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.
Education for sustainable use of cement based materials Cem. Concr. Res. (IF 5.43) Pub Date : 2017-09-24 Wolfram Schmidt, Mark Alexander, Vanderley John
Structural design and application have always been linked to the compressive strength of concrete as the main relevant criterion. This was justifiable in the past, where concrete consisted of water, ordinary Portland cement and aggregates, but this concept is no longer relevant for modern and more sustainable cement and concrete. Despite these new developments, existing standards, guidelines and academic curricula have not been much updated and are still used worldwide. There is a need to change this situation by proper education of the users. This overview describes the challenges that arise at a user level from the higher complexity of modern concrete, and defines needs and requirements for enhanced applicability of sustainable concrete concepts. Furthermore, recommendations are given on how better concrete practice can be communicated to all the involved parties, from civil and design engineers to constructors and site-appliers on the construction site.
Calcined clay limestone cements (LC3) Cem. Concr. Res. (IF 5.43) Pub Date : 2017-11-20 Karen Scrivener, Fernando Martirena, Shashank Bishnoi, Soumen Maity
The use of supplementary cementitious materials (SCMs) to replace part of the clinker in cement is the most successful strategy to reduce CO2 emissions in the global cement industry. However, limited supplies of conventional SCMs make it difficult to take this strategy further unless new types of SCMs become available. The only type of material available in the quantities needed to meet demand is clay containing kaolinite, which can be calcined to produce an effective SCM. Such clays are widely available in countries where most growth in demand for cement is forecast. Calcined clays have previously been used as pozzolans, but calcination makes the economics of substitution marginal in a conventional pozzolanic blend. The major innovation presented here is the possibility to make a coupled substitution of cement with calcined clay and limestone. This allows much higher levels of substitution. Blends where calcined clay is used as a pozzolan, typically have clinker contents around 65–70%. Combination of calcined clay with limestone allows higher levels of substitution down to clinker contents of around 50% with similar mechanical properties and improvement in some aspects of durability. The replacement of clinker with limestone in these blends lowers both the cost and the environmental impact.
Fillers in cementitious materials — Experience, recent advances and future potential Cem. Concr. Res. (IF 5.43) Pub Date : 2018-03-27 Vanderley M. John, Bruno L. Damineli, Marco Quattrone, Rafael G. Pileggi
The paper discusses the potential of fillers in CO2 mitigation in the cement industry. A historical overview of the use of fillers is presented as well as the limits of filler use given in cement standards. Globally, limestone filler currently represents only 7% of average worldwide cement composition. The limits of the current route to adding filler in cement by means of intergrinding are discussed. An innovative technology, that compensates binder dilution by a reduction of the water required for good rheological behavior, is presented; this allows clinker replacement rates of up to 70%. The theory that enables the design of such multimodal particle size distributions with high particle packing and low-water demand is presented, and examples of its application in concrete production are given. The efficiency in terms of CO2 mitigation is demonstrated by comparing concrete formulations designed with this innovative approach with a global benchmark of current technology. New filler minerals, as well as the effects of high-filler content on production processes and on durability are also discussed.
Understanding the behaviour of graphene oxide in Portland cement paste Cem. Concr. Res. (IF 5.43) Pub Date : 2018-06-02 Sam Ghazizadeh, Philippe Duffour, Neal T. Skipper, Yun Bai
This study reports on the effect of graphene oxide (GO) on the hydration of Portland cement (PC) and industrial clinker. GO accelerates PC hydration, whereas it temporarily retards that of clinker. This difference reflects a twofold behaviour of GO in cement pastes. Retardation is due to the interaction of GO with the surface of hydrating grains, while acceleration results from a seeding effect. Gypsum causes this difference. GO is shown to have little effect on the strength of hardened pastes, and this merely relates to the change of hydration degree, as opposed to reinforcing effect formerly assumed. Overall, GO is not particularly active as a nucleation surface, as it aggregates and behaves in a similar way to inert fillers (e.g. quartz). Polycarboxylate-ether copolymer could make GO an active seed in cement pastes, as it prevents GO from aggregating. Nevertheless, this was found to occur only in alite pastes but not PC pastes.
