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Fly ash, an aluminosilicate composite consisting of disordered (major) and crystalline (minor) compounds, is a low-carbon alternative that can partially replace ordinary portland cement (OPC) in the binder fraction of concrete. Therefore, understanding the reactivity of fly ash in the hyperalkaline conditions prevalent in concrete is critical to predicting concrete's performance; including setting and strength gain. Herein, temporal measurements of the solution composition (using inductively coupled plasma-optical emission spectrometry: ICP-OES) are used to assess the aqueous dissolution rate of monophasic synthetic aluminosilicate glasses analogous to those present in technical fly ashes, under hyperalkaline conditions (10 ≤ pH ≤ 13) across a range of temperatures (25°C ≤ T≤45°C). The dissolution rate is shown to depend on the average number of topological constraints per atom within the glass network (n c, unitless), but this dependence weakens with increasing pH (>10). This is postulated to be on account of: (a) time-dependent changes in the glass’ surface structure, that is, the number of topological constraints; and/or (b) a change in the dissolution mechanism (eg from network hydrolysis to transport control). The results indicate that the topological description of glass dissolution is most rigorously valid only at very short reaction times (ie at high undersaturations), especially under conditions of hyperalkalinity. These findings provide an improved basis to understand the underlying factors that affect the initial and ongoing reactivity of aluminosilicate glasses such as fly ash in changing chemical environments, for example, when such materials are utilized in cementitious composites.

In this paper, a nonlinear formulation approach using an artificial neural network (ANN) is proposed and applied for the first time to predict the behavior of rubberized cement composite for which there is neither a design formulation nor a closed-form prediction model. For this purpose, the experimental data on the rubberized composite was collected, which includes mix ingredients, curing condition, and age. The statistical analysis of the data was performed to well characterize the interdependencies. Then, the ANN model was built, and the parameters for the optimum ANN model were found. Afterward, based on the relationships used in the ANN architecture at different steps, such as inputs nodes, neurons of hidden layers, activation functions, and output calculations, the ANN model was formulated, and the optimum weights and biases were determined. Once the closed-form formulation was derived, of the model’s performance was assessed through its prediction correlation and error. It was found that the ANN formulation was very robust to predict the complex behavior of rubberized composites with R2 = 0.98. Finally, the normalized importance of the model’s inputs was determined and the parametric study of the ANN model was also performed to show the effect of each variable on the output. The results showed that the relationships between the inputs and the output were mostly nonlinear, indicating the fact that the system’s behavior is a complex problem, which was well captured by formulation-based ANN model.

Cementitious binders have become increasingly complex in their variety, mineralogical and chemical compositions, and structure over the past several decades, and there is no sign that the trend will stop or reverse in the future. Strategies for handling the complexity fall into one of two categories: (1) increasingly laborious trial-and-error exploration of the design space and mixture qualification process, or (2) systematic development of structure-property relationships, through curation of fundamental material component data and validated modeling based on fundamental scientific principles. The latter approach requires a sustained research investment and has received comparatively little attention in the concrete materials community. Nevertheless, the long-term benefits of determining structure-property relationships arguably far outweigh the costs. This paper describes general measurement and modeling strategies for developing structure-property relationships and suggests how the existence of comprehensive materials data repositories and scientific modeling tools could revolutionize concrete materials design, including design for service life.

The surface normal dissolution flux of beta dicalcium silicate is quantified using in situ digital holographic microscopy. The dissolution flux in highly undersaturated flowing solutions depends on the water activity to an empirically determined power of 1.7. A heterogeneous distribution of reactive surface sites (e.g., crystalline defects) at the surface leads to a distribution of local fluxes. Accounting for the nominally unreactive and slowly reactive surface sites, the median of the macroscopic forward reaction dissolution flux distribution is −1.0 μmol m−2 s−1 with an interquartile range of 0.3 μmol m−2 s−1. Ex situ X-ray photoelectron spectroscopy indicates that the surface calcium-to-silicon ratio decreases by 10% after some dissolution.

This paper studies a selection of fine sands, their particle shape and its effect on the plastic properties of mortars via the influence on the workability and packing of cementitious mixtures. Five sands from different sources were sampled, wet sieved, and classified into six size classes: 20 μm–75 μm, 75 μm–150 μm, 150 μm–300 μm, 300 μm–600 μm, 600 μm to 1.18 mm and 1.18 mm–2.36 mm. Particle shape was measured and various geometrical quantities were determined for particles of each class using a combination of X-ray microcomputed tomography (μCT) and spherical harmonic analysis. Specimen preparation, scanning method, minimum required number of particles in a representative sample, and shape histograms are all discussed, and used to classify and compare the shape of the particles of each material type and size class. This information is then used to establish a connection between sand particle shape and the wet packing density, the workability in the form of slump-flow, and the water demand in mortar mixtures.

