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Non-hydraulic calcium silicate-based cement (CSC) is a newly developed binder that hardens upon carbonation reaction. The primary components of CSC are low-lime calcium silicates, including rankinite, wollastonite and pseudo-wollastonite. During the carbonation, CSC binder forms Ca-modified silica gel and CaCO 3 . Microscopic evaluation of the carbonated CSC paste using the 29 Si { 1 H} CP-MAS NMR revealed that the Ca-modified silica gel phase consists primarily of Q 3 and Q 4 species, indicating a substantially higher degree of polymerization in comparison to that of the C-S-H. The elastic modulus and hardness of this gel phase were found to be close to those of the high density C-S-H. A composite phase, formed by the intermixing of Ca-modified silica gel and CaCO 3 , was also identified using SEM X-ray microanalysis and nanoindentations. The distribution of the microscopic phases in ITZ was similar to that observed in the bulk paste region for this system. [ABSTRACT FROM AUTHOR]

This paper presents the results of the investigation on the effects of Li + ions on the chemical and physical changes in the cementitious system undergoing alkali-silica reaction (ASR). Specifically, this paper focuses on determining which chemical steps of ASR processes are affected by the presence of Li + ions in the pore solution in order to provide better understanding of the role of Li + ions in the mitigation process of ASR. The results presented in this paper strongly support the hypothesis that the Li + ions facilitate the formation of a physical barrier on the surface of reactive silica, and thus prevent further attack on the reactive sites by hydroxyl ions. [ABSTRACT FROM AUTHOR]

This paper presents the results of the investigations on the chemistry of pore solutions, the contents of calcium hydroxide, and the expansions in mortars containing both reactive and non-reactive aggregates. In order to examine the effect of the temperature, experiments were performed at three different temperatures (23 °C, 38 °C and 55 °C). The compositions of the pore solution were measured at short time intervals for a period of up to 130 days in order to capture the kinetics of the chemistry of pore solution. The results showed that the changes in the concentrations of alkali ions can be best explained by the first order reaction. In addition, the proposed rate equation could reasonably simulate the changes in the actual concentrations of alkalis. Finally, the results in this paper suggest that the rate of the alkali–silica reaction in cementitious system containing highly reactive aggregate can be also expressed as the first order reaction. [ABSTRACT FROM AUTHOR]

Addition of lithium nitrate admixture to the fresh concrete mixture helps to minimize potential problems related to alkali-silica reaction. For this admixture to function as an effective ASR control measure, it is imperative that the lithium ions remain in the pore solution. However, it was found that about 50% of the originally added lithium ions are removed from the pore solution during early stages of hydration. This paper revealed that the magnitude of the Li + ion loss is highly dependent on the concentration of Li + ions in the pore solution and the hydration rate of the cementitious systems. Using these findings, an empirical model has been developed which can predict the loss of Li + ions from the pore solution during the hydration period. The proposed model can be used to investigate the effects of mixture parameters on the loss of Li + ions from the pore solution of cementitious system. [ABSTRACT FROM AUTHOR]

Abstract: Conventionally-designed concretes were prepared with different sand particle size distributions, so as to systematically vary the extent of aggregate-cement paste interface and the mean spacing between sand grains. The range of fineness modulus of the sands fully encompassed the range of sands normally used in concretes. The concretes were batched at w/c ratios of 0.30 and 0.50 and cured for various periods before carrying out determinations of mechanical properties and of “rapid chloride permeability”. The conventional notions of the effect of the ITZ on concrete properties would predict that a reduction in strength and an increase in chloride permeability would accompany increased ITZ interfacial area and closer spacing between sand grains. In general, no such influence was found. It appears from this research that the traditional notions of the adverse influence of the ITZ on the properties of conventional concretes may not be accurate, within the realms of conventional concrete and typical inter-aggregate spacings. [Copyright &y& Elsevier]