Role of alcohol-ethylene oxide polymers on the reduction of shrinkage of cement paste Cem. Concr. Res. (IF 5.43) Pub Date : 2018-05-30 Ippei Maruyama, Ellis Gartner, Katsutoshi Beppu, Ryo Kurihara
Concrete drying shrinkage is a major practical problem, but it can be ameliorated by means of “shrinkage-reducing agents” (SRAs). Water vapor sorption isotherms and low temperature DSC measurements suggest that alcohol-ethylene oxide polymers (AEOPs) with a better hydrophilic-lipophilic (HL-) balance are present in meso-pores in which water molecules evaporate from 75% RH to 40% RH. Short-term drying shrinkage measurements show that the better HL-balanced AEOPs reduce irreversible shrinkage, which occurs mainly from 75% RH to 40% RH during the first drying. Based on the similarity between corresponding RH regions, it is concluded that the better HL balanced AEOP in relevant meso-pores hinders agglomeration of C-S-H during the first desorption to mitigate the irreversible shrinkage and reduce the first drying shrinkage.
Hydration and rheology control of concrete for digital fabrication: Potential admixtures and cement chemistry Cem. Concr. Res. (IF 5.43) Pub Date : 2018-05-26 Delphine Marchon, Shiho Kawashima, Hela Bessaies-Bey, Sara Mantellato, Serina Ng
Concrete digital fabrication is an innovative construction approach where infrastructural elements can be built additively without using formwork. This represents a significant advantage, but also introduces materials engineering challenges, as the requirements normally fulfilled by the formwork are now imposed on the concrete. In this paper, it is discussed how admixtures can be employed to achieve the rheological and hydration properties necessary for printable concrete. An overview of various admixtures currently implemented in standard practice is presented. Then, the main required concrete states for extrusion and deposition processes are analyzed with respect to required performances and potential admixtures. Finally, possible side effects and incompatibilities are discussed, as well as how they could be unconventionally used for printable concrete purposes. The main objective is to demonstrate how admixtures will be critical in the development of concrete systems to realize digital fabrication, and to ultimately motivate investigation in the key areas discussed.
Concrete material science: Past, present, and future innovations ☆ Cem. Concr. Res. (IF 5.43) Pub Date : 2018-05-24 Henri Van Damme
Concrete is flying off, but it is simultaneously facing tremendous challenges in terms of environmental impact, financial needs, societal acceptance and image. Based on an historical approach of the science of concrete and reinforced concrete in particular, this paper calls for the exploration of radical changes in three key aspects of concrete use: reinforcement, binder content, and implementation methods. More precisely, it is suggested that, in parallel to the introduction of robotic fabrication methods, digital technologies may be key for the introduction several innovations like (i) rebar-free reinforcement using non-convex granular media; (ii) compression-optimized concrete structures, using topology optimization, architectural geometry, and 3D-printing or origami-patterned formworks; (iii) truly digital concrete through the coupling of massive data collection and deep learning.
Effect of environmental exposure on autogenous self-healing of cracked cement-based materials Cem. Concr. Res. (IF 5.43) Pub Date : 2018-05-21 A.R. Suleiman, M.L. Nehdi
Despite abundant literature on self-healing of cement-based materials, there is dearth of information on how environmental exposure affects self-healing. In this study, self-healing of cracks in cement mortar under different environmental exposure was investigated. Pre-cracked mortar specimens were submerged in water, while identical specimens were exposed to cyclic temperature and relative humidity. Change in crack width was examined using optical microscopy. SEM coupled with energy dispersive X-ray analysis was used to identify healing compounds. Mercury Intrusion Porosimetry, water absorption and permeability were employed to assess porosity. X-ray computed tomography was deployed to explore healing of internal cracks. No significant self-healing occurred in specimens exposed to cyclic T and RH. Although crack self-healing was identified in submerged specimens, X-ray μCT demonstrated that it was limited to exposed surface of specimens. This warrants further research to bridge the gap between laboratory findings on self-healing and actual field performance of ageing civil infrastructure.
Rheological requirements for printable concretes Cem. Concr. Res. (IF 5.43) Pub Date : 2018-05-03 Nicolas Roussel
We study in this paper the rheological requirements for printable concrete in terms of yield stress, viscosity, elastic modulus, critical strain, and structuration rate. We first discuss the extrusion/deposition process at the level of the nozzle from a material perspective. We then focus on the rheological requirements needed to prevent the flow of one layer or the strength-based failure of the rising printed element. We moreover discuss the rheological requirements needed to control the final geometrical dimensions of one layer and of the entire object, including buckling stability and surface cracking. We finally describe the requirement for a proper intermixing of the layers interface and also note that drying of the upper surface of the layer at rest could also play a major role on the interlayer bond. Finally, we evaluate the effect of the use of printing supports (i.e. non-direct printing) on the above rheological requirements.