Alkali-silica reaction (ASR) is a significant cause of the degradation of concrete structures. ASR results from the dissolution of reactive silicate aggregates and the subsequent formation of damaging, expansive ASR gels. Despite the prevalence of ASR, so far only lithium-based chemical admixtures have been shown to be successful in mitigating its deleterious effects. Herein, dissolved calcium nitrate (Ca(NO3)2: CN) is demonstrated to reduce ASR-induced expansion caused by the use of sodium borosilicate (NBS) glass as a model reactive aggregate. The significance of reductions of NBS-glass dissolution and mortar-bar expansion rates is discussed, with special focus on highlighting the mechanism by which calcium nitrate mitigates ASR, i.e., by the inhibition of aggregate dissolution. This study reveals a new and cost-effective pathway to suppress aggregate dissolution and ASR in concrete based on the dosage of soluble alkaline-earth additives.

Setting and strength development of ordinary Portland cement (OPC) binders involves multiple interacting chemical reactions, resulting in the formation of a solid microstructure. A long‐standing yet elusive goal has been to establish a basis for the prediction of the properties and performance of concrete using knowledge of the chemical and physical attributes of its components—PC, sand, stone, water, and chemical admixtures—together with the environmental conditions under which they react. Machine learning (ML) provides a data‐driven basis for the estimation of properties, and has recently been applied to estimate the 28 days (compressive) strength of concrete from knowledge of its mixture proportions (Young et al, Cem Concr Res, 2019, 115:379). Building on this success, the current work uses a diverse dataset of ASTM C150 cements, the chemical composition and other attributes of which have been measured. ML estimators were trained with this dataset to estimate both paste setting time and mortar strength development. The ML estimation errors are typically similar to the measurement repeatability of the relevant ASTM test methods, and are thus able to account for the influence of binder composition and fineness. This creates new opportunities to apply data intensive methods to optimize concrete formulations under multiple constraints of cost, CO2 impact, and performance attributes.

The dissolution rates, normalized by surface area, of two different gypsum powders in aqueous solutions were measured by tracking changes in solution composition as a function of time in a continuously stirred batch reactor. The powders were classified into different size ranges to observe the influence of particle coarseness on dissolution kinetics. Changes in solid surface area during the experiment were modeled as a function of mass loss by measuring the specific surface areas of the powders before and after the experiments. Contrary to the implicit assumption in several previous studies, the total powder surface area increases, typically by a factor of two, prior to decreasing with ongoing dissolution. This phenomenon therefore has a significant influence, up to 50%, on the surface area-normalized rate. The initial solution composition was set to one of several prescribed saturation states so that an empirical rate law could be inferred over a wide range of saturations. The rate coefficient for both types of gypsum depended on size, being lower for coarser particles. Other than the difference in rate coefficients, the dissolution rate of both sources of gypsum can be described well by the same mathematical function of solution saturation, at least to within the uncertainty in the measurements. Data from previous studies of gypsum dissolution appear to follow approximately the same dependence on particle size as observed in this study.

This study follows a first paper written on some three-dimensional geometrical quantities of a selection of sand particles from the United Arab Emirates (UAE), determined using a combination of X-ray microcomputed tomography (μCT) and spherical harmonic analysis, and their correlation to the properties of mortars made with these sands, with no added dispersant. This paper studies other shape metrics of these same sands, the degree of convexity and the Hofman shape entropy, and the influence of particle shape, as judged from these shape parameters, on the water demand and other properties of dispersed mortars. Five sands from different sources were sampled, wet sieved, and classified into six sieve size classes: 20 μm–75 μm, 75 μm–150 μm, 150 μm–300 μm, 300 μm–600 μm, 600 μm to 1.18 mm and 1.18 mm–2.36 mm. The paper discusses the methods of sample preparation, μCT scanning, and the classification of particles based on their shape. The mortars used a dispersant agent enabling experiments to be conducted at low levels of water to cement mass ratios. A cause and effect relation was established between sand particle shape, in the presence of a dispersant, and properties of mortars such as the wet packing density, the workability in the form of slump-flow, and the water demand. A relationship between plastic density, after the peak of packing, the volumetric water to cement mass ratio (vw/c), and workability, as measured by slump-flow, was developed.

Fly ash which encompasses a mixture of glassy and crystalline aluminosilicates is an abundant supplementary cementitious material (SCM), valuable for replacing ordinary portland cement (OPC) in the binder fraction in concrete. Because higher OPC replacement levels are desired, it is critically important to better understand and quantify fly ash reactivity. By combining molecular dynamics (MD) simulations and vertical scanning interferometry (VSI), this study establishes that the reactivity of the glassy fractions in a fly ash with water (i.e., their aqueous dissolution rate) is controlled by the number of constraints placed on atoms within the disordered aluminosilicate network. More precisely, an Arrhenius-like dependence of dissolution rates on the atomic network topology is observed. Such topological controls on fly ash reactivity are highlighted for a range of U.S. commercial fly ashes spanning CaO-enriched and SiO2-enriched compositions. The structure-property relationships reported herein establish an improved framework to control and estimate fly ash-cement interactions in concrete. This article is protected by copyright. All rights reserved.