Abstract: A study of the physicochemical interaction of a high free lime (CaO) content cement kiln dust (CKD) with expansive Na-montmorillonite clay is presented. Moist compacted specimens of the CKD-treated clay, the clay alone, the CKD alone, and (for comparison) the clay treated with 7% CaO were each cured for periods up to 90 days and examined by XRD, TGA, and SEM techniques. The results for the CKD-treated clay indicated that calcium hydroxide, derived from the CaO present in the CKD, was extensively adsorbed on the surfaces of the clay flakes, but apparently only limited pozzolanic reaction occurred. Gypsum was rapidly produced from sulfate-bearing components in the CKD, and subsequently ettringite was produced as well, some of the latter apparently incorporating aluminum derived from the clay. Much of the clay was left unreacted, but the morphology of the clay particle assemblage was significantly modified in response to the CKD treatment. Similar morphological changes were also induced by the CaO treatment, suggesting that similar underlying mechanisms were also active here. [Copyright &y& Elsevier]

Abstract: The interaction of CKDs with a given soil depends on the chemical and physical characteristics of the CKDs. Hence, the characterization of CKDs and their hydration products may lead to better understanding of their suitability as soil stabilizers. In the present article, four different CKD powders are characterized and their hydration products are evaluated. A detailed chemical (X-ray diffraction), thermogravimetric and morphological (scanning electron microscope) analyses of both the CKD powders and the hydrated CKD pastes are presented. In general, high free-lime content (~14–29%) CKDs, when reacted with water produced significant amounts of calcium hydroxide, ettringite and syngenite. These CKDs also developed higher unconfined compressive strength and higher temperature of hydration compared to CKDs with lower amounts of free-lime. An attempt was made to qualitatively correlate the performance of CKD pastes with the chemical and physical characteristics of the original CKD powders and to determine their potential suitability as soil stabilizers. To that effect a limited unconfined compressive strength testing of CKD-treated kaolinite clays was performed. The results of this study suggest that both the compressive strength and the temperature of hydration of the CKD paste can give early indications of the suitability of particular CKD for soil stabilization. [Copyright &y& Elsevier]

Expansions of mortar bars, stored over (but not in) water after simulated steam curing to 85 °C, were related to certain cement compositional parameters. The relationship is expressed in the form of a “delayed ettringite formation (DEF) index.” The DEF index is computed as the joint product of the SO3/Al2O3 molar ratio of the cement, the sum of its SO3 and Bogue C3A percentages divided by 10 and the square root of the alkali content expressed as equivalent % Na2O. The mortars studied were made with 18 different cements, prepared from a set of six representative clinkers by incorporating Terra Alba gypsum to total SO3 contents that were 1% below optimum, at optimum and 1% above optimum (as defined in ASTM C 563). Measurements of expansion were recorded at intervals for up to 1400 days. Severe cracking and prominent DEF-induced expansions were observed in mortar bars derived from four of the six ‘oversulfated’ cements and lesser expansions from three of the six cements prepared at optimum SO3 contents. No expansion was found for cements of DEF index below a threshold value; above this value expansions were approximately proportional to the difference between DEF index and its threshold value. The relationship confirms the significance of all three compositional parameters making up the index, e.g., the SO3/Al2O3 molar ratio, the joint contents of SO3 and C3A, and the alkali content, in influencing the extent of DEF-induced expansion. In these measurements, the apparent pessimum effect for SO3 content previously reported by others was not found, although SO3 contents examined spanned the supposed pessimum value of 4%. Rather, expansion increased with increasing SO3 content for mortars made with all clinkers exhibiting expansion. [Copyright &y& Elsevier]

Mortars were prepared from laboratory cements blended from a set of six representative ground clinkers and Terra Alba gypsum. The addition of gypsum was such that cements containing 1% SO3 less than the optimum SO3 content, the optimum SO3 content, and 1% greater than the optimum SO3 content were produced. Mortar bars and mortar cubes containing each of these cements were exposed to continuous room temperature (23 °C) curing, or to early-age curing cycles involving maximum temperatures of 55 and 85 °C, followed by long-term exposure at 100% RH over water, but not immersed in water. Measurements of expansion, dynamic elastic modulus, and weight gain were recorded at intervals of up to 900 days. Severe cracking and prominent delayed ettringite formation (DEF)-induced expansions were observed in 85 °C cured mortar bars derived from four of the six “oversulfated” cements. Much smaller expansions were observed in mortar bars from two cements with optimum SO3 content cements also cured at 85 °C. No expansion or other visible indication of distress was observed for any of the 55 °C or continuously room-temperature-cured mortars. The dynamic elastic modulus increased progressively on prolonged exposure for the unaffected mortar bars, but it decreased precipitously after the onset of expansion in affected mortar bars. Significant weight increases also accompanied the processes of expansion. Mortars that showed severe cracking and deterioration when exposed as mortar bars suffered almost no visible damage when exposed as cubes. [Copyright &y& Elsevier]

