hou

    This study presents an optimized design method in developing ultra-high performance concrete (UHPC) with high wet packing density, in which the multiply effects of solid and liquid phases on UHPC packing mode are considered. Specifically, a model based on D-optimal method is firstly established to assess the influence of solid granular particles and superplasticizer (SP) on the UHPC packing density. Based on the theoretical analysis, the mixture proportions can be optimized, and UHPC with high wet packing density could be produced. Then, to evaluate the reliability of the math-physical method in developing UHPC, its compressive strength and pore structure are tested. The obtained experimental results show that the designed UHPC contributes high packing density, leading to optimized pore structure and extraordinary compressive strength. The experimental verification further highlights that D-optimal method is a promising and effective approach to design UHPC, and eventually for the development of sustainable UHPC.

    更新日期:2019-10-24
  • NOx degradation by photocatalytic mortars: The underlying role of the CH and C-S-H carbonation
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-09-11
    A.M. Kaja, H.J.H. Brouwers, Q.L. Yu

    This study aims to understand the impact of carbonation mechanism of C-S-H and CH in photocatalytic mortars on NOx removal efficiency. Changes in surface chemistry and microstructure induced by the carbonation of portlandite and C-S-H (AFm/AFt) were correlated with the photocatalytic efficiency of the mortars doped with three types of titania-based photocatalysts. Furthermore, the influence of cementitious matrix on the photocatalytic selectivity was evaluated by studying the capacity of hydration/carbonation products to adsorb NO2. The study revealed that in terms of both photocatalytic efficiency and selectivity, mortars with microsilica addition exhibit superior properties over the pure cement-based mortars upon carbonation. Carbonation of C-S-H (AFm/AFt) gel results in the formation of capillary pores between10–50 nm, which outbalances the shielding effects of carbonates formed, leading to the enhanced photocatalytic properties. Moreover, C-S-H gel maintains its high NO2 adsorption capacity even after carbonation, resulting in the high selectivity of the photocatalysis.

    更新日期:2019-09-11
  • Conceptual design and performance evaluation of two-stage ultra-low binder ultra-high performance concrete
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-09-10
    P.P. Li, Q.L. Yu, H.J.H. Brouwers, W. Chen

    This study proposes a novel concept of two-stage ultra-high performance concrete (TS-UHPC), towards ultra-low binder consumption. The effects of grout and coarse aggregate are investigated and their compatibility is evaluated. Results show that TS-UHPC has a low binder amount (down to 364 kg/m3) and high binder efficiency (up to 0.417 MPa·m3/kg), possessing excellent compressive strength of up to 151.8 MPa at 91 days. Microstructural analysis reveals that grout with a sand-to-powder ratio of 1.0 shows a higher hydration degree, denser structure, and increased later strength. Coarser basalt aggregate tends to slightly lower compressive and splitting tensile strength, 14% and 12% reduction with the maximum size from 8 mm to 25 mm, respectively. The TS-UHPC has an excellent interfacial transition zone that induces a water-permeable porosity of 0.91%–1.32%. New formulas are proposed to describe correlation between compressive and splitting tensile strength of TS-UHPC, and to predict strength of TS-UHPC by grout.

    更新日期:2019-09-10
  • Self-healing of Engineered Geopolymer Composites prepared by fly ash and metakaolin
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-09-10
    Li-li Kan, Ji-wei Lv, Bei-bei Duan, Min Wu

    This work focuses on investigating the self-healing characteristics and mechanism of Engineered Geopolymer Composites using fly ash and metakaolin as the precursors (MFA-EGCs). The hardened specimens were preloaded to different tensile strain levels followed by exposure to air and wet-dry cycles allowing for self-healing. Apart from the cracking characteristics, tensile properties were studied together with chemical and microstructural analyses. The results revealed a pronounced multiple cracking pattern and strain-hardening behavior for the MFA-EGCs. The number of cracks decreases significantly after the self-healing. High recovery ratios of the tensile strains and the ultimate tensile strengths were found, with some studied specimens showing even better tensile properties than the control specimens. It seems that air exposure is more favorable for the self-healing of the MFA-EGCs compared with exposure to the wet-dry cycles. Based on the results from SEM-EDS, XRD and FT-IR, it was concluded that amorphous aluminosilicate phases are the main healing products.