Modelling of transport processes in concrete exposed to elevated temperatures – An alternative formulation for sorption isotherms Cem. Concr. Res. (IF 5.43) Pub Date : 2018-01-19 C.T. Davie, C.J. Pearce, K. Kukla, N. Bićanić
There is a significant need to understand, analyse and assess moisture transport in cementitious materials exposed to elevated temperatures in order to confidently predict the behaviour and ultimately the development of damage in safety critical applications such as nuclear reactor vessels, structures exposed to fire and well bore grouts. In view of this need a rigorous and robust formulation to describe water retention curves (sorption isotherms) as a function of temperature based on the evolution of physical parameters is presented. The model formulation is successfully validated against independent sets of experimental data up to temperatures of 80 °C. It is then further validated under isothermal drying conditions and then high temperature conditions through the numerical reproduction of laboratory experiments following implementation in a fully coupled hygro-thermo-mechanical finite element model. The new formulation is found to work well under a variety of conditions in a variety of cementitious material types.
Determining the free alkali metal content in concrete – Case study of an ASR-affected dam Cem. Concr. Res. (IF 5.43) Pub Date : 2018-01-17 G. Plusquellec, M.R. Geiker, J. Lindgård, K. De Weerdt
In concrete affected by alkali–silica reaction (ASR), aggregates react in the high pH environment and cause deleterious expansion and cracking of the concrete. Leaching of alkali metals from the concrete might therefore locally reduce ASR. However, few data on alkali metals leaching are available in the literature. Our goal was to document the alkali metal leaching and to build-up an alkali inventory (the amount in solid and in solution, and the amount released by the aggregates) in a full-scale structure, the 50-year-old Votna I dam in Norway. Free alkali metal profiles were determined on cores taken at four locations with different exposure conditions: permanently immersed, periodically immersed, exposed to rain, or sheltered. Alkali leaching was observed at all four locations up to a depth of 100 mm. The leached zone exhibited less intense cracking than the non-leached concrete, indicating that the alkali leaching might be limiting ASR.
Modelling of the sulfuric acid attack on different types of cementitious materials Cem. Concr. Res. (IF 5.43) Pub Date : 2018-01-17 Anaïs Grandclerc, Patrick Dangla, Marielle Gueguen-Minerbe, Thierry Chaussadent
A chemical-reactive transport model is used to simulate the sulfuric acid attack of cement pastes based on ordinary Portland cement (CEM I), blended Portland cements (CEM III, CEM IV, and CEM V), and calcium aluminate cement (CAC). This model accounts for the dissolution of cement hydrates (portlandite, C-S-H, hydrogarnet), and the precipitation of deterioration products (ettringite and gypsum). Moreover, diffusion of the aqueous species in the pore space of the material is considered. With this model, the hydrate contents, the porosity, and the deterioration phase contents throughout a sulfuric acid attack are determined. Two indicators are defined to predict the service life of the cementitious materials: the deterioration depth and the dissolved calcium content. These two indicators show that calcium aluminate cement has a better resistance to sulfuric acid attack than that of Portland cements. This better resistance is mainly due to the partial dissolution of CAC hydrate as opposed to the total dissolutions of CH and C-S-H.
Relationship between the particle size and dosage of LDHs and concrete resistance against chloride ingress Cem. Concr. Res. (IF 5.43) Pub Date : 2018-01-17 Z.Y. Qu, Q.L. Yu, H.J.H. Brouwers
The present study investigates the transport properties of cement mortar with Ca-Al-NO3 layered double hydroxides (LDHs). A co-precipitation method is applied to synthesize the Ca-Al-NO3 LDHs and the effect of the synthesis environment on the size and particle shape is studied. The synthesized Ca-Al-NO3 LDHs are analytically characterized by XRD, SEM and FTIR analyses. The relationships between the sizes and addition amount of Ca-Al-NO3 LDHs and the mechanical and transport properties of mortars are investigated. Rapid chloride migration (RCM) tests are performed to the cement mortars with Ca-Al-NO3 LDHs. The results show that permeability of the designed concrete decreased with the addition of Ca-Al-NO3 layered double hydroxides (LDHs). The decrease of chloride migration coefficients can be attributed to the enhanced barrier effect because of the increase of tortuosity. In long-term natural diffusion tests, LDHs present significantly enhanced barrier effect due to the combined chloride binding ability and improved tortuosity.
