Expansion and deterioration of concrete due to ASR: Micromechanical modeling and analysis Cem. Concr. Res. (IF 5.43) Pub Date : 2018-10-16 Tagir Iskhakov, Jithender J. Timothy, Günther Meschke
A multi-scale micromechanics model is proposed to describe the expansion and deterioration of concrete due to Alkali-Silica Reaction (ASR). The mechanics of ASR induced deterioration of a Representative Elementary Volume (REV) of concrete is modeled through a synthesis of distributed microcracking and mean-field homogenization. At the microscale, ASR-gel-pressure induced microcrack growth in and around the reactive aggregates is modeled using the framework of linear elastic fracture mechanics. Mean-field homogenization across multiple scales is used to obtain the overall expansion and degradation of the material. By specifying the spatial distribution of the pressurizing gel, two different ASR mechanisms associated with “slowly” and “rapidly” reactive aggregates can be modeled. Experimental data for concrete degradation as a function of the macroscopic expansion is found to lie within the theoretical upper and lower bounds that characterize the distribution of the gel in the aggregate or the cement paste.
Predicting the time to corrosion initiation in reinforced concrete structures exposed to chlorides Cem. Concr. Res. (IF 5.43) Pub Date : 2018-10-12 Ueli M. Angst
Reliable predictions of the time to onset of corrosion in reinforced concrete are essential for service life modelling, to ensure sufficient durability, and for holistic sustainability assessments of new materials. All existing models are based on the same concept, that is, predicting the development over time of the chloride content at the level of the steel and comparing this numerical result with the critical chloride content for corrosion initiation, Ccrit. This paper presents example calculations utilizing input data derived from both laboratory specimens and from structures, illustrating the poor predictive power of state-of-the-art models. While it is generally assumed that improving chloride ingress models will improve the prediction of the time-to-corrosion, this paper shows that the bottle neck to more reliable predictions are rather i) the lack of fundamental understanding of corrosion initiation, ii) the use of non-representative laboratory results, and iii) ignoring the size-effect in localized corrosion.
Cemdata18: A chemical thermodynamic database for hydrated Portland cements and alkali-activated materials Cem. Concr. Res. (IF 5.43) Pub Date : 2018-10-11 Barbara Lothenbach, Dmitrii A. Kulik, Thomas Matschei, Magdalena Balonis, Luis Baquerizo, Belay Dilnesa, George D. Miron, Rupert J. Myers
Thermodynamic modelling can reliably predict hydrated cement phase assemblages and chemical compositions, including their interactions with prevailing service environments, provided an accurate and complete thermodynamic database is used. Here, we summarise the Cemdata18 database, which has been developed specifically for hydrated Portland, calcium aluminate, calcium sulfoaluminate and blended cements, as well as for alkali-activated materials. It is available in GEMS and PHREEQC computer program formats, and includes thermodynamic properties determined from various experimental data published in recent years. Cemdata18 contains thermodynamic data for common cement hydrates such as C-S-H, AFm and AFt phases, hydrogarnet, hydrotalcite, zeolites, and M-S-H that are valid over temperatures ranging from 0 to at least 100 °C. Solid solution models for AFm, AFt, C-S-H, and M-S-H are also included in the Cemdata18 database.
Effects of severe heating and rehydration on poro-mechanical properties of a mortar Cem. Concr. Res. (IF 5.43) Pub Date : 2018-10-10 Yan Pei, Shu-cai Li, Franck Agostini, Frédéric Skoczylas
A normalized mortar was severely heated at different temperatures up to 600 °C that led to a strong material degradation linked to its loss of water and to the Portlandite decomposition. These heating were followed by pure water rehydration as these operations have proven to be efficient as regards the recovery in the transfer properties. The present study is based on the main material's poromechanical property measurements with gas as the pressurized porous fluid. They are useful to indicate, after heating, the strong increase of the mortar's skeleton compressibility and of its expansion due to internal pressure. These phenomena are due to heating micro-cracking, pore widening and to a material fractioning highlighted with the increase in its Biot's coefficient. Unambiguously, rehydration led to a visible recovery of the mortar's poromechanical properties that is linked to newly formed hydrates.
Carbonation of hardened cement pastes: Influence of temperature Cem. Concr. Res. (IF 5.43) Pub Date : 2018-10-05 Emeline Drouet, Stéphane Poyet, Patrick Le Bescop, Jean-Michel Torrenti, Xavier Bourbon
The impact of temperature on carbonation was investigated in laboratory conditions using a device developed for this purpose. Two hardened cement pastes (CEM I and CEM V/A) were tested between 20 °C and 80 °C at different levels of relative humidity (RH). The carbonation rate of the CEM I increased with temperature, whereas that of CEM V/A reached a maximum at around 50 °C.
Triaxial mechanical behavior of early age concrete: Experimental and modelling research Cem. Concr. Res. (IF 5.43) Pub Date : 2018-10-01 Da Chen, Xiaotong Yu, Ruiwen Liu, Shagang Li, Yan Zhang
Considering the triaxial behavior of concrete is highly related to its hydration process with curing temperature and time, the triaxial mechanical behavior of early age concrete under different curing conditions is investigated. Uniaxial/triaxial compression tests of a C30 concrete cured at representative temperature and time are performed while its pore characteristic is determined. Based on experimental analysis, a curing factor indicative of the effect of curing conditions is proposed and discussed by empirical functions. It is found that the uniaxial/triaxial failure strength and elastic modulus increase with curing factor, but the deformation decreases slightly. The pore structures evolve with curing factor. Furthermore, the influence of curing factor on the strength decreases with the confining pressure. Finally, a hydration dependent modelling is proposed by introducing curing factor. Numerical simulations show that the proposed modelling can well predict the triaxial mechanical behavior of early age concrete under different curing conditions.
Mineralogical and microstructural evolution of Portland cement paste/argillite interfaces at 70 °C – Considerations for diffusion and porosity properties Cem. Concr. Res. (IF 5.43) Pub Date : 2018-09-29 Philippines Lalan, Alexandre Dauzères, Laurent De Windt, Juuso Sammaljärvi, Danièle Bartier, Isabelle Techer, Valéry Detilleux, Marja Siitari-Kauppi
A Portland cement (CEM I) paste was poured onto an argillite disk in diffusion cells with reservoirs filled by alkaline water and argillite pore water. The system evolution was followed over the course of 415 days. The imposed temperature of 70 °C affected the mineralogy (precipitation of crystallized C-S-H) and mechanical strength of the interface that became brittle over time. The interface consisted of a calcite/tobermorite/C-A-S-H layer, whose thickness increased at a growth rate of 0.3 μm/d. Contrary to calcite crusts that formed in immersion tests or when hardened cement was placed in contact with argillite, this layer had no significant effect on the diffusion properties during the one-year duration of the experiment due to its microporous structure and rather small thickness (100 μm). The argillite mineralogy was altered over 100 μm. In the cement paste, the total porosity decreased because carbonation was enhanced with temperature, which counterbalanced the effect of decalcification over 400 μm.
Interplay between silicate and hydroxide ions during geopolymerization Cem. Concr. Res. (IF 5.43) Pub Date : 2018-09-29 Julien Aupoil, Jean-Baptiste Champenois, Jean-Baptiste d'Espinose de Lacaillerie, Arnaud Poulesquen
Two sets of activating solutions with increasing sodium hydroxide contents were prepared either with or without silicates. Their buffer capacities, i.e. their ability to resist changes in pH, were determined and compared using the Hammett acidity function, a measure of acidity appropriate for concentrated solutions. This is the first time the Hammett acidity function of sodium silicate solutions has been measured. The effects of the buffer capacity and of the initial Hammett acidity function on the reactivity of metakaolin-based pastes were assessed using isothermal conduction micro-calorimetry. The reactivity of metakaolin in sodium hydroxide solutions is shown to be directly related to the initial Hammett acidity function, whereas for sodium silicate mixtures, the buffer capacity is a more pertinent parameter. The mechanism deduced for the role of hydroxide ions during geopolymerization also highlights the role of silicate species as a hydroxide reservoir that nurtures the dissolution process.