    更新日期:2019-09-10
  • Micropillar compression investigation of size effect on microscale strength and failure mechanism of Calcium-Silicate-Hydrates (C-S-H) in cement paste
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-09-09
    Rahnuma Shahrin, Christopher P. Bobko

    The compressive strength and failure of concrete and cement exhibit strong size effect over various length scales. To investigate possible size effect on compressive strength and failure mechanism of Calcium-Silicate-Hydrates (C-S-H) in cement paste, micropillar compression experiments were performed on micropillar geometries fabricated by focused ion beam milling on potential C-S-H locations identified through coupled backscatter electron imaging (BSE) and energy dispersive spectroscopy (EDS) analysis. The compressive strength of C-S-H (181–1145 MPa) measured from C-S-H micropillars of varying diameters indicated presence of a size effect with strong increase in strength with decreasing diameter. The deformation mode at failure also exhibited size effect: the dominant failure mode changed from axial splitting to plastic crushing as the pillar diameter was decreased. The observed relationship between strength and pillar diameter can be modeled by an inverse square root dependency which closely corresponds to Bazant's scaling law of quasi-brittle failure.

    更新日期:2019-09-09
  • Carbonation of steel slag and gypsum for building materials and associated reaction mechanisms
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-09-07
    Xue Wang, Wen Ni, Jiajie Li, Siqi Zhang, Michael Hitch, Rodrigo Pascual

    The carbonation of steel slag to produce building material is a useful way to increase the utilization of steel slag and absorb carbon dioxide. In this study, gypsum, steel slag, and water were mixed, compaction-shaped, and carbonation-cured as a means of improving the strength of the steel slag. It was observed that gypsum promoted an increase in both the compressive strength and the CO2 uptake of steel slag. CO2 uptake was positively correlated with strength. Microanalysis indicated that the main hydration product were C-S-H phases and ettringite, while the main carbonation products were calcite and monocarbonate (C3A. CaCO3.11H2O). Gypsum is speculated to promote the rapid hydration of steel slag to form ettringite (C3A.3CaSO4.32H2O), which then reacts with CO2 to produce monocarbonate; thus, gypsum plays a catalytic role in this system. The results of this study therefore provide theoretical guidance for the preparation of steel slag–gypsum carbide building materials.

    更新日期:2019-09-07
  • Interaction mechanisms between organic and inorganic phases in calcium silicate hydrates/poly(vinyl alcohol) composites
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-09-05
    Yang Zhou, Luping Tang, Jiaping Liu, Changwen Miao

    Poly (vinyl alcohol) (PVA) emulsion is commonly used in cement-based materials to obtain higher performance. The present work investigates the interaction mechanisms between PVA and calcium silicate hydrates (C-S-H) using ambient and high pressure X-ray diffraction (XRD), 29Si magic angle spinning nuclear magnetic resonance (MAS NMR), nano-indentation, and molecular dynamics simulation. In a C-S-H nano-particle, small-size PVA molecules can be intercalated into the interlayer region of the calcium silicate sheets, lowering the interlayer incompressibility and the overall bulk modulus of C-S-H at the crystal length scale. At a larger length scale, the large-size PVA chains can be in the midst among the C-S-H nano-particles, decreasing the porosity, thus the indentation modulus of the meso-composite is improved.

    更新日期:2019-09-06
  • Formation of carbonate phases and their effect on the performance of reactive MgO cement formulations
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-09-06
    N.T. Dung, A. Lesimple, R. Hay, K. Celik, C. Unluer

    This study investigated the formation of different phases and their influence on the microstructural and mechanical development of carbonated reactive MgO cement formulations. Hydrated magnesium carbonates (HMCs) were identified and quantified via XRD, TG-DTG and SEM. Visual observations of the 3D internal structure at different depths were performed by X-ray computed tomography and confocal Raman microscopy. Elastic modulus of different components was obtained by nano-indentation. The expansive formation of HMCs led to dense microstructures composed of well-connected carbonate networks. The transition of artinite to hydromagnesite/nesquehonite was observed over time. The initial formation of a dense carbonate layer on sample surface inhibited continuous diffusion of CO2, resulting in a slower strength development at later ages. Despite this limitation, the continuation of hydration and evolution of carbonate morphology led to 28-day strengths of 60 MPa in concrete samples, supported by the higher elastic modulus of HMCs than brucite (19.4 vs. 2.5 GPa).

    更新日期:2019-09-06
  • Review of fundamental assumptions of the Two-Phase model for aggregate interlocking in cracked concrete using numerical methods and experimental evidence
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-09-05
    Mohit Pundir, Max Tirassa, Miguel Fernández Ruiz, Aurelio Muttoni, Guillaume Anciaux

    Aggregate interlocking allows transferring shear and normal stresses through open cracks, and is considered to significantly contribute to the force transfer in cracked concrete. The complex phenomenon depends on the roughness of cracked surfaces, where material protruding from one side may engage with the opposite one. Two-Phase models were established in the 1980s by Walraven to estimate the force transfer, distinguishing between cement matrix and spherical aggregates. The approach leads to good results but has several shortcomings. In this paper, the fundamental assumptions are reviewed using specific numerical and experimental investigations. Special tests respecting the geometrical assumptions are presented and the results compared with numerically calculated estimates. The model is extended to address some shortcomings and investigate the physical nature of the main parameters. Positive aspects of Two-Phase models and a number of limitations are highlighted, allowing a consistent step forward in the understanding of aggregate interlocking.