Digital holographic microscopy in reflection mode is used to track in situ, real-time nanoscale topography evolution of cleaved (104) calcite surfaces exposed to flowing or static deionized water. The method captures full-field holograms of the surface at frame rates of up to 12.5 s-1. Numerical reconstruction provides 3D surface topography with vertical resolution of a few nanometers and enables measurement of time-dependent local dissolution fluxes. A statistical distribution, or spectrum, of dissolution rates is generated by sampling multiple area domains on multiple crystals. The data show, as has been demonstrated by Fischer et al. (2012), that dissolution is most fully described by a rate spectrum, although the modal dissolution rate agrees well with published mean dissolution rates (e.g., 0.1 µmol m-2 s-1 to 0.3 µmol m-2 s-1). Rhombohedral etch pits and other morphological features resulting from rapid local dissolution appear at different times and are heterogeneously distributed across the surface and through the depth. This makes the distribution in rates measured on a single crystal dependent both on the sample observation field size and on time, even at nominally constant undersaturation. Statistical analysis of the inherent noise in the DHM measurements indicates that the technique is robust and that it likely can be applied to quantify and interpret rate spectra for the dissolution or growth of other minerals.

The partial replacement of ordinary portland cement (OPC) by fine mineral fillers accelerates the rate of hydration reactions. This acceleration, known as the filler effect, has been attributed to enhanced heterogeneous nucleation of C‐S‐H on the extra surface provided by fillers. This study isolates the cause of the filler effect by examining how the composition and replacement levels of two filler agents influence the hydration of tricalcium silicate (T1‐Ca3SiO5; C3S), a polymorph of the major phase in ordinary portland cement (OPC). For a unit increase in surface area of the filler, C3S reaction rates increase far less than expected. This is because the agglomeration of fine filler particles can render up to 65% of their surface area unavailable for C‐S‐H nucleation. By analysis of mixtures with equal surface areas, it is hypothesized that limestone is a superior filler as compared to quartz due to the sorption of its aqueous CO32− ions by the C‐S‐H—which in turn releases OH− species to increase the driving force for C‐S‐H growth. This hypothesis is supported by kinetic data of C3S hydration occurring in the presence of CO32− and SO42− ions provisioned by readily soluble salts. Contrary to prior investigations, these results suggest that differences in heterogeneous nucleation of the C‐S‐H on filler particle surfaces, caused due to differences in their interfacial properties, have little if any effect on C3S hydration kinetics.

Fly ash is a critical material for partial replacement of ordinary portland cement (OPC) in the binder fraction of a concrete mixture. However, significant compositional variability currently limits fly ash use. For example, the performance of OPC-fly ash blends cannot be estimated a priori using current characterization standards (e.g., ASTM C618). In this study, fly ashes spanning a wide compositional range are characterized in terms of glassy and crystalline phases using a combination of X-ray fluorescence (XRF), X-ray diffraction (XRD), and scanning electron microscopy with X-ray energy dispersive spectroscopy (SEM-EDS) techniques. The compositional data are distilled to a unitless parameter, the network ratio (Nr), which represents the network behavior of atoms that form alkali/alkaline earth-aluminosilicate glasses that make up fly ashes. Nr is correlated to known composition-dependent features, including the glass transition temperature and amorphous XRD peak (“hump”) position. Analysis of heat release data and compressive strengths are used to evaluate the impact of fly ash compositions on reaction kinetics and on the engineering properties of cement-fly ash blends. It is shown that fly ashes hosting glasses with a high network ratio (i.e., having a less stable glass structure) are more reactive than others. This article is protected by copyright. All rights reserved.

Recent topography measurements of gypsum dissolution have not reported the absolute dissolution rates, but instead focus on the rates of formation and growth of etch pits. In this study, the in situ absolute retreat rates of gypsum (010) cleavage surfaces at etch pits, at cleavage steps, and at apparently defect-free portions of the surface are measured in flowing water by reflection digital holographic microscopy. Observations made on randomly sampled fields of view on seven different cleavage surfaces reveal a range of local dissolution rates, the local rate being determined by the topographical features at which material is removed. Four characteristic types of topographical activity are observed: 1) smooth regions, free of etch pits or other noticeable defects, where dissolution rates are relatively low; 2) shallow, wide etch pits bounded by faceted walls which grow gradually at rates somewhat greater than in smooth regions; 3) narrow, deep etch pits which form and grow throughout the observation period at rates that exceed those at the shallow etch pits; and 4) relatively few, submicrometer cleavage steps which move in a wave-like manner and yield local dissolution fluxes that are about five times greater than at etch pits. Molar dissolution rates at all topographical features except submicrometer steps can be aggregated into a continuous, mildly bimodal distribution with a mean of 3.0 μmol m −2 s −1 and a standard deviation of 0.7 μmol m −2 s −1.

To compute any physical quantity for a random particle, one needs to know the mathematical shape of the particle. For regular particles like spheres and ellipsoids, the mathematics are straightforward. For random particles, with realistic shapes, mathematically characterizing the shape had not been generally done. But since about the year 2002, a method has been developed that combines X-ray computed tomography and spherical harmonic analysis to give analytical, differentiable mathematical functions for the three-dimensional shape of star-shape particles, which are a wide class of particles covering most industrial particles of interest, ranging from micrometer scale to millimeter scale particles. This review article describes how this is done, in some detail, and then gives examples of applications of this method, including a contact function that is suitable for these random shape particles. The purpose of this article is to make these ideas widely available for the general powder researcher who knows that particle shape is important to his/her applications, and especially for those researchers who are just starting out in their particle science and technology careers.