Natural fluorapatite as a raw material for Portland clinker Cem. Concr. Res. (IF 5.43) Pub Date : 2018-01-17 Sonia Boughanmi, Islem Labidi, Adel Megriche, Mohamed El Maaoui, André Nonat
The present work focuses on the mineralogy and the reactivity of clinkers made from the integration of natural fluorapatite in the raw meals with percentages ranging from 0 up to15%. The samples were characterized by infrared spectrometry, X-ray diffraction and microscopy techniques. The distribution of phosphorous and fluorine into clinkers minerals was determined by MEB/EDS mapping. The mineralogical composition was determined by XRD/Rietveld and the samples reactivity followed by isothermal microcalorimetry. It has been found that fluorine stabilizes C3S and β-C2S which still are found with high levels of P2O5. Cements obtained from up to 8% natural fluorapatite incorporation still present acceptable properties. Natural fluorapatite could have an important potential as decarbonised source of CaO contributing to decrease the CO2 emission in cement industry.
Development of silica-enriched cement-based materials with improved aging resistance for application in high-temperature environments Cem. Concr. Res. (IF 5.43) Pub Date : 2018-01-17 Konrad J. Krakowiak, Jeffrey J. Thomas, Simon James, Muhannad Abuhaikal, Franz.-J. Ulm
Understanding the effects of high temperature (HT) and high pressure (HP) conditions on the microstructure of cement-based materials is critical to the construction and safe operation of deep oil and gas wells. Under such conditions, the persistence of calcium-silicate-hydrate (C-S-H) gel is compromised by ongoing crystallization that, if not controlled, may adversely affect the durability of the cement sheath. This work investigates the effect of silica content >35% by-weight-of-cement (BWOC), silica particle size, and solid volume fraction (SVF) on the microstructure and phase composition of cement-silica blends cured hydrothermally at 200 °C and 20.7 MPa. The results of X-ray diffraction and electron microprobe analysis revealed significant impact of these three mix design parameters on the final phase assembly, and on the conversion rate of semi-crystalline C-S-H to gyrolite and 11 Å tobermorite. Incorporation of more fine siliceous material suppressed dissolution of coarse silica particles, resulting in a matrix with improved homogeneity and dominated by fine gel pores. Mixes with lower SVF showed greater formation of 11 Å tobermorite, a higher degree of crystallinity and/or greater crystallite size. Prolonged HTHP curing of all systems (up to three months in this study), irrespective of the initial SVF, increased the fraction of capillary pores, indicating void coalesce caused by crystal growth. However, we find that this coarsening is less pronounced in systems with less pore space available for crystallization.
Characterization of novel blast-furnace slag cement pastes and mortars activated with a reactive mixture of MgO-NaOH Cem. Concr. Res. (IF 5.43) Pub Date : 2018-01-12 Oswaldo Burciaga-Díaz, Irma Betancourt-Castillo
This paper reports results of strength and reaction products of blast-furnace slag cements activated using a mixture of MgO-NaOH with a ratio 50/50 incorporated at 4, 6 and 8% relative to the slag mass. Pastes and mortars were prepared and cured at 20 °C and 60 °C-24 h-20 °C up to 90 days. The results indicated that the combination of MgO-NaOH acts as an effective alkaline activator of the slag, as samples cured at 20 °C developed 54 MPa at 90 days, while those at 60 °C-24 h-20 °C had a rapid strength gain at early ages but in the long run strengths of ~42 MPa at 90 days were observed. The microstructures consisted of (C-(A)-S-H), third aluminate hydrates and hydrotalcite. The formation of the latter increased at higher MgO:NaOH concentration lowering the Al uptake by C-S-H. MgO and Mg(OH)2 were not detected, indicating an effective incorporation of Mg2+ ions into the atomic structure of the reaction products.