    更新日期:2019-09-05
  • pH dependent leaching characterization of major and trace elements from fly ash and metakaolin geopolymers
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-09-04
    Zengqing Sun, Anya Vollpracht, Hans A. van der Sloot

    The leaching characteristics of Cl−, SO42−, Na, K, Al, Si, Ca, Mg, Ba, Cd, Co, Cr, Cu, Mn, Mo, Ni, Pb, Sb, Se, Sr, Tl, V, Zn from fly ash and metakaolin geopolymer mortars were characterized in a broad pH range (1–14) in this work. Apart from revealing the partitioning of chemical species between the solid and aqueous phase, the acid neutralization property of both geopolymers were studied. In contrast with the virtually pH independent characteristic of chloride and sulfate, the leaching of the other elements showed pH dependent features, which can be further divided into amphoteric, cationic and oxyanionic patterns. Formation of montmorillonite and amorphous silica were detected by X-ray diffraction after the leaching. The leaching results and mineralogical measurement were coupled with geochemical modelling to identify the solubility controlling phases of Al, Si, Ca and Mg.

    更新日期:2019-09-05
  • Numerical and analytical modeling of fiber-matrix bond behaviors of high performance cement composite
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-09-04
    Chaohui Zhang, Caijun Shi, Zemei Wu, Xue Ouyang, Kai Li

    Use of steel fiber in high performance cement composite (HPCC) can enhance its strength and toughness, which is achieved through stress transfer from fiber to matrix at interface. In this study, a numerical model in Finite Element Method and three analytical models were proposed to predict bond behaviors between HPCC matrix and three different shaped (straight, hooked, and corrugated) steel fibers at different ages. The proposed numerical model considered an enhancement factor based on the modified interfacial friction law (MIFL). In the analytical models, frictional force following a power function was considered. Predicted results from those numerical and analytical models were compared with existing models and corresponding experimental results. The comparisons indicated that the enhancement factor employed in the MIFL efficiently reflected the change in fiber-matrix bond properties with age. The predicted pullout load-slip relationships and energies from those proposed models showed higher accuracy than those from the existing models.

    更新日期:2019-09-04
  • Experimental database of mixed-mode crack propagation tests performed on mortar specimens with a hexapod and full-field measurements. Part II: interactive loading
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-08-31
    A. Carpiuc-Prisacari, C. Jailin, M. Poncelet, K. Kazymyrenko, H. Leclerc, F. Hild

    This second paper presents a series of 4 crack propagation tests with the same experimental protocol as in a companion paper, but with some significant loading modifications. The first difference is that the loading is composed of in-plane rotation in addition to tension and shear translations. The second difference is that the loading is manually changed during the tests, depending on the crack tip location. This leads to tests with several bifurcations, and/or different loading ratios during the same test. One of them leads to mode I+II, and then mode I+III crack propagation. Some tests end with instabilities while others are controlled to be stable up to the complete failure of the specimen. In some cases, crack closure and friction between the crack faces occur.

    更新日期:2019-09-03
  • Stability criterion for fresh cement foams
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-08-31
    Blandine Feneuil, Patrick Aimedieu, Mario Scheel, Jonathan Perrin, Nicolas Roussel, Olivier Pitois

    We prepare and study cement foam samples with well-controlled structure, i.e. containing monodisperse bubbles. We observe that the foam structure often changes before cement setting and identify ripening as the major destabilization mechanism at stake. Drainage plays only a minor role in cement foam destabilization except when bubble size is large. Then we show that a single stability criterion can be defined, for a large range of cement foams with different formulations. This criterion involves the bubble radius and the yield stress of the cement paste such as confined by and between the bubbles, at a given characteristic time after sample preparation.

    更新日期:2019-09-03
  • Impact of temperature on expansive behavior of concrete with a highly reactive andesite due to the alkali–silica reaction
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-08-31
    Yuichiro Kawabata, Cyrille Dunant, Kazuo Yamada, Karen Scrivener

    The influence of temperature on the expansive behavior and microstructural damage of concrete due to the alkali–silica reaction (ASR) is studied experimentally by applying a new testing protocol, the Alkali-Wrapped Concrete Prism Test (AW-CPT). Early-stage expansion is highly dependent on temperature, more so than expansion in later stages. In the early stages of expansion, the ASR gel is well restrained so that the early-stage expansion rate mainly depends on reaction kinetics. In later stages, since the viscosity of the ASR gel is lower at higher temperatures, it flows out of the concrete prism through small cracks or pores, resulting in reduced paste cracking and an increased silica content detected in the wrapping cloth at higher temperatures. Consequently, the late-stage expansion rate is found to be reduced at higher temperatures. Therefore, greatly accelerated test conditions may lead to reduced expansion of the concrete prism during the AW-CPT.