The dissolution of silicate minerals and glasses in aqueous solutions is important in many natural and engineered contexts including mineral weathering, nuclear waste stabilization, cementation, and infrastructure degradation. The influences of electrolytes on dissolution rates have been extensively studied, but previous studies have used widely varying minerals and electrolytes, experimental conditions, and measurement techniques. Comparatively fewer studies have been conducted in hyperalkaline solutions that are encountered in concrete, geopolymers, and nuclear stabilization systems. This study seeks to control many of these variables to isolate the effects of electrolyte composition on altering the degree of dissolution of a soda lime silicate glass in hyperalkaline electrolyte solutions. A glass powder is dissolved in one of a series of static solutions having 10 mmol L–1 NaOH (pH 12) at (25 ± 0.2) °C and either an organic or an inorganic electrolyte at a concentration of 1 or 10 mmol L–1. The solution series was designed to reveal qualitatively how ion identity and concentration alter the glass’ degree of dissolution after prescribed exposure times. The results indicate that a relative dissolution enhancement of no greater than about 2.4 times can be induced at pH 12, with the greatest enhancements being observed for Na-benzoate, Na-citrate, and Na-malonate. The degree of dissolution is unaffected by most of the other salts examined. Broadly, the nucleophilic attack by OH– on Si–O bonds and the formation of O––Na+ surface complexes appear to be the most important factors influencing the dissolution rate at high pH. The adsorption and the influence on dissolution of other electrolyte ions are comparatively weak.

The measured dissolution rate of a granular medium depends on its surface area and how the surface area changes during the course of the measurement. Moreover, the assumption that the specific surface area either remains constant or initially increases during dissolution is not always valid. This paper demonstrates that when the particle size distribution has sufficient variance, the instantaneous change in surface area during dissolution can be negative, even before the smallest particles dissolve away. The concept is explained using spherical particles, extended for use with prismatic particles, and demonstrated experimentally with gypsum powder. For the commercial gypsum powder used, the specific surface area decreases by about 50% during the first 10% of mass loss in water, so this effect may have practical importance and have a significant impact on the uncertainty in reported dissolution rates measured with batch reactors.

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

Tricalcium silicate does not undergo hydration at relative humidities (RH's) below 80%. But, the rate at which its hydration rate decreases as a function of the RH has not yet been elucidated. By invoking correspondence between RH and water activity (aH, unitless), both of which are related to the chemical potential of water, the reaction evolutions of triclinic tricalcium silicate (i.e., T1‐Ca3SiO5 or C3S) are tracked in water + isopropanol (IPA) mixtures, prepared across a wide range of water activities. Emphasis is placed on quantifying the: (a) rate of hydration as a function of aH, and (b) the critical (initial, aH0c or the achieved) water activity at which hydration effectively ceases, i.e., does not progress; here identified to be ≈ 0.70. The hydration of tricalcium silicate is arrested even when the system remains near saturated with a liquid phase, such that small, if any, capillary stresses develop. This suggests that changes in chemical potential induced via a vapor‐phase or liquid‐phase route both induce similar suppressions of C3S hydration. A phase boundary nucleation and growth (pBNG) model is fit to measured hydration rates from the onset of the acceleration period until well beyond the rate maximum when the water activity is altered. The simulations suggest that for a fixed hydrate nucleation density, any water activity reductions consistently suppress the growth of hydration products. Thermodynamic considerations of how water activity changes may influence reactions/hydrate evolutions are discussed. The outcomes improve our understanding of the chemical factors that influence the rate of Ca3SiO5 hydration.

The great reactivity of tricalcium aluminate (C3A) with water and subsequent rapid precipitation of insoluble products are serious challenges to learning how salts affect its interfacial chemistry and physics during dissolution. Those challenges are addressed here with rapid in situ surface nanotopography monitoring and characterization of hydration products as C3A dissolves in aqueous sulfate solutions (Na2SO4, MgSO4, and CaSO4) with varying pH. These measurements shed light on how pure dissolution and early hydration of C3A are influenced by sulfates. Bulk dissolution experiments confirm a cation-specific effect of sulfates on the inhibition of C3A dissolution and of etch pit evolution. Soon after the initial contact with sulfate solutions, precipitation of hydration products occurred and the morphology and composition of products were strongly affected by the types of sulfates.

In the present paper, the design of optimized rubber concrete composite containing silica fume (SF) and zeolite (ZE) was undertaken using the literature, and the properties were assessed through destructive and non-destructive (NDT) methods. In order to optimize the rubberized cement composite, the optimum tradeoff between compressive strength as the main objective and rubber content, as well as the optimum fractions of the admixtures were taken into account. Main tests including workability, compressive strength, elastic modulus, and ultrasonic tests were carried out to fully assess the effects of rubber, ZE, SF, curing, and age on the rubberized composite behavior. Primary and secondary wave velocities, i.e. Vp and Vs were determined from ultrasonic test to characterize different mixtures. Static modulus results obtained from NDT were compared, and it was found that NDT results were in very good agreement with those of destructive test results. Moreover, the dynamic elastic modulus determined from compression and shear wave velocities (Vp, Vs) conforming to ASTM were compared with those estimated from six different relationships including BS, EN and ACI relationships along with other well-known equations available in the literature. In order to predict the compressive strength of the rubberized cement composite as a function of the influencing variables, a comprehensive comparative modeling was performed and different predictive models were developed using regressions and machine-learning (ML) techniques, i.e. nonlinear multi-variable regression (NMVR), Artificial neural network (ANN), genetic programming (GP), adaptive neuro-fuzzy inference system (ANFIS), and support-vector machine (SVM). Closed- form formulations were derived for NMVR, ANN, and GP models, and parametric study was conducted for ML models. Performance criteria such as root mean squared error (RMSE), mean absolute percentage error (MAPE), and coefficient of determination (R2) were used to compare the models’ performance. It was found that SVM outperformed the other models with the highest R2 and the lowest RMSE equal to 0.989 and 1.393, respectively.