Rietveld quantitative phase analyses of SRM 2686a: A standard Portland clinker Cem. Concr. Res. (IF 5.43) Pub Date : 2018-09-26 M. García-Maté, G. Álvarez-Pinazo, L. León-Reina, A.G. De la Torre, M.A.G. Aranda
SRM 2686a is a NIST reference Portland clinker with reported mineralogical analysis from powder diffraction and electron microscopy. This sample is used in ASTM C1365 test method for Rietveld quantitative phase analysis validation procedure. Here, we have analysed SRM 2686a by using three state-of-the-art powder diffraction configurations: i) strictly monochromatic CuKα1 radiation in flat reflection geometry; ii) strictly monochromatic MoKα1 radiation in flat transmission geometry; and iii) synchrotron radiation in rotating capillary transmission geometry. The silicate and aluminate enriched residues have also been studied by CuKα1 powder diffraction. All the powder patterns were analysed by Rietveld method with the best available protocols. The results indicate that belite in SRM 2686a is composed of two polymorphs (β- and α'H-) that must be included in the analyses. The use of a unique phase for describing belite (β-polymorph) and improper peak shape modelling could explain the problems found for implementing ASTM C1365 in some cement manufacturing plants. Furthermore, all the patterns are deposited as open data access at Zenodo, so interested laboratories can analyse these data to verify their protocols.
The chemistry and structure of calcium (alumino) silicate hydrate: A study by XANES, ptychographic imaging, and wide- and small-angle scattering Cem. Concr. Res. (IF 5.43) Pub Date : 2018-09-26 Jiaqi Li, Guoqing Geng, Rupert Myers, Young-Sang Yu, David Shapiro, Carlo Carraro, Roya Maboudian, Paulo J.M. Monteiro
Calcium (alumino)silicate hydrate (C-(A-)S-H) is the main binding phase in blended cement concrete. Understanding the chemistry and structure of C-(A-)S-H is essential to optimizing concrete properties such as compressive strength and durability; yet questions remain around the coordination environments of Ca and Al in its structure with various chemical compositions and equilibration temperatures. C-(A-)S-H with Ca/Si = 0.6–1.6, Al/Si = 0–0.1, and equilibrated at 7–80 °C is studied by nanoscale soft X-ray spectroscopy at the Ca L2,3- and Si K-edges. Highly distorted CaO7 complexes occur in the intralayer of C-(A-)S-H irrespective of Ca/Si, Al/Si, and temperature. Zeolitic Ca in the interlayer of C-(A-)S-H is highly distorted from an ideal octahedral coordination. Third aluminate hydrate is either not Ca-bearing or its Ca is structurally similar to C-(A-)S-H and does not resemble the Ca in AFm-phases. Increasing aluminosilicate chain polymerization in C-(A-)S-H shifts the Si K-edge to higher energies, implying Al uptake in the bridging and/or cross-linked sites, as well as a contraction of SiO bond lengths. C-(A-)S-H exhibits a foil-like morphology, with individual foils comprised of nano-sized platelets with comparable thickness regardless of Ca/Si or Al/Si at 7–50 °C. Coarser C-(A-)S-H foils occur at 80 °C and higher Al/Si ratios relative to lower temperatures and Al content.
The use of rice husk ash as reactive filler in ultra-high performance concrete Cem. Concr. Res. (IF 5.43) Pub Date : 2018-09-26 Sung-Hoon Kang, Sung-Gul Hong, Juhyuk Moon
In this study, rice husk ash (RHA)-based reactive filler was used in ultra-high performance concrete (UHPC) to improve mechanical properties without heat-treatment. This strategy replaces inert quartz filler with the reactive RHA filler, to increase the amorphous silica content while maintaining the physical role of the micron-sized quartz filler. Due to the high porosity of RHA, internal curing is effective, which promotes the hydration reaction over a long period of time. Experimental results show an outstanding strength around 200 MPa after 91 days, under ambient conditions (20 °C and 60% relative humidity). This was possible due to the promotion of pozzolanic reaction by additional water and amorphous silica provided by the porous (i.e., internal curing effect) and reactive filler, respectively; hence, the volume of capillary pores was reduced. The result reported herein will further promote the utilization of agricultural byproduct for the development of reactive RHA-based construction materials.
Can the compressive strength of concrete be estimated from knowledge of the mixture proportions?: New insights from statistical analysis and machine learning methods Cem. Concr. Res. (IF 5.43) Pub Date : 2018-09-27 Benjamin A. Young, Alex Hall, Laurent Pilon, Puneet Gupta, Gaurav Sant
The use of statistical and machine learning approaches to predict the compressive strength of concrete based on mixture proportions, on account of its industrial importance, has received significant attention. However, previous studies have been limited to small, laboratory-produced data sets. This study presents the first analysis of a large data set (>10,000 observations) of measured compressive strengths from actual (job-site) mixtures and their corresponding actual mixture proportions. Predictive models are applied to examine relationships between the mixture design variables and strength, and to thereby develop an estimate of the (28-day) strength. These models are also applied to a laboratory-based data set of strength measurements published by Yeh et al. (1998) and the performance of the models across both data sets is compared. Furthermore, to illustrate the value of such models beyond simply strength prediction, they are used to design optimal concrete mixtures that minimize cost and embodied CO2 impact while satisfying imposed target strengths.
Autogenous healing of fiber-reinforced reactive magnesia-based tensile strain-hardening composites Cem. Concr. Res. (IF 5.43) Pub Date : 2018-09-27 Jishen Qiu, Shaoqin Ruan, Cise Unluer, En-Hua Yang
Reactive magnesia-based cement (RMC) is an emerging group of alternative binder to Portland cement. Recently, the first fiber-reinforced RMC-based strain-hardening composites (SHC) have been developed by the authors. The current work investigated the feasibility of the PC-free RMC-based SHC formulations to engage autogenous healing. Results showed that crack sealing and significant mechanical recovery can be realized through proper environmental conditioning. The presence of water is necessary to engage autogenous healing and elevated CO2 concentration leads to the formation of HMCs that can seal larger crack. However, ample supply of CO2 results in fast sealing of crack on the near surface region, which blocks the pathway for further carbonation and healing of interior region of cracks. Microstructure analysis reveals that the healing products are hydrated magnesium carbonates (HMCs) and different conditioning regimes lead to different types of HMCs as the healing products.
The needle model: A new model for the main hydration peak of alite Cem. Concr. Res. (IF 5.43) Pub Date : 2018-09-25 Alexandre Ouzia, Karen Scrivener
This article presents a new model for the main hydration peaks of tricalcium silicate and alite on the assumption that C-S-H nucleates and grows as needles. The model relies only on directly observable quantities and reproduces the transition from the acceleration to deceleration periods without assuming a diffusion barrier or impingement. Simulations of the model based on measured characteristics of the grains and needles are compared with experimental data and satisfactory agreement is found across a wide range of experiments. Because the model disentangles the impact of each input parameter, it sheds new light on some aspects of alite hydration. In particular, the model provides a quantification of the impact of the consumption of the small grains and shows that it should not be neglected as soon as several tens of percent of the hydration reaction is reached. Finally, the article provides a critical examination of other models.
