Modelling of transport processes in concrete exposed to elevated temperatures – An alternative formulation for sorption isotherms Cem. Concr. Res. (IF 4.762) 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 4.762) 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 4.762) 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 4.762) 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 4.762) 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 4.762) 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 4.762) 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 4.762) 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 4.762) 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 4.762) 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 4.762) 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 4.762) 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 4.762) 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.
Calcined clay limestone cements (LC3) Cem. Concr. Res. (IF 4.762) 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.
Water absorption measurement of fine porous aggregates using an evaporative method: Experimental results and physical analysis Cem. Concr. Res. (IF 4.762) 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 4.762) 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 4.762) 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 4.762) 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 4.762) 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 4.762) 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 4.762) 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 4.762) 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 4.762) 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.
Education for sustainable use of cement based materials Cem. Concr. Res. (IF 4.762) 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.
Carbon dioxide reduction potential in the global cement industry by 2050 Cem. Concr. Res. (IF 4.762) 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.
Vegetable ashes as Supplementary Cementitious Materials Cem. Concr. Res. (IF 4.762) 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.
Earth concrete. Stabilization revisited Cem. Concr. Res. (IF 4.762) 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.
Admixtures and sustainability Cem. Concr. Res. (IF 4.762) 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.
Alkali-activated materials Cem. Concr. Res. (IF 4.762) 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.
Alternative cement clinkers Cem. Concr. Res. (IF 4.762) 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.
Hydrate failure in ITZ governs concrete strength: A micro-to-macro validated engineering mechanics model Cem. Concr. Res. (IF 4.762) 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 4.762) 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.
A 12 year EDF study of concrete creep under uniaxial and biaxial loading Cem. Concr. Res. (IF 4.762) Pub Date : 2017-11-04 Laurent Charpin, Yann Le Pape, Éric Coustabeau, Éric Toppani, Grégory Heinfling, Caroline Le Bellego, Benoît Masson, José Montalvo, Alexis Courtois, Julien Sanahuja, Nanthilde Reviron
This paper presents a 12-year-long creep and shrinkage experimental campaign on cylindrical and prismatic concrete samples under uniaxial and biaxial stress, respectively. The motivation for the study is the need for predicting the delayed strains and the pre-stress loss of concrete containment buildings of nuclear power plants. Two subjects are central in this regard: the creep strain's long-term evolution and the creep Poisson's ratio. A greater understanding of these areas is necessary to ensure reliable predictions of the long-term behavior of the concrete containment buildings. Long-term basic creep appears to evolve as a logarithm function of time in the range of 3 to 10 years of testing. Similar trends are observed for drying creep, autogenous shrinkage, and drying shrinkage testing, which suggests that all delayed strains obtained using different loading and drying conditions originate from a common mechanism. The creep Poisson's ratio derived from the biaxial tests is approximately constant over time for both the basic and drying creep tests (creep strains corrected by the shrinkage strain). It is also shown that the biaxial non-drying samples undergo a significant increase in Young's modulus after 10 years.
Propagation of corrosion-induced cracks of the RC beam exposed to marine environment under sustained load for a period of 26 years Cem. Concr. Res. (IF 4.762) Pub Date : 2017-11-03 Wenjun Zhu, Raoul François, Chengping Zhang, Dingli Zhang
This paper presents the corrosion-induced cracking process of a corroded reinforced concrete beam exposed to a chloride environment for 26 years. The cracking maps of the beam were drawn during different corrosion periods. The first corrosion-induced cracks in the compression zone occurred at about the 5th year and then developed significantly. The corrosion-induced cracks in the tension zone appeared at about the 7th year, followed by the stirrup zones in the transversal sections. At about the 14th year, the width of the cracks in the tension zone exceeded that in the compression zone. The cracks in the tension zone became connected almost throughout the span. The top-bar effect, bleeding and the “top surface ponding effect” led to the appearance of corrosion-induced cracks first in the compression zones, while the corrosion-induced cracks in the tension zone increased more significantly in both length and width as a result of the sustained load.