Historically, sparingly soluble mineral dissolution in aqueous solutions is measured at macroscopic scales, mostly by indirect methods that track the solution composition over times ranging from minutes to years. Relatively fewer measurements have been made of dissolution rates of highly soluble minerals and their variability at microscopic length scales and shorter times prior to the products precipitation. Here, we report in-situ, real-time measurements of the initial absolute dissolution rate of an extremely reactive mineral, cubic tricalcium aluminate (C3A), in pure water, by monitoring its surface retreat with nanometer resolution. The data reveal that C3A has an extremely fast dissolution rate within the instantaneous contact with water, in the range of (0.5 to 2) mmol·m−2·s−1 and several orders of magnitude greater than that of gypsum or calcite. Soon after that initial dissolution, nucleation and rapid growth of calcium aluminate hydrate would occur, leading to a dramatic decrease of the C3A dissolution rate by blocking its access to the solution. To our knowledge, this is the first time the real dissolution rate of C3A is measured in water, which is urgently needed for the fundamental modelling of the reaction mechanisms, kinetics, and microstructure development during cement hydration.

Solution flow profiles near a mineral surface can exert significant influences on the local thermodynamic driving force and, potentially, the rate-controlling kinetic mechanism of its dissolution or precipitation. These influences are investigated here both by in situ digital holographic microscopy of calcite in flowing pure water and by lattice Boltzmann reaction-transport simulations using close to the same macroscopic flow rates and sample geometry as the experiments. The measurements show that the median dissolution rate along the crystal surface decreases significantly with distance along the flow direction. At lower flow rates localized advancement of the surface, rather than retreat, is sometimes observed near the trailing edge of a macroscopic crystal, which suggests some precipitation even though pure water is injected near the crystal's leading edge. The simulations confirm that the lower rates are caused by an accumulation of dissolved species as the solution flows from the leading to the trailing edge. The experiments and simulations both indicate that the effect is lessened as the flow rate increases but never becomes negligible within the range of flow rates examined. This study implies that dissolution rates are significantly influenced by near-surface conditions even under relatively high flow rates where a surface process, not diffusion, controls the rate.

Influences of alkali or alkaline earth sulfates on the hydration of cubic tricalcium aluminate (C3A) were evaluated by in situ dissolution rate measurements, by ex situ near-surface composition measurements with secondary ion mass spectrometry, and by in situ synchrotron X-ray diffraction to monitor precipitation of hydration products. Both slight reductions in dissolution rate and cation-specific interactions with the solid were observed. The near-surface Ca/Al ratio is significantly lower after some dissolution and the electrolyte cations are incorporated within the surface with different affinities (Mg2+ > K+ > Na+). An interfacial dissolution-reprecipitation mechanism may explain the observations as well as, or better than, a simple cation exchange. The sulfate concentration in solution affects the rates of both C3A dissolution and precipitation of hydration products. Sulfate ions likely adsorb at the hydrous Al-rich surface layer, thereby reducing the dissolution rate of aluminates and delaying the precipitation of aluminate hydration products.

The reaction kinetics of gypsum dissolution are important for establishing an optimum gypsum content in portland cement. In this paper, the temperature dependence of dissolution rates was measured by tracking changes in solution composition and surface area with time. The rate coefficient increases with temperature and the specific surface area decreases with ongoing dissolution. The dissolution rate depends on the solution's saturation index to an exponent of approximately 1.5. The apparent activation enthalpy is approximately 34 kJ mol−1, independent of saturation index and surface area, suggesting that dissolution is controlled by surface reaction rather than mass transport. The activation enthalpy is interpreted as the energy required to cooperatively break all the ionic bonds at occupied kink sites of the gypsum-water interface. Activation enthalpies similar in magnitude have been reported for other calcium salts, which may indicate that all of these processes involve breaking ionic calcium-oxygen bonds of similar strengths.

Advances in nano-computed X-ray tomography (nCT), nano X-ray fluorescence spectrometry (nXRF), and high-performance computing have enabled the first direct comparison between observations of three-dimensional nanoscale microstructure evolution during cement hydration and computer simulations of the same microstructure using HydratiCA. nCT observations of a collection of triclinic tricalcium silicate (Ca(3)SiO(5)) particles reacting in a calcium hydroxide solution are reported and compared to simulations that duplicate, as nearly as possible, the thermal and chemical conditions of those experiments. Particular points of comparison are the time dependence of the solid phase volume fractions, spatial distributions, and morphologies. Comparisons made at 7 h of reaction indicate that the simulated and observed volumes of Ca(3)SiO(5) consumed by hydration agree to within the measurement uncertainty. The location of simulated hydration product is qualitatively consistent with the observations, but the outer envelope of hydration product observed by nCT encloses more than twice the volume of hydration product in the simulations at the same time. Simultaneous nXRF measurements of the same observation volume imply calcium and silicon concentrations within the observed hydration product envelope that are consistent with Ca(OH)(2) embedded in a sparse network of calcium silicate hydrate (C−S−H) that contains about 70 % occluded porosity in addition to the amount usually accounted as gel porosity. An anomalously large volume of Ca(OH)(2) near the particles is observed both in the experiments and in the simulations, and can be explained as originating from the hydration of additional particles outside the field of view. Possible origins of the unusually large amount of observed occluded porosity are discussed.