Copper and cobalt improve the acid resistance of alkali-activated cements Cem. Concr. Res. (IF 5.43) Pub Date : 2018-09-22 Juan Pablo Gevaudan, Alejandro Caicedo-Ramirez, Mark T. Hernandez, Wil V. Srubar
Experimental evidence of a new acid degradation mechanism in alkali-activated cements (AACs) micro-doped with copper (Cu) and cobalt (Co) is presented in this work. Cu and Co incorporation into binary metakaolin and basic oxygen furnace (BOF) slag-based AACs reduced bulk permeable porosity and acid penetration and retarded the formation of calcium sulfate phases upon exposure to acid. Analysis of microstructural evolution and elemental mobility using X-ray diffraction and electron microprobe analysis (EMPA) showed that Cu and Co doping was associated with major differences in AAC leaching patterns when exposed to sulfuric acid. Converging lines of evidence suggest that acid resistance is improved by the preferential mobilization of Cu and Co, along with other multivalent cations (i.e., magnesium), at the acid degradation front(s), stabilizing the AAC binder and inhibiting further deterioration.
Preferred orientation of calcium aluminosilicate hydrate induced by confined compression Cem. Concr. Res. (IF 5.43) Pub Date : 2018-09-13 Guoqing Geng, Roman Nikolayevich Vasin, Jiaqi Li, Mohammad Javad Abdolhosseini Qomi, Jinyuan Yan, Hans-Rudolf Wenk, Paulo J.M. Monteiro
The existing macroscale models of the calcium (alumino)silicate hydrate (C-(A-)S-H), the main binder of concrete, assume that the nanocrystallites maintain random orientation under any loading conditions. However, using synchrotron-radiation-based XRD, we report the development of preferred orientation of nanocrystalline C-A-S-H, from random at ambient pressure to strongly oriented under uniaxial compression with lateral confinement. The c-axes of the nanocrystals tend to align with the primary load. This preferred orientation is preserved after removing of external loading. The texture, quantified using a standard Gaussian fiber orientation distribution function (ODF), was used to calculate the averaged bulk elastic tensor of oriented C-(A-)S-H. It changes from isotropic (without texture) to transversely isotropic (with texture). Our results provide direct evidence of the reorientation of nanocrystalline C-(A-)S-H as a mesoscale mechanism to the irreversible deformation of cement-based material. The implications of these results for modeling the mechanical property of C-(A-)S-H at the macroscale are discussed.
Modeling of water transport in highly saturated concrete with wet surface during freeze/thaw Cem. Concr. Res. (IF 5.43) Pub Date : 2018-09-11 Fuyuan Gong, Stefan Jacobsen
Wet frost exposure is a usual environmental condition for cement-based materials (CBM) during winter and the most common way of frost testing in the lab. In this study, the temperature gradient, pressure gradient, and the liquid flow under wet frost exposure are modeled for highly saturated CBM with different amount of entrained air. It is found that the water uptake happens at the melting stage, and for non-air-entrained CBM, the hydraulic pumping effect is dominant and will suck the water from wet surface. While for air-entrained CBM, the cryosuction pressure is the main driving force of the inward flow. The results are compared with experimental data from rapid freeze/thaw testing of various types of concrete in water, showing a satisfactory agreement. Sensitivity analysis also indicates that the hydraulic induced flow depends on the amount of entrained air, while the cryosuction induced flow mainly relies on the permeability and temperature gradient.
Pore network of cement hydrates in a High Performance Concrete by 3D FIB/SEM — Implications for macroscopic fluid transport Cem. Concr. Res. (IF 5.43) Pub Date : 2018-09-11 Yang Song, C.A. Davy, D. Troadec, X. Bourbon
Alite dissolution and C-S-H precipitation rates during hydration Cem. Concr. Res. (IF 5.43) Pub Date : 2018-09-07 C. Naber, F. Bellmann, T. Sowoidnich, F. Goetz-Neunhoeffer, J. Neubauer
An extensive dataset suitable for kinetic calculations was collected for the hydration of monoclinic alite at water to solid ratios of 10 and 0.5. The measurement methods used to investigate the hydration behavior are XRD, TGA, solid state NMR, BET, SEM and ICP-OES. A comparison of the two water to solid ratios shows only minor effects of the w/s-ratio on the reaction kinetics. Reaction rates for the two kinetic steps alite dissolution and C-S-H precipitation are calculated from this dataset and compared to the measured reaction rates. Computed reaction rates from C-S-H precipitation show a good match with the measured reaction rates, indicating that the computation incorporates all relevant factors related to C-S-H precipitation kinetics during hydration.
Moisture ingress in cracked cementitious materials Cem. Concr. Res. (IF 5.43) Pub Date : 2018-09-08 A. Michel, B.J. Pease
Wedge split test specimens were conditioned to a relative humidity of 50%, deformed to various damage states, and exposed to liquid water. Water ingress was monitored using x-ray attenuation measurements and compared to numerical predictions. The transport model accounts for the damage state using the cracked hinge model and a simplified approach whereby a crack is considered to consist of two distinct parts: 1) a coalesced crack behaving as a free surface for moisture ingress, and 2) an area of isolated micro cracks behaving as bulk material. Comparison of experimental and model results shows the simplified crack geometry approach applied in the transport model is capable of predicting the ingress of moisture and influence of cracks. The model was found to underestimate the vertical extent of moisture ingress in the largest CMOD samples (0.20 and 0.40 mm), which was likely due to instability of the large cracks in these samples.
On the mechanism of prevention of explosive spalling in ultra-high performance concrete with polymer fibers Cem. Concr. Res. (IF 5.43) Pub Date : 2018-09-08 Dong Zhang, Aravind Dasari, Kang Hai Tan
It has been a common practice to use polymer fibers to reduce susceptibility of explosive spalling in ultra-high performance concrete (UHPC). However, to-date, despite the proposition of different mechanisms through which polymer fibers enhance gas permeability and reduce explosive spalling, there are many unanswered questions and unjustified claims on the proposed mechanisms. Therefore, the major emphasis of this work is to thoroughly re-examine and understand the exact role of polymer fibers in the prevention of explosive spalling of UHPC. A range of analytical and microscopic tools are used to realize this objective. It is concluded that melting of polymer fibers and creation of empty channels are not required for enhancing the permeability of gases or water vapor through concrete. In fact, it is the thermal mismatch between embedded fibers and matrix that is critical in obtaining an interconnected network of cracks in the matrix. This occurs even before melting of polypropylene (PP) fibers. The network of cracks is responsible for enhancing permeability, thereby reducing the susceptibility of explosive spalling of UHPC.
In-situ investigation of superplasticizers: From fluorescence microscopy to concrete rheology Cem. Concr. Res. (IF 5.43) Pub Date : 2018-09-08 J. Arend, A. Wetzel, B. Middendorf
Concretes with advanced strength and durability necessitates organic polymers to receive a homogeneous concrete mixture due to a low amount of mixing water and a high content of fine fillers with large inner surface. The basic working mechanisms of those so-called superplasticizers are widely understood in theory, but still there are unclear phenomena like incompatibilities with certain additives. This study presents a novel method to investigate the interaction of superplasticizers with mineral particles. With the application of fluorescence microscopy, it is possible for the first time to localize and quantify the adsorption of superplasticizers to particles and to vary experimental parameters during the measurement. The presented results prove this method, which enables first time to investigate the superplasticizer interaction with mineral surfaces in situ. The important role of calcium ions for the necessary adsorption of those polymers to mineral particle surfaces is obvious in the shown fluorescence microscopic experiment. Furthermore, the detected amount of adsorbed polymers depending in the concentration of calcium ions present in the used suspension correlates with the development of the zeta potential of various minerals. Finally, a correlation between the microscopically measured adsorption of two different superplasticizers to rheological properties of a test mortar succeeded.