Unsupervised and supervised pattern recognition of acoustic emission signals during early hydration of Portland cement paste Cem. Concr. Res. (IF 4.762) Pub Date : 2017-11-03 Lateef Assi, Vafa Soltangharaei, Rafal Anay, Paul Ziehl, Fabio Matta
Several studies have been conducted to investigate early age Portland cement hydration using acoustic emission technique, with different mechanisms attributed by different authors. In the proof-of-concept research presented in this paper, acoustic emission (AE) was employed to explore relationships between recorded signals associated with elastic stress waves and potential mechanisms associated with cement hydration. Ordinary Portland cement paste samples having water/cement ratio of 0.3 and 0.5 were monitored during the first 72 h of curing using broadband AE sensors. The acoustic emission signals were analyzed using unsupervised and supervised pattern recognition algorithms to address limitations of acoustic emission parameter analysis. Wavelet analysis was utilized as a complementary method, which can be considered as a map for identification of patterns in the signal set. Unsupervised methods are useful when there is no history or background data concerning the pattern of a phenomenon such as the hydration process.
Isothermal calorimetry and in-situ XRD study of the NaOH activated fly ash, metakaolin and slag Cem. Concr. Res. (IF 4.762) Pub Date : 2017-11-02 Zengqing Sun, Anya Vollpracht
Much is unknown about the reaction processes responsible for the formation and polycondensation of geopolymers and other alkali activated materials. In this work, isothermal calorimeter and in-situ XRD were adopted to study the heat and mineral evolution of NaOH activated fly ash, metakaolin and ground granulated blast furnace slag. Both activator concentration and temperature have profound influences on duration of exothermal geopolymerization peaks. NaOH activated fly ash is more temperature dependable, with much higher activation energy than metakaolin and slag. The dissolution of source precursor is rapid and the formation of new phases can be detected by the end of the initial dissolution period. The in-situ XRD measurement together with the PONKCS analysis method promotes quantitative estimation of amorphous evolution during alkali activation.
Can calcium aluminates activate ternesite hydration? Cem. Concr. Res. (IF 4.762) Pub Date : 2017-10-31 M. Montes, E. Pato, P.M. Carmona-Quiroga, M.T. Blanco-Varela
Aluminum hydroxide (AH3) has recently been shown to be able to activate hydration in ternesite, a phase found in some calcium sulfoaluminate (CSA) cements. This study explored the capacity of a number of calcium aluminates (C3A, C12A7, CA and C4A3 S ¯ ) to activate ternesite hydraulic reactivity. After laboratory synthesis, the aluminates were blended with ternesite at a ratio of 1:2 and their hydration was monitored with isothermal conduction calorimetry for 7 days at 25 °C. The resulting pastes were analysed with XRD, FTIR and DTA. The presence of ternesite in the pastes altered the aluminate heat flow curves, shortening the induction period and bringing the reaction peak forward, an indication of hastened hydration. Ternesite also altered the reaction products, which included calcium monosulfoaluminate hydrate and strätlingite.
Micro- and nano-structural evolutions in white Portland cement/pulverized fuel ash cement pastes due to deionized-water leaching Cem. Concr. Res. (IF 4.762) Pub Date : 2017-10-31 Shanshan Jia, Ian G. Richardson
Thin slices of white Portland cement-low calcium pulverized fuel ash (pfa) blended cement pastes containing 30 or 50% pfa were leached progressively in de-ionized water. The paste with 50% pfa was aged 13 years prior to leaching and those with 30% pfa were aged 1 and 13 years. Pastes were leached for up to 75 days and were characterized using thermal analysis, X-ray diffraction, analytical scanning and transmission electron microscopy, and solid-state nuclear magnetic resonance spectroscopy. Leaching affected the pastes in the following sequence: (i) crystals of Ca(OH)2 large enough to be resolved by backscattered electron imaging were removed completely prior to any effect on C-A-S-H; (ii) the Ca/Si ratio of C-A-S-H reduced from ≈ 1.4 to ≈ 1.0 whilst the aluminosilicate structure was unaffected; (iii) further reduction in the Ca/Si ratio of C-A-S-H was accompanied by lengthening of the aluminosilicate chains; (iv) the Ca/Si ratio of C-A-S-H reduced ultimately to ≈ 0.6.