The viscoelastic/viscoplastic behavior of cement paste may occur due to intrinsic calcium silicate hydrate (C-S-H) viscoelasticity/viscoplasticity and cement grain dissolution during the hydration process. A numerical model that combines a microstructure model and a finite element calculation model has been developed to predict the time-dependent behavior of cementitious materials based on these two mechanisms, while incorporating C-S-H intrinsic aging. The simulation results from the model suggest that when considering C-S-H aging, the time-dependent properties of C-S-H are capable of generating the aging effect of cement paste, and can become a significant mechanism leading to the overall relaxation of cement paste.

A model for an ideal solid solution, developed by Nourtier-Mazauric et al. [Oil & Gas Sci. Tech. Rev. IFP, 60 [2] (2005) 401], is applied to calcium-silicate-hydrate (C-S-H). Fitting the model to solubility data reported in the literature for C-S-H yields reasonable values for the compositions of the end-members of the solid solution and for their equilibrium constants. This model will be useful in models of hydration kinetics of tricalcium silicate because it is easier to implement than other solid solution models, it clearly identifies the driving force for growth of the most favorable C-S-H composition, and it still allows the model to accurately capture variations in C-S-H composition as the aqueous solution changes significantly at early hydration times.

The stabilities of Al2O3-Fe2O3-mono (AFm) and -tri (AFt) phases in the Ca-Al-S-O-H system at 25 °C are examined using Gibbs energy minimization as implemented by GEM-Selektor software coupled with the Nagra/PSI thermodynamic database. Equilibrium phase diagrams are constructed and compared to those reported in previous studies. The sensitivity of the calculations to the assumed solid solubility products, highlighted by the example of hydrogarnet, is likely the reason why some studies, including this one, predict a stable SO4-rich AFm phase while others do not. The majority of the effort is given to calculating the influences on AFm and AFt stability of alkali and carbonate components, both of which are typically present in cementitious binders. Higher alkali content shifts the equilibria of both AFt and AFm to lower Ca but higher Al and S concentrations in solution. More importantly, higher alkali content significantly expands the range of solution compositions in equilibrium with AFm relative to AFt phases. The introduction of carbonates alters not only the stable AFm solid solution compositions, as expected, but also influences the range of solution pH over which SO4-rich and OH-rich AFm phases are dominant. Some experimental tests are suggested that could provide validation of these calculations, which are all the more important because of the implications for resistance of portland cement binders to external sulfate attack.

Simulations of tricalcium silicate (C 3 S) hydration using a kinetic cellular automaton program, HydratiCA, indicate that the net rate depends both on C 3 S dissolution and on hydration product growth. Neither process can be considered the sole rate-controlling step because the solution remains significantly undersaturated with respect to C 3 S yet significantly supersaturated with respect to calcium silicate hydrate (C–S–H). The reaction rate peak is attributed to increasing coverage of C 3 S by C–S–H, which reduces both the dissolution rate and the supersaturation of C–S–H. This supersaturation dependence is included in a generalized boundary nucleation and growth model to describe the kinetics without requiring significant impingement of products on separate cement grains. The latter point explains the observation that paste hydration rates are insensitive to water/cement ratio. The simulations indicate that the product layer on C 3 S remains permeable; no transition to diffusion control is indicated, even long after the rate peak.

Ordinary Portland cement (OPC) prehydrates during storage or handling in moist environments, forming hydration products on or near its particles' surfaces. Prehydration is known to reduce OPC reactivity, but the extent of prehydration has not yet been quantitatively linked to reaction rate and mechanical property changes. A series of experiments are performed to develop a better understanding of prehydration, by intentionally exposing an OPC powder to either water vapor or liquid water, to investigate the extent to which premature contact of OPC with water and other potential reactants in the liquid and/or vapor state(s) can induce differing surface modifications on the OPC grains. Original results obtained using isothermal calorimetry, thermogravimetric analysis and strength measurements are correlated to a prehydration index, which is defined for the first time. The addition of fine limestone to a mixture formed using prehydrated cement is shown to mitigate the detrimental effects of prehydration.

Finite element procedures combined with microstructure development modeling are integrated to quantitatively predict the viscoelastic/viscoplastic relaxation of cement paste due to intrinsic calcium silicate hydrate viscoelasticity and microstructure evolution associated with the hydration process. The combined models are implemented in a computational routine to predict time-dependent stress and strain fields in cement paste. The model simulations suggest that inherent viscoelastic deformation caused by calcium silicate hydrate is not necessarily the primary mechanism leading to the overall early-age viscoelastic/viscoplastic behavior of cement paste. The effect of time-dependent dissolution of cement grains occurring during the hydration process is substantial and should be considered as a significant mechanism for the apparent viscoelasticity/viscoplasticity of cement paste.