Effect of highly carboxylated colloidal polymers on cement hydration and interactions with calcium ions Cem. Concr. Res. (IF 5.43) Pub Date : 2018-09-05 Zichen Lu, Xiangming Kong, Chaoyang Zhang, Yi Cai
Polymer latexes are often found to retard cement hydration. Interaction of polymer particles with Ca2+ in pore solution and adsorption of polymer on cement were proposed as possible mechanisms for the retardation effect. This paper aims at disclosing the retardation mechanism of colloidal polymers using a highly carboxylated polystyrene latex. Interaction between colloidal particles and Ca2+ was studied by post-treatment of the latex with Ca(NO3)2. Techniques including calorimetry, adsorption test, ICP-OES, SEM, are involved to investigate effects of the post-treated latexes on hydrations of cement and C3S. It is found that enrichment of Ca2+ on surface of colloidal particles doesn't contribute to the retardation effect of polymer on cement hydration. Furthermore, the acceleration effect of the two-step treated latex using Ca(NO3)2 and Na2SiO3 suggests that nucleation inhibition of the adsorbed polymer layer on cement surface is the more conceivable mechanism responsible for the retardation effect of the highly carboxylated latex.
Durability for concrete structures in marine environments of HZM project: Design, assessment and beyond Cem. Concr. Res. (IF 5.43) Pub Date : 2018-09-05 Kefei Li, Dongdong Zhang, Quanwang Li, Zhihong Fan
This paper reviews the fundamentals of the durability of structural concrete, and then applies the concepts to the durability design, quality control, performance assessment and maintenance planning of concrete structures in Hong Kong-Zhuhai-Macau (HZM) sea link project with design working life of 120 years. The durability design adopts a multi-level philosophy for different durability risks and uses model-based approach to determine design parameters. The long-term exposure data of 30 years were used to calibrate the chloride ingress model and quantify the modelling uncertainty. The durability quality control was realized through converting the design values to laboratory characterization values, especially for the chloride diffusivity. With the data collected in construction phase, a full-probabilistic assessment is performed on the achieved performance against the chloride ingress, serving as basis for maintenance planning. The open issues related to the durability of concrete infrastructures with long service life are elaborated in the end.
Vision of 3D printing with concrete — Technical, economic and environmental potentials Cem. Concr. Res. (IF 5.43) Pub Date : 2018-08-07 Geert De Schutter, Karel Lesage, Viktor Mechtcherine, Venkatesh Naidu Nerella, Guillaume Habert, Isolda Agusti-Juan
A vision is presented on 3D printing with concrete, considering technical, economic and environmental aspects. Although several showcases of 3D printed concrete structures are available worldwide, many challenges remain at the technical and processing level. Currently available high-performance cement-based materials cannot be directly 3D printed, because of inadequate rheological and stiffening properties. Active rheology control (ARC) and active stiffening control (ASC) will provide new ways of extending the material palette for 3D printing applications. From an economic point of view, digitally manufactured concrete (DFC) will induce changes in the stakeholders as well as in the cost structure. Although it is currently too ambitious to quantitatively present the cost structure, DFC presents many potential opportunities to increase cost-effectiveness of construction processes. The environmental impact of 3D printing with concrete has to be seen in relation to the shape complexity of the structure. Implementing structural optimization as well as functional hybridization as design strategies allows the use of material only where is structurally or functionally needed. This design optimization increases shape complexity, but also reduces material use in DFC. As a result, it is expected that for structures with the same functionality, DFC will environmentally perform better over the entire service life in comparison with conventionally produced concrete structures.
Rheology of activated phosphorus slag with lime and alkaline salts Cem. Concr. Res. (IF 5.43) Pub Date : 2018-08-06 Hamideh Mehdizadeh, Ebrahim Najafi Kani, Angel Palomo Sanchez, Ana Fernandez-Jimenez
The present study aimed to determine the rheological behavior of activated phosphorus slag (AAPS) with a combination of Ca(OH)2 and Na2CO3 (CNC activator) or Na2SO4 (CNS activator) compared to OPC paste. The findings showed that the rheology of AAPS pastes fit with the Herschel-Bulkley model and behaved like a shear-thinning fluid. The AAPS pastes rheology is affected by concentration and nature of activator. The shear stress in AAPS pastes was lower than OPC paste, and increased with increasing the activator concentration. In early ages, reaction products of PS/CNC pastes included calcite-type compounds with the formed gel while gypsum compounds formed in the PS/CNS pastes. This resulted in higher shear stress and lower workability for PS/CNC pastes compared to PS/CNS pastes. The formed gel due to interaction between Ca2+ ions of CNC activator and Si4+ ions of Phosphorus slag caused an increase in the shear stress for PS/CNC during the time.
Water vapor sorption isotherms, pore structure, and moisture transport characteristics of alkali-activated and Portland cement-based binders Cem. Concr. Res. (IF 5.43) Pub Date : 2018-08-02 M. Babaee, A. Castel
Moisture transport plays a key role in determining the different durability-related features of cementitious materials. In this paper, moisture sorption in a range of low-calcium (geopolymer-type) and calcium-rich alkali-activated binders are studied and compared with that of Portland cement-based binders. Through the analysis of water vapor sorption isotherms (WVSI) and mercury intrusion porosimetry (MIP) test results, two vastly different pore structures were observed. Fly ash-based geopolymer-type binders showed a very porous structure where a large volume of mesopores coexisted with a significant volume of macropores. Alkali-activated slag binders, however, had a very fine pore structure, with a relative lack of large macropores. The different pore structure of fly ash-based and slag-based binders led to amplification of pore blocking and cavitation in blended systems by the addition of slag. Analysis of the sorption kinetics showed the prominent effect of the presence of calcium in the matrix to reduce the permeability.
Chloride induced reinforcement corrosion behavior in self-healing concrete with encapsulated polyurethane Cem. Concr. Res. (IF 5.43) Pub Date : 2018-08-02 Bjorn Van Belleghem, Sylvia Kessler, Philip Van den Heede, Kim Van Tittelboom, Nele De Belie
Cracks in reinforced concrete structures accelerate the ingress of chlorides and therefore cause a higher risk for corrosion. In this research, autonomous healing of cracks by encapsulated polyurethane was investigated as a possible method to reduce reinforcement corrosion. Reinforced concrete beams were exposed weekly to a chloride solution and electrochemical parameters were measured to determine the influence of the self-healing mechanism on the corrosion process. The rebars were visually examined afterwards. For the cracked beams an active state of corrosion was detected within an exposure period of 10 weeks and clear pitting corrosion was observed on the rebars. Autonomous crack healing with low viscosity polyurethane could significantly reduce the corrosion in the propagation stage. For these specimens no visual damage to the rebars was detected. In conclusion, the application of self-healing concrete with a low viscosity polyurethane is able to enhance the durability of reinforced concrete structures in marine environments.
Particle-bed 3D printing in concrete construction – Possibilities and challenges Cem. Concr. Res. (IF 5.43) Pub Date : 2018-07-30 Dirk Lowke, Enrico Dini, Arnaud Perrot, Daniel Weger, Christoph Gehlen, Benjamin Dillenburger
This article provides an overview of different particle-bed 3D printing techniques for the production of concrete elements. A classification is proposed which considers the direct production of concrete components, the production of formwork as well as composite components by means of a permanent formwork. Three techniques are considered as relevant for concrete construction, i.e. selective binder activation, selective paste intrusion and binder jetting. Design as well as material aspects are addressed. The underlying physics of fluid infiltration into the particle-bed and its effect on the properties of the hardened material are discussed on the basis of recent research results. Finally, the first applications of particle-bed 3D printing are presented which demonstrate the potential of this technique in concrete construction.