Microstructural packing- and rheology-based binder selection and characterization for Ultra-high Performance Concrete (UHPC) Cem. Concr. Res. (IF 4.762) Pub Date : 2017-10-31 Aashay Arora, Matthew Aguayo, Hannah Hansen, Cesar Castro, Erin Federspiel, Barzin Mobasher, Narayanan Neithalath
This paper reports strategies to design the binder phase of ultra-high performance concretes (UHPC) from commonly available cement replacement (fly ash, slag, microsilica, metakaolin) and fine filler (limestone) materials. A packing algorithm is used to extract the number density, mean centroidal distance, and coordination number of the microstructure. Similarly, rheological studies on the pastes provide yield stress, plastic viscosity, and min-slump spread. The selection criteria involves using the three microstructural and three rheological parameters individually or in combination to define packing and flow coefficients. The selection criteria is flexible enough to allow users modify the constraints depending on the application. In this study, only ternary and quaternary blends, with a total cement replacement of 30% by mass are selected for further characterization. The highly efficient microstructural packing in these mixtures and better workability that facilitated dispersion of particles to enhance the reactivity results in beneficial pore structure and mechanical properties.
Development of MgO concrete with enhanced hydration and carbonation mechanisms Cem. Concr. Res. (IF 4.762) Pub Date : 2017-10-27 N.T. Dung, C. Unluer
This study proposed the use of hydration agent (HA) and seeds to improve the hydration and carbonation of reactive magnesium cement (RMC)-based concrete formulations. Hydration of RMC was evaluated by isothermal calorimetry. Water absorption and compressive strength results were used to assess the mechanical performance of RMC-based concrete samples. Quantification of hydrate and carbonate phases was performed via XRD and TGA. Formation and morphology of carbonates were observed via BSE and SEM. In addition to increasing the utilization of RMC in the carbonation reaction and facilitating early strength development, the use of HA formed large carbonate phases, while the addition of seeds improved sample microstructures via the development of dense carbonate networks. The improvements in morphology, microstructure and carbonate content in samples involving the simultaneous use of HA and seeds resulted in 56% lower water absorption values and 46% higher 28-day compressive strengths (70 MPa) in comparison to the control sample.
The impact of carbonation on bulk and ITZ porosity in microconcrete materials with fly ash replacement Cem. Concr. Res. (IF 4.762) Pub Date : 2017-10-27 J.L. Branch, R. Epps, D.S. Kosson
Scanning electron microscopy was used to evaluate the porosity patterns found in microconcrete materials to better understand the gaseous diffusion pathways and reaction of CO2 within the bulk cement paste and interfacial transition zone (ITZ) of microconcrete materials containing different fly ash replacement types. Image segmentation was applied to evaluate the porosity as a function of distance from an exposed surface in the bulk cement paste and from aggregate boundaries in the ITZ in both carbonated and non‑carbonated microconcrete materials. The ITZ region remained more porous compared to the bulk cement paste for all microconcrete types after carbonation despite a change in the porosity profile across the ITZ region. Results indicate that both the carbonation reaction capacity and porosity before carbonation impact the decrease in porosity observed as a result of carbonation in the ITZ and bulk cement paste regions.
The effect of mixing on the performance of macro synthetic fibre reinforced concrete Cem. Concr. Res. (IF 4.762) Pub Date : 2017-10-27 J.O. Lerch, H.L. Bester, A.S. Van Rooyen, R. Combrinck, W.I. de Villiers, W.P. Boshoff
Concrete suffers from brittle failure due to its low tensile strength. This drawback can be compensated for by adding reinforcement bars and/or steel fibres, and more recently, macro synthetic fibres. When mixing concrete with these fibres the aggregates could damage the fibres. This paper presents work done on the effect of mixing on the performance of macro synthetic fibre reinforced concrete. Single-fibre pull-out tests were conducted on various fibres in both the original and mixed state. Furthermore, flexural tests were performed to investigate the influence of mixing time and mixer type on the performance. It can be concluded that mixing is beneficial for flat type fibres, but the performance of crimped or embossed fibres remains the same. Furthermore, longer mixing times (> 10 min) in a pan mixer are detrimental to the performance, while the performance in a tilting drum mixer remains unchanged even after a mixing time of 60 min.