Pore-size resolved water vapor adsorption kinetics of white cement mortars as viewed from proton NMR relaxation Cem. Concr. Res. (IF 5.43) Pub Date : 2018-01-10 Chunsheng Zhou, Fangzhou Ren, Qiang Zeng, Lizhi Xiao, Wei Wang
The dynamic pore structure of partially saturated cement-based material is rather essential to quantify its mass transport and many other properties. To clarify the influence of water removal and re-entry, the low-field nuclear magnetic resonance technique is employed to characterize the pore structure of two white cement mortars during isopropanol exchange and water vapor adsorption kinetics. It is found that, after isopropanol replacement the pore structures of mortars become remarkably coarsen due to the significant collapse of C-S-H interlayer pores, which are only partly reversible at water re-saturation. Consistently, water vapor adsorption is not a simple progressively filling process from finer to coarser pores, but accompanied by continuous expansion of C-S-H gel adsorbing water in priority. Furthermore, the irrecoverable contraction of C-S-H gel is the physical root of irreversible shrinkage. The smaller BET surface area using nitrogen than water is also attributed to the collapse of interlayer pores after drying.
Limitations of the hydrotalcite formation in Portland composite cement pastes containing dolomite and metakaolin Cem. Concr. Res. (IF 5.43) Pub Date : 2018-01-10 Alisa Machner, Maciej Zajac, Mohsen Ben Haha, Knut O. Kjellsen, Mette R. Geiker, Klaartje De Weerdt
This study focuses on the reaction of dolomite powder in combination with metakaolin in Portland composite cement pastes. We studied paste samples cured at 20 °C, 38 °C, and 60 °C for up to 1 year. In these systems, the only magnesium-containing hydration phase of dolomite observed was hydrotalcite. Dolomite reacted notably already after 90 days when cured at 60 °C, whereas at lower curing temperatures the reaction was limited. The increased availability of aluminium due to the addition of metakaolin did not contribute to the formation of hydrotalcite. The refined pore space due to the metakaolin addition did not inhibit the hydrotalcite formation. However, the almost total absence of portlandite due to the pozzolanic reaction of the metakaolin inhibited the dolomite reaction, even in pastes with high porosity. Portlandite seems to be the driving force for the reaction as its absence is inhibiting the reaction to take place.
Effect of graphite nanoplatelets and carbon nanofibers on rheology, hydration, shrinkage, mechanical properties, and microstructure of UHPC Cem. Concr. Res. (IF 5.43) Pub Date : 2018-01-08 Weina Meng, Kamal H. Khayat
This study evaluates the effects of two types of graphite nanoplatelet (GNP-C and GNP-M) and one type of carbon nanofiber (CNF) on rheological properties, hydration kinetics, autogenous shrinkage, and pore structure of ultra-high-performance concrete (UHPC). The dispersion method was optimized to secure uniform dispersion of the nanomaterials in the UHPC. The plastic viscosity decreased with the nanomaterials content as the content was increased from 0 to 0.05%. As the nanomaterials content increased from 0 to 0.3%, the duration of induction period was extended by the addition of CNF, but shortened by use of GNP-C or GNP-M; cumulative hydration heat release was increased by introduction of nanomaterials; the autogenous shrinkage of UHPC with CNF, GNP-C, and GNP-M was increased by 30%, 20%, and 20%, respectively. The use of 0.3% CNFs reduced the total porosity of the UHPC by approximately 35%, indicating that the presence of CNFs refined the microstructure of UHPC.
Microstructural effects in the simulation of creep of concrete Cem. Concr. Res. (IF 5.43) Pub Date : 2018-01-06 Alain B. Giorla, Cyrille F. Dunant
The influence of the microstructure on the visco-elastic properties of concrete is investigated using finite element simulations at the meso-scale. We first derive a constitutive law for the creep of the cement paste which accounts for both recoverable and permanent deformations, as well as the influence of temperature and internal relative humidity. The model is calibrated on a set of experiments at the cement paste scale, and then validated after upscaling to the concrete scale. The model is then applied to study the influence of the microstructure on the macroscopic creep of concrete. We show that materials with finer particles exhibit less creep, and that the anisotropy of creep can be explained with the shape and orientation of the aggregates. Furthermore, the acceleration of the stress relaxation in the presence of damage is explained by the micro-mechanical interaction between the aggregates, the cement paste, and the micro-cracks.