Water penetration through cracks in self-healing cementitious materials with superabsorbent polymers studied by neutron radiography Cem. Concr. Res. (IF 5.43) Pub Date : 2018-07-27 D. Snoeck, P. Van den Heede, T. Van Mullem, N. De Belie
SuperAbsorbent Polymers (SAPs) are a promising admixture to obtain self-sealing and self-healing cementitious materials. They are able to physically block water penetration through cracks due to their swelling ability. This is very useful in a cementitious material which is prone to cracking leading to a reduced durability. The effectiveness of counteracting water penetration is of concern. Neutron radiography was performed in order to investigate the influence of SAPs on the water permeability through healed cracks with respect to a reference material without SAPs. The gravimetrically measured capillary absorption corresponded with the obtained results after image analysis and a quantitative analysis could be made. SAPs enhance the water impermeability and are able to seal and heal a crack effectively up to 100 μm. The healing was a main factor to impede water movement through cracks. This will lead to less water ingress in building infrastructures and a possible longer service life.
Graphic statics and their potential for digital design and fabrication with concrete Cem. Concr. Res. (IF 5.43) Pub Date : 2018-07-26 Joseph Schwartz
The paper describes the influence of the lower bound theorem of theory of plasticity on the design of structures during the last 70 years. Starting with the fundamental theorems developed in the middle of the last century, the influence of rigid plastic (discontinuous) stress fields on the design of reinforced beam, wall, slab and shell structures is worked out. The potential of graphic statics for digital design and fabrication is illustrated by the results of several research projects on form finding and design of reinforced concrete structures carried out during the last decade. It is shown that the design methods allow for simultaneous considerations about structure and architecture. This leads to leaner structure and reduced material consumption. However, while a current limitation of manufacturing objects in these “nonstandard” shapes is presently cost limiting, digital fabrication may change this and open a much greater scope for the use and application of graphic statics in a near future.
The synthesis and hydration of ternesite, Ca5(SiO4)2SO4 Cem. Concr. Res. (IF 5.43) Pub Date : 2018-07-20 Solon Skalamprinos, Gabriel Jen, Isabel Galan, Mark Whittaker, Ammar Elhoweris, Fredrik Glasser
The hydration and strength evolution of two multi-phase ternesite-based cements with 15 and 29 wt% ternesite are reported. The synthesis and hydration properties of single phase ternesite, nominally Ca5(SiO4)2SO4, are also reported including both chemically-pure ternesite and preparations doped with sodium, potassium, phosphorous, magnesium, manganese, strontium, zinc and titanium oxides. Hydration of the samples at 25 °C was studied by calorimetry and quantitative X-ray diffraction. Unconfined compressive strength development was determined for up to 1 year. Single-phase chemically activated ternesite hydrated rapidly at 25 °C achieved compressive strengths of ≈30 and ≈65 MPa at 28 and 90 days respectively, with C-S-H and gypsum as hydration products. The multi-phase ternesite-based cements reached 7 day strengths of ≈30 MPa. It is concluded that ternesite reacts with water, exhibiting strength gain. The future of calcium sulfoaluminate and sulfosilicate cements is discussed and it is suggested that a considerable, and as yet unrealised scope exists for simultaneously optimising cementing properties while lowering production costs and reducing CO2 emissions.
Nucleation seeding with calcium silicate hydrate – A review Cem. Concr. Res. (IF 5.43) Pub Date : 2018-07-20 Elisabeth John, Thomas Matschei, Dietmar Stephan
The development of green cements, with the aim of reducing CO2 emissions, often results in reduced hydration activity, especially during the first hours and days. Nucleation seeding with C-S-H has enormous potential to accelerate hydration, which can compensate for the above-mentioned effect without compromising the long-term strength of seeded cements. In this work, the effects of calcium silicate hydrate are reviewed in detail, with a focus on synthesis, as well as their influence on the hydration mechanism and the development of mechanical properties, such as early and long-term compressive strength and porosity.
Experimental and modeling study of calcium carbonate precipitation and its effects on the degradation of oil well cement during carbonated brine exposure Cem. Concr. Res. (IF 5.43) Pub Date : 2018-07-18 Edward N. Matteo, Bruno Huet, Carlos F. Jové-Colón, George W. Scherer
Decalcification of cement in solutions of carbonated brine is important to a host of engineering applications, especially in subsurface service environments where cementitious materials are frequently utilized as engineered barriers for wellbore seals, as well as shaft and drift seals and waste forms for nuclear waste disposal. Analysis of leaching simulations and experiments shows that, depending on solution compositions (dissolved CO2 concentration, pH, Ca ion concentration), calcite precipitation occurring during leaching of cement in contact with carbonated brine can have a significant impact on cement reactivity, in some instances leading to complete arrest of reactivity via calcium carbonate “pore-clogging”. We present modeling and experimental results that examine the range of solution conditions that can lead to pore-clogging. Analysis of the results shows that distinct regimes of leaching behavior, based on pH and pCO2, can be used to form a framework to better understand the occurrence of pore-clogging.
May reversible water uptake/release by hydrates explain the thermal expansion of cement paste? — Arguments from an inverse multiscale analysis Cem. Concr. Res. (IF 5.43) Pub Date : 2018-07-18 Hui Wang, Christian Hellmich, Yong Yuan, Herbert Mang, Bernhard Pichler
Quasi-instantaneous thermal expansion of cement pastes is governed by the relative humidity (RH) within their air-filled pores and by the decrease/increase of this internal RH resulting from a temperature decrease/increase. The latter effect is traced back to quasi-instantaneous water uptake/release by cement hydrates, using microporomechanics and a three-scale representation of mature cement pastes. Partially saturated gel and capillary pores are considered to be connected and spherical, with radii following exponential distributions. The Mori-Tanaka scheme provides the scale transition from effective pore pressures to eigenstrains at the cement paste level. This modeling approach, together with considering mass conservation of water, allows for downscaling macroscopic thermal expansion coefficients, so as to identify the molecular water uptake/release characteristics of the hydrates. The latter characteristics are mixture-independent, as shown by their use for predicting the thermal expansion coefficients of different mature cement pastes, with w/c-ratios ranging from 0.50 to 0.70.
Effect of alkali dosage and silicate modulus on carbonation of alkali-activated slag mortars Cem. Concr. Res. (IF 5.43) Pub Date : 2018-07-17 Zhenguo Shi, Caijun Shi, Shu Wan, Ning Li, Zuhua Zhang
The long-term durability and their mechanisms of alkali-activated cement based materials have remained largely elusive. In this paper, carbonation of alkali-activated slag (AAS) mortars activated by NaOH and waterglass with different alkali dosages and silicate moduli has been investigated after exposure to 3 ± 0.2% (v/v) CO2 at 20 ± 2 °C/65 ± 5% RH for 56 days. The results show that carbonation resistance of the AAS mortars increases with increase of not only alkali dosage but also silicate modulus. In addition to the higher pore solution alkalinity and slag reaction extent, the relatively higher carbonation resistance of the AAS mortars is attributed to the lower porosity and average pore size. The loss of compressive strength for the waterglass activated slag mortars after carbonation is due to decalcification of C-A-S-H phase, whereas the carbonation of katoite contributes to the increase of compressive strength of the NaOH activated slag mortars.
Performance of 25-year-old silica fume and fly ash lightweight concrete blocks in a harsh marine environment Cem. Concr. Res. (IF 5.43) Pub Date : 2018-07-14 E.G. Moffatt, M.D.A. Thomas
This paper presents the long-term durability performance of semi-lightweight concrete containing various levels of supplementary cementing materials (SCM) and steel fibers when exposed to a harsh marine environment for up to 25 years. Concrete specimens (305 × 305 × 915 mm [1 × 1 × 3 ft.]) were casting using W/CM in the range of 0.26 to 0.60.The depth of chloride penetration was greater than 90 mm (3.5 in.) for all the control specimens (without fly ash and silica fume). However, mixes containing both silica fume and fly ash (with W/CM ranging from 0.40 to 0.60) performed very well resulting in chloride penetration of approximately 40 mm (1.6 in.) during the same period. The results from the chloride permeability testing also indicate significant increases in the resistance to chloride-ion penetration for ternary concrete containing fly ash and silica fume.