    更新日期:2019-08-31
  • Mechanism of sulfate attack on alkali-activated slag: The role of activator composition
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-08-30
    Hailong Ye, Zhijian Chen, Le Huang

    In this work, the degradation mechanisms of alkali-activated slag (AAS), prepared with various activator compositions, exposed to 5% Na2SO4 and MgSO4 solutions are studied. The results show that upon Na2SO4 attack, little ettringite formation and compositional alteration occur in AAS with little length expansion. However, upon MgSO4 attack, brucite, gypsum, and magnesium-aluminosilicate-hydrate (M-A-S-H) are observed in altered layers of hardened AAS pastes. The sulfate-activated slag shows a weaker resistance against MgSO4 attack than NaOH- and Na2CO3-counterparts due to a lower pH value in pore solution and thus absence of brucite protective layer. The reacted products in hardened AAS pastes have a high capacity against dealumination in sulfate environments, contributing to superior sulfate resistance over traditional ordinary Portland cement (OPC).

    更新日期:2019-08-30
  • Physicochemical and mechanical changes of thermally damaged cement pastes and concrete for re-curing conditions
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-08-26
    Hong Jae Yim, Sun-Jong Park, Yubin Jun

    Thermally damaged concrete recovers its material properties under specific post-heating curing conditions. Among the previous studies to investigate the effects of post-heating curing, the nonlinear resonance vibration method was recently proposed to evaluate contact-type defects of concrete induced by high temperature although the use of physicochemical analysis is not reported to investigate the rehydration products of concrete on post-damage curing conditions. This study performed destructive and nondestructive tests, such as the measurement of tensile strength, dynamic elastic modulus, and hysteretic nonlinear parameter, to evaluate the degree of thermal damage of concrete and its restoration. Various types of concrete samples were prepared according to the different exposures of high temperature and relative humidity for re-curing. In addition, XRD analysis of heated cement pastes was performed to identify the formed and disappeared hydration products, and physicochemical characteristics of thermally damaged concrete was discussed with respect to various exposed temperature and re-curing conditions.

    更新日期:2019-08-26
  • The microstructure development during bleeding of cement paste: An NMR study
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-08-20
    Yanliang Ji, Leo Pel, Zhenping Sun

    NMR was applied to investigate the microstructure development of cement paste during bleeding. In this study, we have measured the void ratio and the T1 relaxation, reflecting the pore structure development, at two fixed positions during the bleeding process of cement paste, whereas the void ratio was also measured over the complete sample during hydration. Here we have compared Portland and blast furnace slag cement and looked at various factors such as water-cement ratio, slag content and the use of water reducer. It was found that the void ratio at the top almost remains constant, and the void ratio at the bottom gradually decreased until it reached a constant value during bleeding. The results indicate that the finite-strain model is best suited to predict the structure development during a consolidation process. Moreover, it was found that the porosity is not totally linearly related to the distance as predicted by the consolidation model because of the existence of the transition zone.

    更新日期:2019-08-21
  • Development of the strain field along the crack in ultra-high-performance fiber-reinforced concrete (UHPFRC) under bending by digital image correlation technique
    Cem. Concr. Res. (IF 5.618) Pub Date : 2019-08-19
    Yanfei Niu, Haoliang Huang, Jie Zhang, Wen Jin, Jiangxiong Wei, Qijun Yu

    The digital image correlation (DIC) technique was applied to the analysis of displacement fields along the surface cracks of ultra-high-performance fiber-reinforced concrete (UHPFRC) under a bending load. Analysis was performed on the fiber bridging forces acting on a unit area of cracked sections belonging to different regions (perfect bonding region, partial debonding region, and full debonding region), according to the fiber distribution characterization and DIC data. By using the DIC technique, the visualization and quantification of the UHPFRC fracture properties could be determined. The length of the uniform zone increased, whereas the fracture process decreased under the external load of the limit of proportionality (LOP). With the appearance of macro-cracks under the external load of the modulus of rupture (MOR), the length of the uniform and fracture process zones increased as the fiber content increased. However, the localization zone decreased. The steel fiber volume fractions had limited influence on the tensile strain at cracking and the complete debonding of fibers.

    更新日期:2019-08-20
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