Corrosion rate of carbon steel in carbonated concrete – A critical review Cem. Concr. Res. (IF 4.762) Pub Date : 2017-10-27 M. Stefanoni, U. Angst, B. Elsener
Reinforced concrete with lower environmental footprint (lower CO2 emission) can be obtained by reducing the clinker content in the cements. As the carbonation of concrete is faster, corrosion of steel in carbonated concrete during the propagation phase is becoming important both for science and practice. The present literature review summarizes the state of the art, reporting corrosion rate data for a broad range of cement types, w/b ratios and environmental conditions. Correlations between corrosion rate and the main influencing parameters are elaborated and discussed. It confirms that the corrosion rate of steel in carbonated concrete is not under ohmic control. More important are the degree of pore saturation and the effective steel area in contact with water filled pores. It also emerges that the new blended cements have to be systematically studied with respect to the corrosion behavior of steel in carbonated concrete in order to make reliable service life prediction.
Influence of carbonation on “maximum phenomenon” in surface layer of specimens subjected to cyclic drying-wetting condition Cem. Concr. Res. (IF 4.762) Pub Date : 2017-10-27 Honglei Chang, Song Mu, Pan Feng
Numerous researches have reported that there is a tendency chloride content first climbs to the maximum then declines with depth increasing in the surface layer of concrete under cyclic drying-wetting environments, which is temporally called ‘maximum phenomenon’ in this paper. This research focuses on the impact of different carbonation conditions on this phenomenon for both pastes and mortars. The distribution of chloride suggests that coupled effect of carbonation and capillary suction/moisture evaporation is more likely to lead to the formation of maximum phenomenon than the merely effect of capillary suction/moisture evaporation. Furthermore, analysis of pore structure and phase composition reveals that this particular phenomenon is directly related to the release of bound chloride fixed in Friedel's salt triggered by carbonation. In addition, the forming process of maximum phenomenon is proposed in the end based on Friedel's salt decomposition under cyclic drying and wetting condition.
Corrosion resistance of steel fibre reinforced concrete - A literature review Cem. Concr. Res. (IF 4.762) Pub Date : 2017-10-26 Victor Marcos-Meson, Alexander Michel, Anders Solgaard, Gregor Fischer, Carola Edvardsen, Torben Lund Skovhus
Steel fibre reinforced concrete (SFRC) is increasingly being used in the construction of civil infrastructure. However, there are inconsistencies among international standards and guidelines regarding the consideration of carbon-steel fibres for the structural verification of SFRC exposed to corrosive environments. This paper presents a review of the published research regarding carbonation- and chloride-induced corrosion of SFRC, and proposes a deterioration theory for cracked SFRC exposed to chlorides and carbonation, based on the damage at the fibre-matrix interface. The review confirms an overall agreement among academics and regulators regarding the durability of uncracked SFRC exposed to chlorides and carbonation. Contrariwise, the durability of cracked SFRC is under discussion at the technical and scientific level, as there is a large dispersion on the experimental results and some of the mechanisms governing the corrosion of carbon-steel fibres in cracks and its effects on the fracture behaviour of SFRC are not fully understood.
The mechanism of hydration of MgO-hydromagnesite blends Cem. Concr. Res. (IF 4.762) Pub Date : 2017-10-23 C. Kuenzel, F. Zhang, V. Ferrándiz-Mas, C.R. Cheeseman, E.M. Gartner
The hydration of reactive periclase (MgO) in the presence of hydromagnesite (Mg5(CO3)4(OH)2·4H2O) was investigated by a variety of physical and chemical techniques. Hydration of pure MgO-water mixtures gave very weak pastes of brucite (Mg(OH)2), but hydration of MgO-hydromagnesite blends gave pastes which set quickly and gave compressive strengths of potential interest for construction applications. The strengths of the blends increased with hydration time at least up to 28 days, and were not significantly decreased by increasing the hydromagnesite content up to 30%. Raman spectroscopy suggests that an amorphous phase, of composition between that of brucite, hydromagnesite and water, may form. Small amounts of calcite also form due to CaO in the MgO source. Thermodynamic calculations imply that the crystalline phase artinite (MgCO3·Mg(OH)2·3H2O) should be the stable product in this system, but it is not observed by either XRD or FTIR techniques, which suggests that its growth may be kinetically hindered.