Digital in situ fabrication - Challenges and opportunities for robotic in situ fabrication in architecture, construction, and beyond Cem. Concr. Res. (IF 5.43) Pub Date : 2018-07-14 Jonas Buchli, Markus Giftthaler, Nitish Kumar, Manuel Lussi, Timothy Sandy, Kathrin Dörfler, Norman Hack
While a consensus exists that advanced digital and mechatronic technology is on the cusp of profoundly impacting virtually every manufacturing and industry sector, there are some industries that seem to have profited far less from this ongoing ‘revolution’. One prominent example of this is the construction sector and, in particular, building construction. In this paper, we aim at discussing some of the reasons for this apparent lack, and some reasons why this might change in the near future. We introduce the problem of digital in situ fabrication as both a significant challenge and a huge opportunity. We support the discussion with an example of a robotically-fabricated digital concrete wall. Overall, we find that solving in situ fabrication constitutes an inherently multidisciplinary challenge.
Impact of initial CA dissolution on the hydration mechanism of CAC Cem. Concr. Res. (IF 5.43) Pub Date : 2018-07-12 F. Hueller, C. Naber, J. Neubauer, F. Goetz-Neunhoeffer
Usually, for basic investigations on the hydration behaviour of CA (monocalcium aluminate, CaAl2O4) preferably pure samples without the presence of other hydraulic phases are used. In the presented study we apply an alternative approach investigating mixes that contain tiny to low amounts of CA (0.5–20 wt%) besides CA2 (CaAl4O7) or C6AF2 (Ca6Al2Fe4O15). The hydration of the mixes is investigated by isothermal heat flow calorimetry, in-situ XRD and pore solution analysis at 23 °C. A modified thermodynamic database enables the calculation of saturation indices for relevant hydrate phases in the investigated systems. CA similarly influences the hydration of both CA2 and C6AF2 although the exact impact strongly depends on the initial CA/water ratio in the mixes. Up to a certain amount CA is completely consumed during the first minutes after mixing. Beyond this amount the presence of undissolved CA causes an induction period and retards the hydration of CA2 or C6AF2.
Simulating mixing processes of fresh concrete using the discrete element method (DEM) under consideration of water addition and changes in moisture distribution Cem. Concr. Res. (IF 5.43) Pub Date : 2018-07-04 Knut Krenzer, Viktor Mechtcherine, Ulrich Palzer
The article presents a new model for the use in the framework of the Discrete Element Method to simulate the mixing processes of fresh concrete. The model provides a representation of liquid transfer from fluid entities or moist solid particles to dryer solid particles, including volume adaptation and mass conservation. The parameters and laws of force interactions between two particles are defined locally and depend on the amount and the properties of the liquid phase in the contact area. If two dry particles are in contact, friction and elastic forces are computed based on the Hertz-Mindlin model. If two particles with low moisture content are in contact, additional liquid bridge forces act on the particles. When the suspension state is reached, a Bingham-based model is applied. The simulation enables realistic estimation of the moisture distribution during the mixing processes, the corresponding changes in forces acting on the particles and consequently the effect on the rheological properties of the concrete mixture.
Reactive transport modelling of a cement backfill for radioactive waste disposal Cem. Concr. Res. (IF 5.43) Pub Date : 2018-07-03 James C. Wilson, Steven Benbow, Richard Metcalfe
Cementitious materials are included in many geological disposal facilities for radioactive wastes, including the UK candidate backfill material Nirex Reference Vault Backfill (NRVB). As part of an ongoing programme of work to assess NRVB performance, 1-D reactive-transport models have been constructed to simulate reaction with different illustrative groundwater compositions. Variant cases were also produced to explore the effect of model assumptions on the predicted behaviour of the backfill. Depending on groundwater composition, cement alteration pathways included: carbonation; external sulphate attack; chloride attack; the formation of magnesium-rich solids; and the precipitation of secondary aluminosilicate minerals. In general, the models suggest that the pore space in backfill associated with radioactive waste disposal systems is likely to become clogged (to some degree) over time close to backfill-rock interfaces resulting in a reduction in capacity for solute transport. However, the models do not include all relevant process couplings which is potentially, an area for further work.
Effects of alkali dosage and silicate modulus on alkali-silica reaction in alkali-activated slag mortars Cem. Concr. Res. (IF 5.43) Pub Date : 2018-07-03 Zhenguo Shi, Caijun Shi, Shu Wan, Zuhua Zhang
The effects of silicate modulus and alkali dosage on alkali-silica reaction (ASR) of alkali-activated slag (AAS) mortars are investigated under accelerated mortar bar testing conditions as specified in ASTM C1260. At a given alkali dosage, an intermediate silicate modulus with respect to highest ASR expansion is observed, which increases from 1.0 to >2.0 with increase of alkali dosage. The effect of silicate modulus on ASR expansion is found to be governed by the pore solution alkalinity. At a given higher silicate modulus such as 1.5 and 2.0 at 14 day, the ASR expansion tends to increase with increase of alkali dosage, whereas it decreases with increase of alkali dosage at a given lower silicate modulus such as 0–0.5 at 28 day. The mechanism governing the effect of alkali dosage on ASR expansion is more complicated and cannot be explained by only the pore solution alkalinity.
Analysis of moisture migration in concrete at high temperature through in-situ neutron tomography Cem. Concr. Res. (IF 5.43) Pub Date : 2018-06-30 Dorjan Dauti, Alessandro Tengattini, Stefano Dal Pont, Nikolajs Toropovs, Matthieu Briffaut, Benedikt Weber
Spalling, which is a phenomenon encountered when high-performance concrete is exposed to high temperature, can lead to large economical damage and can be a major safety hazard. Moisture distribution in concrete during exposure to fire is of paramount importance for understanding the complex mechanism of this phenomenon. To capture in its fullness this mechanism, it is crucial to account for the heterogeneous nature of concrete.In this paper, the first 3D analysis of moisture distribution in concrete at high temperature through in-situ neutron tomography is presented. The world-leading flux at the Institute Laue Langevin in Grenoble, France allowed capturing one 3D scan per minute, which is sufficient to follow the fast dehydration process. The paper describes the experimental setup with the heating system and discusses in detail the framework of the neutron tomography test. Quantitative analysis showing the effect of the aggregate size on the moisture distribution is presented.
In-situ pore size investigations of loaded porous concrete with non-destructive methods Cem. Concr. Res. (IF 5.43) Pub Date : 2018-06-28 H. Rifai, A. Staude, D. Meinel, B. Illerhaus, G. Bruno
Subject of this investigation is the in-situ evolution of pore volume and pore size distribution in Ytong (a porous concrete material) under increasing pressure with two different non-destructive analytical methods: Nuclear Magnetic Resonance (NMR) and X-ray Computed Tomography (CT). For both methods special strain devices to apply external pressure were constructed. The results from the two techniques yield complementary information on the pore size distribution and allows covering different pore size regions.
Effect of slag content and activator dosage on the resistance of fly ash geopolymer binders to sulfuric acid attack Cem. Concr. Res. (IF 5.43) Pub Date : 2018-06-29 Timothy A. Aiken, Jacek Kwasny, Wei Sha, Marios N. Soutsos
Geopolymer (GP) binders are an appealing alternative to Portland cement (PC) binders as they have the potential to reduce the CO2 emissions associated with the cement and concrete industry. However, their durability in aggressive environments needs thorough examination if they are to become a viable alternative to traditional PC materials. This study investigated the effect of increasing slag content and activator dosage on the sulfuric acid resistance of fly ash GP binders. Their performance was also compared with that of neat PC mixes using various physical and microstructural techniques. The results show that increasing the slag content of fly ash GPs decreases porosity, but makes the reaction products more susceptible to sulfuric acid attack. It was also found that increasing the alkaline activator dosage of fly ash GPs has little impact on sulfuric acid resistance. Finally, GP binders displayed superior sulfuric acid resistance than their PC counterparts.