A new model of 3D cement paste microstructure development is described and used to simulate the influence of leaching on hydrated cement pastes. In contrast to recent leaching models that have used empirical rules for phase dissolution, this model uses continual thermodynamic speciation and phase stability calculations to guide the microstructural changes that happen throughout hydration and subsequent exposure to low-pH solutions. This novel aspect of the model enables it to predict not only the well-known phase instability of calcium hydroxide at the onset of leaching, but also the detailed compositional and volumetric changes of C–S–H gel and other calcium, aluminate, and sulfate phases. Besides tracking the compositional and microstructural changes, we use the evolving microstructure as input to calculate changes in the relative diffusivity and effective Young’s modulus of the binder using established finite difference and finite element models. The results are broadly consistent with previous experimental and modeling investigations of leaching. In particular, the leaching process can be roughly divided into initial, intermediate, and final stages, each of which has distinct degradation characteristics and consequences for mechanical and transport properties. The thermodynamic basis of the model makes it readily extensible to simulate a wide range of cementitious materials and degradation phenomena, so we discuss its potential as a virtual microprobe for use with continuum-scale service life models of concrete elements.

Truncated spherical harmonic expansions are used to approximate the shape of 3D star-shaped particles including a wide range of axially symmetric ellipsoids, cuboids, and over 40.000 real particles drawn from seven different material sources. This mathematical procedure enables any geometric property to be calculated for these star-shaped particles. Calculations are made of properties such as volume, surface area, triaxial dimensions, the maximum inscribed sphere, and the minimum enclosing sphere, as well as differential geometric properties such as surface normals and principal curvatures, and the values are compared to the analytical values for well-characterized geometric shapes. We find that a particle's Krumbein triaxial dimensions, widely used in the sedimentary geology literature, are essentially identical numerically to the length, width, and thickness dimensions that are used to characterize gravel shape in the construction aggregate industry. Of these dimensions, we prove that the length is a lower bound on a particle's minimum enclosing sphere diameter and that the thickness is an upper bound on its maximum inscribed sphere diameter. We examine the “true sphericity” and the shape entropy, and we also introduce a new sphericity factor based on the radius ratio of the maximum inscribed sphere to the minimum enclosing sphere. This bounding sphere ratio, which can be calculated numerically or approximated from macroscopic dimensions, has the advantage that it is less sensitive to surface roughness than the true sphericity. For roundness, we extend Wadell's classical 2D definition for particle silhouettes to 3D shapes and we also introduce a new roundness factor based on integrating the dot product of the surface position unit vector and the unit normal vector. Limited evidence suggests that the latter roundness factor more faithfully captures the common notion of roundness based on visual perception of particle shapes, and it is significantly simpler to calculate than the classical roundness factor.

Portland cement is a fundamental structural and binding material for industry and society. Its structural and physical properties at different scales show a complexity that can presently be barely managed through experimental and computational methods. State of the art problems and trends in cement studies will be briefly assessed from the point of view of the processes at the molecular scale.

Electrical properties are frequently measured in the concrete construction industry as a part of mixture qualification and quality-control testing. Whereas there are several factors that influence the electrical response of concrete, one of the most important factors is its degree of saturation. Although current standard tests rely on the concrete’s being saturated, this can be difficult to accomplish, is time-consuming, and can artificially increase the degree of hydration of the test sample in comparison with that of concrete in field structures (when the test samples are stored in water). Although some studies have measured the electrical response of concrete for samples with different moisture content (i.e., stored at different relative humidities), a single expression has not been proposed that predicts how drying changes the electrical response. This paper suggests that a saturation function should be considered as a possible method to account and to correct for less than complete saturation...

Finely ground mineral powders are known to accelerate cement hydration rates. This “filler effect” has been attributed to the effects of dilution (w/c increase) when the cement content is reduced or to the provision of additional surface area by fine powders. The latter contribution (i.e., surface area increase) is speculated to provide additional sites for the nucleation of the hydration products, which accelerates reactions. Through extensive experimentation and simulation this study describes the influence of surface area and mineral type (e.g., quartz or limestone) on cement reaction rates. Simulations using a boundary nucleation and growth (BNG) model and a multiphase reaction ensemble (MRE) indicate that the extent of the acceleration is linked to the: (1) magnitude of surface area increase and (2a) capacity of the filler's surface to offer favorable nucleation sites for hydration products. Other simulations using a kinetic cellular automaton model (HydratiCA) suggest that accelerations are linked to: (2b) the interfacial properties of the filler that alters (increases or decreases) its tendency to serve as a nucleant, and (3) the chemical composition of the filler and the tendency for its dissociated ions to participate in exchange reactions with the calcium silicate hydrate product. The simulations are correlated with accelerations observed using isothermal calorimetry when fillers partially replace cement. The research correlates and unifies the fundamental parameters that drive the filler effect and provides a mechanistic understanding of the influence of filler agents on cementitious reaction rates.