Automated coupling of NanoIndentation and Quantitative Energy-Dispersive Spectroscopy (NI-QEDS): A comprehensive method to disclose the micro-chemo-mechanical properties of cement pastes Cem. Concr. Res. (IF 4.762) Pub Date : 2017-09-22 William Wilson, Luca Sorelli, Arezki Tagnit-Hamou
Engineering cement-based composites requires a comprehensive understanding of the microstructure features governing macroscopic properties. This work aims to foster the latest chemo-mechanical technique to disclose the micro-mechanical properties of intimately intermixed phases. Considering a typical Portland cement paste as a study case, we compared the so-far developed nanoindentation analysis methods with a new automatic method coupling Nano-Indentation and Quantitative Energy-Dispersive Spectroscopy (NI-QEDS). Besides evincing advantages and disadvantages of previous methods, the NI-QEDS analyses enabled distinguishing chemical phases having strongly overlapping mechanical properties. Results suggested the presence at the sub-micrometre scale of extra calcium, aluminum and sulfur into (or in the vicinity of) outer C–S–H. The quantitative chemistry allowed identifying the rare occurrence of nanoindentation volumes located on “pure” phases (e.g., inner C–S–H). Finally, NI-QEDS provided new knowledge on cement pastes' micro-chemo-visco-mechanical properties and showed to be a powerful tool for investigating even more heterogeneous systems.
Response to the discussion by Hongyan Ma and Ying Li of the paper “Characterization of magnesium potassium phosphate cement blended with fly ash and ground granulated blast furnace slag” Cem. Concr. Res. (IF 4.762) Pub Date : 2017-08-05 Laura J. Gardner, Susan A. Bernal, Samuel A. Walling, Claire L. Corkhill, John L. Provis, Neil C. Hyatt
We recently reported the first comprehensive investigation of magnesium potassium phosphate cements (MKPCs) blended with supplementary cementitious materials (pulverized fuel ash and granulated blast furnace slag) for the encapsulation of radioactive wastes [Gardner et al., Cem. Concr. Res. 74 (2015) 78–87]. Using a combination of characterization techniques, we demonstrated the important role of the reaction of the supplementary cementitious materials in contributing to the development of the microstructure and strength of MKPC composites. Here, we clarify aspects of our experimental design, and elaborate on the interpretation of our data, following discussion by Ma and Li.
Discussion of the paper “Characterisation of magnesium potassium phosphate cement blended with fly ash and ground granulated blast furnace slag” by L.J. Gardner et al. Cem. Concr. Res. (IF 4.762) Pub Date : 2017-08-03 Hongyan Ma, Ying Li
A recently published paper authored by Gardner et al. [Characterisation of magnesium potassium phosphate cement blended with fly ash and ground granulated blast furnace slag, Cem. Concr. Res. 74 (2015) 78–87] is discussed. This work is of great significance in understanding the reaction mechanism of magnesium potassium phosphate cement blended with supplementary cementitious materials. However, several points in this work need to be further discussed, as shown in this discussion, including the mix proportion, paste morphology, mechanism of the synergy between MKPC and fly ash/ground granulated blast furnace slag.
Analysis of C-S-H growth rates in supersaturated conditions Cem. Concr. Res. (IF 4.762) Pub Date : 2017-05-23 Frank Bellmann, George W. Scherer
The growth rate of calcium-silicate-hydrate (C-S-H) was analyzed by following the evolution of calcium and silicon concentrations in supersaturated solutions. In these experiments, the supersaturated solution was produced by mixing a saturated calcium hydroxide solution and a solution obtained from the hydration of tricalcium silicate. A continuous decrease of the silicon concentration over time was observed during the experiments and the C-S-H formation rate was calculated from the amount of silicon that was precipitated between two consecutive analyses. The data obtained in this study demonstrate that the interfacial growth rate of C-S-H depends mainly on the supersaturation with respect to this phase, the availability of calcite as a substrate for heterogeneous nucleation and the calcium concentration in solution. A mean value of approximately 10 nmol of C-S-H per m2 per second was obtained for the interfacial growth rate of C-S-H in conditions that are relevant for the hydration of tricalcium silicate.
Kinetic analysis of C-S-H growth on calcite Cem. Concr. Res. (IF 4.762) Pub Date : 2016-12-23 George W. Scherer, Frank Bellmann
Data for the rate of formation of calcium-silicate-hydrate on a calcite substrate are fitted to a model for growth of ellipsoidal particles. The time-dependent driving force for growth is calculated using a solid solution model for calcium-silicate-hydrate. The nucleation density and particle shape are estimated from SEM photos. Good agreement is found for the rate of growth and the fractional coverage of the substrate. The growth rate is found to have a cubic dependence on the supersaturation, and to be sensitive to the concentration of calcium hydroxide when that component is undersaturated.
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