Effect of the hydration temperature on the pore structure of cement paste: Experimental investigation and micromechanical modelling Cem. Concr. Res. (IF 5.43) Pub Date : 2018-06-27 Sara Bahafid, Siavash Ghabezloo, Pamela Faure, Myriam Duc, Jean Sulem
The hydration temperature has a considerable effect on the microstructure of cement paste. The microstructural changes with increasing curing temperature result in a significant decrease of cement mechanical properties. The present work aims at studying these phenomena via micromechanical modelling, based on an experimental characterization of cement microstructure. The pore structure of a class G oil-well cement paste (American Petroleum Institute classification) hydrated at different temperatures, between 7 °C and 90 °C is characterized. Then, the influence of the microstructure variations on the elastic properties of the hardened paste is explored through micromechanical modelling. A previous analysis of the microstructure enabled determining the volume fractions of different phases of a class G cement paste hydrated under different temperatures . A multiscale self-consistent homogenization model is used based on these results to simulate the variations of the mechanical properties with hydration temperature. The results are compared with macro-scale elastic properties obtained from uniaxial compression tests. It is shown that the increasing capillary porosity with elevating hydration temperature, is not sufficient to fully explain this drop of elastic properties. The latter originates mainly from the decrease of the elastic properties of the foam composed of the porous C-S-H intermixed with capillary pores. The pore structure analysis coupled with the micromechanical modelling permitted a back-analysis of the intrinsic porosities of high density and low density C-S-H showing an almost constant LD intrinsic porosity and a significantly decreasing HD intrinsic porosity.
Eco-efficient cements: Potential economically viable solutions for a low-CO2 cement-based materials industry ☆ Cem. Concr. Res. (IF 5.43) Pub Date : 2018-06-28 Karen L. Scrivener, Vanderley M. John, Ellis M. Gartner
The main conclusions of an analysis of low-CO2, eco-efficient cement-based materials, carried out by a multi-stakeholder working group initiated by the United Nations Environment Program Sustainable Building and Climate Initiative (UNEP-SBCI) are presented, based on the white papers published in this special issue.We believe that Portland-based cement approaches will dominate in the near future due to economies of scale, levels of process optimisation, availability of raw materials and market confidence. Two product-based approaches can deliver substantial additional reductions in their global CO2 emissions, reducing the need for costly investment in carbon capture and storage (CCS) over the next 20–30 years:1.Increased use of low-CO2 supplements (SCMs) as partial replacements for Portland cement clinker.2.More efficient use of Portland cement clinker in mortars and concretes.However, other emerging technologies could also play an important role in emissions mitigation in the longer term, and thus merit further investigation.
Wick action in mature mortars with binary cements containing slag or silica fume – The relation between chloride and moisture transport properties under non-saturated conditions Cem. Concr. Res. (IF 5.43) Pub Date : 2018-06-23 Nilla Olsson, Fatmawati Abdul Wahid, Lars-Olof Nilsson, Charlotte Thiel, Hong S. Wong, Véronique Baroghel-Bouny
Moisture and ionic transport under non-saturated condition is an important, but poorly understood transport phenomena particularly for mature systems containing supplementary cementitious materials. This paper investigates the moisture and chloride profiles of 3-year old mortars containing Portland cement (OPC), slag and silica fume (SF) after long-term (30–48 months) wick action exposure in 1.09 M NaCl solution. Moisture profiles were measured with 1H NMR relaxometry and chloride profiles with microXRF. The measured profiles were discussed in relation to moisture dependent material properties such as chloride diffusion coefficients, moisture diffusion coefficients, and desorption isotherms. Results show that the combination of different cementitious materials, e.g. the cementitious binder, is the key factor affecting chloride penetration depth. The cementitious binder also strongly affects chloride diffusion coefficient, moisture diffusion coefficient and chloride binding properties, which are all important parameters for the prediction of chloride ingress.
Enhancing thixotropy of fresh cement pastes with nanoclay in presence of polycarboxylate ether superplasticizer (PCE) Cem. Concr. Res. (IF 5.43) Pub Date : 2018-06-24 Ye Qian, Geert De Schutter
Nanoclay, a thermal treated, purified attapulgite clay, has been used to increase thixotropy. However, the effect of nanoclay in presence of water reducing agent, such as PCE, on rheology, and the compatibility between nanoclay and PCE have not been well studied. The dynamic yield stress, thixotropic index, characteristic time, and microstructure of fresh cement pastes with combination of nanoclay and PCE addition are measured. It is found that nanoclay has a good compatibility with PCE. Nanoclay increases the dynamic yield stress and enhances the thixotropy of fresh cement pastes with and without PCE addition. The nanoclay addition agglomerates the microstructure at high PCE addition and increases the thixotropic index from bottom value to a high value. This study gives insight of achieving low dynamic yield stress yet high static yield stress and thixotropy mixtures.
Synchrotron X-ray nanotomographic and spectromicroscopic study of the tricalcium aluminate hydration in the presence of gypsum Cem. Concr. Res. (IF 5.43) Pub Date : 2018-06-22 Guoqing Geng, Rupert J. Myers, Young-Sang Yu, David A. Shapiro, Robert Winarski, Pierre E. Levitz, David A.L. Kilcoyne, Paulo J.M. Monteiro
The rheology of modern Portland cement (PC) concrete critically depends on the correct dosage of gypsum (calcium sulfate hydrate) to control the hydration of the most reactive phase - tricalcium aluminate (C3A). The underlying physio-chemical mechanism, however, remains unsolved mainly due to the lack of high-spatial-resolved and chemistry-sensitive characterization of the C3A dissolution frontier. Here, we fill this gap by integrating synchrotron-radiation based crystallographic, photon-energy-dependent spectroscopic and high-resolution morphological studies of the C3A hydration product layer. We propose that ettringite (6CaO·Al2O3·SO3·32H2O) is the only hydration product after the initial reaction period and before complete gypsum dissolution. We quantify the 2D and 3D morphology of the ettringite network, e.g. the packing density of ettringite at various surface locations and the surface dissolution heterogeneity. Our results show no trace of a rate-controlling diffusion barrier. We expect our work to have significant impact on modeling the kinetics and morphological evolution of PC hydration.
Chloride diffusivity, chloride threshold, and corrosion initiation in reinforced alkali-activated mortars: Role of calcium, alkali, and silicate content Cem. Concr. Res. (IF 5.43) Pub Date : 2018-06-22 M. Babaee, A. Castel
The aim of this study is to investigate systematically the chloride diffusivity and chloride threshold of a wide range of calcium-rich and fly ash-dominated alkali-activated samples in light of their compositional differences. To this end, the effects of various fly ash (FA)-to-slag ratios, of alkali concentrations and of silicate content in the activator were investigated. The electrochemical aspects of the passive samples were also assessed. Results show the prominent role of calcium in the matrix to reduce the chloride diffusivity. While higher alkali concentration increased the porosity and chloride diffusivities in general, lower modulus ratios provided considerably better performance in the FA-dominated samples. Chloride threshold values range between 0.19 (wt% binder mass) for calcium-rich mortars fabricated at low levels of alkalinities and 0.69 for FA-dominated mortars fabricated with highly alkaline activators. Half-cell potential and polarization resistance of alkali-activated samples were in general lower than their Portland cement counterparts.