Using a spherical harmonic series, the three-dimensional shape of star-shaped particles (e.g. most powders, sand, gravel) can be represented mathematically as readily as can a sphere, cube, or ellipsoid. In principle, any particle parameter, such as volume, surface area, moment of inertia tensor, or integrated mean curvature, can be easily computed. In this paper, we extend this list by developing three important algorithms, which have been found useful for regular particles, and adapt them to the case of random star-shaped particles using spherical harmonic series. These three algorithms are: a two-particle contact function, how to add a uniform-thickness shell to a single particle, and estimation of the convexity of a single particle using an algorithm for computing the convex hulls of non-convex particles. A derivation and numerical examples are shown for each algorithm. The two-particle contact function may be used in random particle placement programs, so that random-shaped particles may be used in suspension models as readily as spheres or ellipsoids. New results are shown for how the volume of a uniform-thickness shell, normalized by the surface area of the original particle, depends on the shell thickness t. For a sphere, the exact expression is cubic in t, with the linear coefficient unity and the quadratic coefficient equal to the reciprocal of the sphere radius. For star-shaped particles, the coefficient of the linear term is also unity. For all star-shaped particles considered, the coefficient of the quadratic term in t is approximately equal to the reciprocal of the radius of the volume-equivalent sphere, to within a few percent. The convexity measure, using convex hulls, adds to the list of useful shape parameters for random star-shaped particles and is shown to distinguish between different particle sets.

The electrical resistivity of cement-based materials can be used in quality control or service life prediction as an indicator of the fluid transport properties of these materials. Although electrical tests have the advantage of being easy and rapid to perform, several key factors can influence the results: (a) specimen geometry, (b) specimen temperature, and (c) sample storage and conditioning. This paper addresses these issues and compares the measurements from several commercially available testing devices. First, the role of sample geometry is explained with the use of three common geometries: surface, uniaxial, and embedded electrodes. If the geometry is properly accounted for, measurements from different test geometries result in electrical resistivity values that are similar. Second, the role of sample temperature is discussed for both pore solution and uniaxial tests on cylinders. Third, the paper examines the importance of sample curing, storage, and conditioning. Sample storage and conditioning influence both the degree of hydration and the degree of saturation. The role of sample volume to solution volume is discussed, as this ratio may influence alkali leaching and pore solution conduction. This paper is intended to identify factors that influence the results of rapid electrical test measurements and to help identify areas of future research that are needed so that robust specifications and standard test methods can be developed. Standardization will enable electrical tests to provide rapid, accurate, repeatable measurements of concrete's electrical properties.

When cementitious materials are dried, internal stresses are generated that lead to desiccation shrinkage, a portion of which is irreversible. Previous research has indicated that, while a cementitious composite is subjected to a state of stress, dissolution of cement grains and precipitation of hydrates can yield irreversible creep strains, and it is hypothesized that the same process can lead to irreversible shrinkage during drying. To evaluate this hypothesis, a computationally implemented model integrating a microstructural evolution model with a finite element calculation routine was utilized. This computationally implemented model is capable of predicting the magnitude of shrinkage deformation of cement paste during drying conditions as a result of cement grain dissolution and hydrate precipitation. From the simulation results, the mechanism of cement grain dissolution and hydrate precipitation can lead to significant shrinkage behavior of cement paste, and it is also a potential mechanism resulting in the irreversible component of desiccation shrinkage at early ages (e.g., while the hydration rate is significant). The predicted irreversible shrinkage decreases with the age at which drying is initiated as a result of the decreasing hydration reaction rate.

X-ray powder diffraction (XRD) has been used for several decades to identify and measure the mass fractions of various crystalline phases in portland cement. More recently, a combination of scanning electron microscopy with X-ray microanalysis (SEM/XMA) and image processing has been shown to enable the quantitative characterization of microstructural features in these materials. Each technique can furnish some information that is not accessible from the other. For example, SEM/XMA can identify the microstructural location and morphology of calcium sulfate minerals, while only XRD can determine the relative abundance of the different forms of calcium sulfate, such as gypsum (CaSO(4) · 2H(2)O), bassanite [Formula: see text] , and anhydrite (CaSO(4)). This document describes how XRD and SEM/XMA can be used together to establish and validate the portland cement phase composition and microstructure. Particular emphasis is laid on step-by-step procedures and best practices for XRD specimen preparation, data collection, and intepretation. Similar detail has been given recently for SEM/XMA [Stutzman et al., NIST Tech Note 1877, U.S. Department of Commerce, April 2015]. The methods are demonstrated for three portland cement powders, through which apparent discrepancies between the results of the two methods are identified and procedures are described for resolving the discrepancies and quantifying uncertainty.

The current state of knowledge of cement hydration mechanisms is reviewed, including the origin of the period of slow reaction in alite and cement, the nature of the acceleration period, the role of calcium sulfate in modifying the reaction rate of tricalcium aluminate, the interactions of silicates and aluminates, and the kinetics of the deceleration period. In addition, several remaining controversies or gaps in understanding are identified, such as the nature and influence on kinetics of an early surface hydrate, the mechanistic origin of the beginning of the acceleration period, the manner in which microscopic growth processes lead to the characteristic morphologies of hydration products at larger length scales, and the role played by diffusion in the deceleration period. The review concludes with some perspectives on research needs for the future.