Fundamentals of alkali-silica gel formation and swelling: Condensation under influence of dissolved salts Cem. Concr. Res. (IF 5.43) Pub Date : 2017-12-26 J.H.M. Visser
The current theory of ASR gel formation in concrete between silicic acid anions with cations and is unable to explain the differences in concentration required for gel formation between e.g. potassium and sodium or the effect of calcium on the gel formation. In this paper, a new gelation mechanism is explained by means of a condensation between silicic acid anions. The role of salts on the ASR gel formation and gel time is described by their charge screening capacity of the silicic acid, resulting in the decreasing concentration range for gelation: Ca < Mg < Cs < Rb < K < Na < Li. A Cation Gelling Strength (CGS) is defined to assess the cation strength when different cations are present. Silica gel formation will moreover in many cases form a calcium silica precipitate rather than a gel that will be difficult to distinguish from CSH formed during hydration.
Thixotropy of SCC—A model describing the effect of particle packing and superplasticizer adsorption on thixotropic structural build-up of the mortar phase based on interparticle interactions Cem. Concr. Res. (IF 5.43) Pub Date : 2017-12-14 Dirk Lowke
This article considers the thixotropy of SCC mortars and the responsible mechanisms. The objective is a model describing the thixotropic structural build-up based on interparticle interactions and hydration kinetics of the fresh binder paste. In the experimental studies, the formation of thixotropic structure in SCC mortars was varied by changing the composition of the binder paste and quantified rheologically. At the same time, the surface coverage of particles by superplasticizer polymers and the particle packing in the suspension were determined to characterize interparticle interactions. Based on the results, a qualitative model as well as a calculation method were developed to describe the thixotropic behaviour. It is shown that the contribution of colloidal surface interactions and hydration reactions to thixotropy both increase with decreasing surface coverage and therefore decreasing particle separation. In addition, thixotropy increases with increasing contact interactions resulting from a higher solid volume fraction or a lower maximum packing density.
Water absorption measurement of fine porous aggregates using an evaporative method: Experimental results and physical analysis Cem. Concr. Res. (IF 5.43) Pub Date : 2017-11-20 Jennifer Naël-Redolfi, Emmanuel Keita, Nicolas Roussel
Assessment of the so-called “saturated-surface-dried” state shows a strong dependency on the test operator in the case of fine porous aggregates. This leads to low reliability and reproducibility of water absorption measurement for this family of particles. Evaporative methods for water absorption measurements seem to be a promising alternative. In this paper, we aim at evaluating the robustness and the limit of such methods while understanding the general underlying physical processes. We therefore measure the drying kinetics of non-porous and porous, real and model particles with various sizes and morphologies. We study specifically the unexpected effects of angularity and roughness of particles on drying rate changes over time. Our results suggest that this method is not suitable for all porous particles but shall apply well to crushed fine porous particles such as crushed sand and recycled sand.
Influence of set retarding admixtures on calcium sulfoaluminate cement hydration and property development Cem. Concr. Res. (IF 5.43) Pub Date : 2017-11-20 Lisa E. Burris, Kimberly E. Kurtis
Little published data is available to guide specification calcium sulfoaluminate cement (CSA) mixtures with citric acid retarder dosage rates capable of achieving adequate field working times, nor to understand the effect of retarder dosage on hydration and property development, representing a significant barrier to widespread CSA concrete use. Thus, this study investigated the use of citric acid with two commercially-available CSA cements, tracking the effects of dosage on phase development, hydration, setting, and compressive strengths. Key findings were that: citric acid successfully retarded initial set past 120 min for both cements despite significantly different chemistries; increasing cement anhydrite content reduced retarder effectiveness and altered hardened binder microstructure, reducing compressive strengths; greater retarder dosages did not negatively affect cumulative hydration, nor strengths; and the time at which the maximum rate of heat evolution occurred correlated with final setting, a relationship useful for predicting field mixture behavior based on laboratory testing.
A novel method to predict internal relative humidity in cementitious materials by 1H NMR Cem. Concr. Res. (IF 5.43) Pub Date : 2017-11-15 Zhangli Hu, Mateusz Wyrzykowski, Karen Scrivener, Pietro Lura
In sealed cementitious materials, the internal relative humidity (RH) decrease is mainly due to the water activity depression caused by menisci formation in partially-saturated pores. This process can be conveniently described with the evolution of the Kelvin radius. To obtain the Kelvin radius, a novel method based on the evolution of pore volumes quantified by 1H nuclear magnetic resonance (NMR) and mercury intrusion porosimetry (MIP) was developed. This approach was validated against experimental results for cementitious materials with a range of water to cement ratios from 0.30 to 0.46. A comparison between the Kelvin radius calculated with this approach with a previously published method using data obtained from MIP and chemical shrinkage was presented. A sensitivity analysis for the new prediction method was performed using a bootstrapping technique.