Influence of fly ash and metakaolin on the microstructure and compressive strength of magnesium potassium phosphate cement paste Cem. Concr. Res. (IF 5.43) Pub Date : 2018-06-22 Liwu Mo, Liming Lv, Min Deng, Jueshi Qian
The influences of fly ash and metakaolin added as substitutions (by up to 50 wt%) of magnesium potassium phosphate cement (MKPC) on the microstructures and compressive strengths of the MKPC pastes were investigated. The results indicate that the aluminosilicate fractions of both fly ash and metakaolin are involved in the acid-base reaction of MKPC system, leading to a preferential formation of irregular crystalline struvite-K incorporated with Al and Si elements and/or amorphous aluminosilicate phosphate products. Metakaolin is more reactive than fly ash in the MKPC system. For the same addition dosage, the MKPC pastes containing metakaolin exhibit higher compressive strengths than the pastes containing fly ash. This is attributed to the formation of more highly reinforced microstructures and denser interfaces between the metakaolin particle and hydration products (e.g. struvite-K) in the MKPC paste containing metakaolin. Addition of 30 wt% metakaolin increases the compressive strengths of MKPC pastes at all test ages.
3D printing using concrete extrusion: A roadmap for research Cem. Concr. Res. (IF 5.43) Pub Date : 2018-06-14 R.A. Buswell, W.R. Leal de Silva, S.Z. Jones, J. Dirrenberger
Rethinking reinforcement for digital fabrication with concrete Cem. Concr. Res. (IF 5.43) Pub Date : 2018-06-11 Domenico Asprone, Costantino Menna, Freek P. Bos, Theo A.M. Salet, Jaime Mata-Falcón, Walter Kaufmann
The fabrication of novel reinforced concrete structures using digital technologies necessarily requires the definition of suitable strategies for reinforcement implementation. The successful integration of existing reinforcement systems, such as steel rebar, rods, wires, fibres or filaments, will indeed allow for printed concrete structures to be designed using standard structural codes. However, reinforcement integration has to be compatible with either the specific printing technique adopted for the structural element production or with its shape. This paper provides a systematic overview of a number of digital fabrication techniques using reinforced concrete that have been developed so far, proposing a possible organization by structural principle, or place in the manufacturing process.
Alternative cement clinkers Cem. Concr. Res. (IF 5.43) Pub Date : 2017-02-21 Ellis Gartner, Tongbo Sui
This article reviews proposed technical approaches for the manufacture and use of alternatives to Portland Cement Clinker as the main reactive binder component for ordinary concrete construction in non-specialty applications, while giving lower net global CO2 emissions in use. A critical analysis, taking into account a wide range of technical considerations, suggests that, with the exception of alkali-activated systems, (treated in a separate paper in this issue,) there are only four classes of alternative clinker system that deserve serious attention with respect to global reductions in concrete-related CO2 emissions: (A) Reactive Belite-rich Portland cement (RBPC) clinkers (B) Belite-Ye'elimite-Ferrite (BYF) clinkers (C) Carbonatable Calcium Silicate clinkers (CCSC) (D) Magnesium Oxides derived from Magnesium Silicates (MOMS) A and B are “hydraulic” clinkers, (i.e. clinkers which harden by reaction with water,) C is a “carbonatable” clinker, (i.e. one which hardens by reaction with CO2 gas) and D can fall into both categories.
Alkali-activated materials Cem. Concr. Res. (IF 5.43) Pub Date : 2017-03-02 John L. Provis
This paper, which forms part of the UNEP White Papers series on Eco-Efficient Cements, provides a brief discussion of the class of cementing materials known as ‘alkali-activated binders’, which are identified to have potential for utilization as a key component of a sustainable future global construction materials industry. These cements are not expected to offer a like-for-like replacement of Portland cement across its full range of applications, for reasons related to supply chain limitations, practical challenges in some modes of application, and the need for careful control of formulation and curing. However, when produced using locally-available raw materials, with well-formulated mix designs (including in particular consideration of the environmental footprint of the alkaline activator) and production under adequate levels of quality control, alkali-activated binders are potentially an important and cost-effective component of the future toolkit of sustainable construction materials.
Admixtures and sustainability Cem. Concr. Res. (IF 5.43) Pub Date : 2017-05-05 Josephine Cheung, Lawrence Roberts, Jiaping Liu
Cement additives and concrete admixtures are construction chemicals that reliably improve the sustainability of a broad range of cement-based systems. This improvement can be derived from enhancing durability and strength through water reduction, and catalyzing the cement hydration process to enable replacement of clinker with supplementary cementitious materials. Through current best practices, incorporation of these potent chemicals at a fraction of a percent of the entire concrete mix affords a clinker replacement of about 20% by weight. This replacement amount can be more than doubled with correct mix proportioning. Innovative technologies such as pre-test tools and smart concrete management systems further improve the effectiveness of admixtures applied in both current and new cementitious materials. Provided longer set times, a consequence of SCM incorporations, can be tolerated, further reduction in CO2 emissions can be attained. Making trade-offs such as this will be essential to optimizing the sustainability of cement-based materials.
Earth concrete. Stabilization revisited Cem. Concr. Res. (IF 5.43) Pub Date : 2017-05-11 Henri Van Damme, Hugo Houben
Not surprisingly, with the increased awareness of environmental issues, construction with raw (crude, unbaked) earth (subsoil) is gaining renewed interest. However, it suffers from a poor image and from the difficulty to meet modern productivity standards and to pass some durability tests designed for industrial materials. The recent trend is to overcome these drawbacks by “stabilizing” the material most often with Portland cement (PC). Here we show that stabilization with PC is in general neither technically nor environmentally advisable. It brings only moderate mechanical improvement at a high environmental cost. Rather than massively transforming crude earth into a low quality concrete, it would be more appropriate to adapt the architectural practice and/or to look for milder ways to improve properties. In this respect, the recent successful attempts to improve the workability and the strength of raw earth by controlling the dispersion of its fine fraction seem to be particularly promising.
Vegetable ashes as Supplementary Cementitious Materials Cem. Concr. Res. (IF 5.43) Pub Date : 2017-09-01 F. Martirena, J. Monzó
Approximately 140 billion metric tons of biomass are produced every year in the world from agriculture. The ashes resulting from firing agricultural wastes such as rice husk, sugar cane and others can be used as Supplementary Cementitious Materials (SCM). They can be mixed with lime alone or in ternary mixtures with Portland cement and lime. If fired at temperatures around 600–700 °C the agricultural ashes exhibit good reactivity. Despite extensive research work carried out on the use of agricultural ashes as source of SCMs, few success stories are reported on practical applications on an industrial scale. Details of the firing technology should be re-assessed, with special emphasis on the scale at which the technology begins to be economically suitable. Further research is also needed to understand the mechanisms of structural transformation of amorphous silica during calcination, and the impact of the ashes on cement hydration in blended systems.
Carbon dioxide reduction potential in the global cement industry by 2050 Cem. Concr. Res. (IF 5.43) Pub Date : 2017-09-14 Sabbie A. Miller, Vanderley M. John, Sergio A. Pacca, Arpad Horvath
This paper, which is a contribution to the UNEP series on Eco-Efficient Cements, examines the role of material-based solutions to reducing CO2 emissions from cement production considering factors that could influence implementation. Global urbanization has led to an increase in demand for cement and cement-based materials. With its growth in consumption, the associated CO2 emissions from its production are raising concern. However, the role of mitigation strategies in a global context that account for regional material availability and degree of market adoption have yet to be considered. This work shows that the 2 °C scenario targets for 2050 can be met through increased use of calcined clay and engineered filler with dispersants. The introduction of new Portland clinker-based cement alternatives, use of alkali-activated materials, and improvement of efficiency of cement use could further contribute to reduction goals. There are currently-available technologies for reduction that could be rapidly implemented.
Some contents have been Reproduced by permission of The Royal Society of Chemistry.
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