A mechanistic study on mitigation of alkali-silica reaction by fine lightweight aggregates Cem. Concr. Res. (IF 5.43) Pub Date : 2017-11-14 Chang Li, Michael D.A. Thomas, Jason H. Ideker
Previous studies have shown that fine lightweight aggregate (FLWAs) may be a possible solution for ASR mitigation. However, the mechanisms of mitigation have not yet been fully elucidated. This study investigated how three commonly used FLWAs, expanded slate, shale, and clay can mitigate ASR in concrete using the accelerated mortar bar test (AMBT), concrete prism test (CPT), pore solution analysis and scanning electron microscope (SEM) analysis. The AMBT and CPT results showed that expanded clay was the most effective in reducing the expansion caused by ASR. Pore solution analysis showed that FLWAs, especially expanded shale and clay can reduce the alkalinity as well as increase aluminum content in the pore solution. SEM analysis revealed that infilling reaction products formed in the pores of the FLWAs, whose composition was closer to C-A-S-H, rather than alkali-silica reaction product.
Estimating the mechanical properties of hydrating blended cementitious materials: An investigation based on micromechanics Cem. Concr. Res. (IF 5.43) Pub Date : 2017-11-14 F. Lavergne, A. Ben Fraj, I. Bayane, J.F. Barthélémy
The hydration model of Parrot & Killoh (1984) has been extended to blended cements and coupled to a micromechanical scheme similar to that of Pichler & Hellmich (2011) to estimate the Young modulus and the compressive strength of cementitious materials as a function of time. A finite aspect ratio of 7 is introduced to describe the shape of the hydrates and improve the estimate of the early age strength by the micromechanical scheme. Furthermore, accounting for the stress fluctuations in the cement paste partly explains the fact that the compressive strength of a concrete can be lower than that of its cement paste. Finally, the estimated physical properties are compared to numerous experimental measurements from the literature and new experimental measurements on blended cement pastes featuring significant weight fractions of limestone filler, fly ash or silica fume. It is shown that the present model slightly overestimates the dilution effect.
Discussion of the paper “A new view on the kinetics of tricalcium silicate hydration,” by L. Nicoleau and A. Nonat, Cem. Concr. Res. 86 (2016) 1–11 Cem. Concr. Res. (IF 5.43) Pub Date : 2017-11-13 Ellis Gartner
In contradiction to the proposition of Nicoleau and Nonat, it is shown that the rate of hydration of anhydrous tricalcium silicate (C3S) cannot be controlled by the rate of dissolution of the anhydrous phase in typical C3S pastes except at extremely early ages. The strongest evidence for this is the observation that the hydration of C3S in pastes is strongly accelerated by the addition of afwillite seeds, to the extent that it hydrates almost completely by 7 days. It is suggested that the hydration rate at later ages is primarily controlled by the porosity and permeability of the hydration products.
Evaluation of the concrete prisms test method for assessing the potential alkali-aggregate reactivity of recycled concrete aggregates Cem. Concr. Res. (IF 5.43) Pub Date : 2017-11-12 S. Beauchemin, B. Fournier, J. Duchesne
The concrete prism test (CPT) is a widely used method for assessing the potential alkali-aggregate reactivity of any given natural aggregate. This paper presents the evaluation of the current method and its applicability to assess the alkali-silica reactivity of recycled concrete aggregates (RCA). The testing was carried out on two different types of RCA: manufactured from > 15 years old blocks disposed outdoors and from a demolished overpass infrastructure. Following the analysis of the different parameters that make RCA different than original aggregates, it has been found that the residual mortar content of the RCA particles and their absorption condition prior to their addition to mixtures are considerably important. The CPT is reliable to access RCA reactivity, however it is therefore recommended to use saturated RCA as higher expansion is obtained under these conditions. It is also recommended to use the aggregate dry rodded bulk density to determine the coarse aggregate-to-sand ratio, instead of using a fixed 60 to 40 ratio. Moreover, no pessimum effect was observed using RCA; in fact, the expansion of the test prisms was increasing with increasing RCA content. The extent of damaged suffered by the concrete from which the RCA are manufactured and the geology of the original virgin aggregates incorporated in the RCA are also important parameters to consider while using the CPT to assess correctly each RCA reactivity.
Uncovering the role of micro silica in hydration of ultra-high performance concrete (UHPC) Cem. Concr. Res. (IF 5.43) Pub Date : 2017-11-12 Nam Kon Lee, K.T. Koh, Min Ook Kim, G.S. Ryu
This study aims to clarify the role of micro silica in hydration of UHPC. The behavior of two different types of micro silica in UHPC was investigated with regard to their filler effect and pozzolanic reaction by using analytical techniques; XRD, TG, 29Si and 27Al NMR spectroscopy, and MIP. The micro silica with high pozzolanic activity led to the higher level of Al-substitution for Si in C-S-H and the denser structure, thus increasing compressive strength, whereas that facilitating filler effect limited the consumption of Ca(OH)2 even after high temperature curing and led to the higher amount of AFm phase. In the UHPC showing high filler effect, additional hydration of C3S and C2S occurred at later ages, increasing the fraction of Q1 site in C-S-H and decreasing the porosity in the pore diameter region below 10 nm. Consequently, a significant increase in the compressive strength of this UHPC was achieved.
Rate-limiting reaction of C3S hydration - A reply to the discussion “A new view on the kinetics of tricalcium silicate hydration” by E. Gartner Cem. Concr. Res. (IF 5.43) Pub Date : 2017-10-31 Nicoleau Luc, Nonat André, Daval Damien
In the case of coupled solids-solution reactions, any mean accelerating or decelerating one of the reaction, will also change the other reaction(s) in the same way, through the coupling mediated by the solution. The observation of any kinetic change by one of these means should not lead to too rapid conclusion on the limitation of kinetics and it must be done with great caution. Contrary to what Gartner mentioned, the acceleration of C3S hydration by the addition of calcium silicate hydrate seeds, is not a trivial evidence suggesting that hydration kinetics “has to be” limited by the C-S-H precipitation and that the C3S dissolution can be neglected. In our paper, efforts have been made to highlight again the importance of C3S dissolution, based on the Gartner's discussion points.
Hydrate failure in ITZ governs concrete strength: A micro-to-macro validated engineering mechanics model Cem. Concr. Res. (IF 5.43) Pub Date : 2017-11-10 Markus Königsberger, Michal Hlobil, Brice Delsaute, Stéphanie Staquet, Christian Hellmich, Bernhard Pichler
Ever since the early days of Féret (1892) and Abrams (1919), concrete research has targeted at relating concrete composition to uniaxial compressive strength. While these activities were mainly characterized by empirical fitting functions, we here take a more fundamental approach based on continuum micromechanics. The loading applied at the concrete level, is first concentrated (“downscaled”) to maximum stresses related to cement paste volumes which are directly adjacent to the aggregates, i.e. to the interfacial transition zones (ITZ). These maximum stresses are further “downscaled” to the micron-sized hydrates, in terms of higher-order stress averages. The latter enter a Drucker-Prager failure criterion with material constants derived from nanoindentation tests. The model is successfully validated across the hydrate-to-concrete scales. Strength magnitude is governed by ITZ stress concentrations, and the water-to-cement ratio is its dominant mixture design parameter.
One-part alkali-activated materials: A review Cem. Concr. Res. (IF 5.43) Pub Date : 2017-11-06 Tero Luukkonen, Zahra Abdollahnejad, Juho Yliniemi, Paivo Kinnunen, Mirja Illikainen
Alkali-activated materials (AAM) are recognized as potential alternatives to ordinary Portland cement (OPC) in order to limit CO2 emissions as well as beneficiate several wastes into useful products. However, the alkali activation process involves concentrated aqueous alkali solutions, which are corrosive, viscous, and, as such, difficult to handle and not user friendly. Consequently, the development of so-called one-part or “just add water” AAM may have greater potential than the conventional two-part AAM, especially in cast-in-situ applications. One-part AAM involves a dry mix that consists of a solid aluminosilicate precursor, a solid alkali source, and possible admixtures to which water is added, similar to the preparation of OPC. The dry mix can be prepared at elevated temperatures to facilitate the reactivity of certain raw materials. This review discusses current studies of one-part AAMs in terms of raw materials, activators, additives, mechanical and physical properties, curing mechanisms, hydration products, and environmental impacts.
Some contents have been Reproduced by permission of The Royal Society of Chemistry.
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