Abstract: The study reported in this article deals with understanding the physical, chemical and microstructural differences in sulfate attack from seawater and groundwater. Portland cement mortars were completely immersed in solutions of seawater and groundwater. Physical properties such as length, mass, and compressive strength were monitored periodically. Thermal analysis was used to study the relative amounts of phases such as ettringite, gypsum, and calcium hydroxide, and microstructural studies were conducted by scanning electron microscopy. Portland cement mortars performed better in seawater solution compared to groundwater solution. The difference in performance could be attributed to the reduction in the quantity of the expansive attack products (gypsum and ettringite). The high Cl concentration of seawater could have played an important role by binding the C3A to form chloroaluminate compounds, such as Friedel''s salt (detected in the microstructural studies), and also by lowering the expansive potential of ettringite. Furthermore, the thicker layer of brucite forming on the specimens in seawater could have afforded better protection against ingress of the solution than in groundwater. [Copyright &y& Elsevier]

Abstract: Enhanced Porosity Concrete (EPC) is manufactured by gap grading coarse aggregates to create interconnected porosity in the system. The porosity and physical features of the pore network are characterized in this paper using Electrical Impedance Spectroscopy (EIS). Porosity alone was found to be an inaccurate indicator of the electrical conductivity of the sample. While several studies have shown that a conventional form of Archie''s law can describe porous systems, it was observed that Archie''s law did not completely describe the electrical conductivity of the EPC system. Therefore, a modified version of Archie''s law was used that incorporated the matrix conductivity, which described the system more accurately than the conventional form. The pore connectivity factor determined using EIS is found to be linearly related to the acoustic absorption of the material. Similarly, conductivity results determined from EIS were used with total porosity to compute the hydraulic connectivity factor. This factor was related to intrinsic permeability calculated from hydraulic conductivity (measured using a falling head permeameter). Based on these studies, it appears that a single electrical impedance test could provide information for the design, quality control/quality assurance, and utilization of EPC. [Copyright &y& Elsevier]

Abstract: The results of an investigation on the effect of dry and prewetted lightweight aggregates on the microstructure and durability of mortar are presented in this paper. The results are compared with those obtained for normal aggregate mortar. There appears to be only a small difference in the microstructure of the interfacial transition zone (ITZ) between dry and prewetted lightweight aggregate mortars. The porous ITZ surrounding lightweight aggregate appears to extend for about 10 and 15 μm from the aggregate surface for dry and prewetted lightweight aggregates, respectively. The ITZ for dry and prewetted lightweight aggregates seems to be surrounded by dense paste that extends from 10 to about 50 μm from the aggregate surface. This dense paste has lower porosity than that observed in the bulk paste located 50 μm and farther from aggregate surface. The normal aggregate mortar prepared with the same water/cement ratio appears to have porous ITZ that extends beyond 35 μm from the aggregate surface. The dry and prewetted lightweight aggregate mortars seem to have a lower sorptivity and electrical conductivity than does the normal aggregate mortar. Lightweight aggregate mortars also appear to have excellent resistance to sulfate attack as compared with normal aggregate mortar. [Copyright &y& Elsevier]

This paper presents the results of an investigation on the effect of water–cement ratio (w/c), aggregate size, and age on the microstructure of the interfacial transition zone (ITZ) between normal weight aggregate and the bulk cement paste. Backscattered electron images (BSE) obtained by scanning electron microscope were used to characterize the ITZ microstructure. The results suggest that the w/c plays an important role in controlling the microstructure of the ITZ and its thickness. Reducing w/c from 0.55 to 0.40 resulted in an ITZ with characteristics that are not distinguishable from those of the bulk paste as demonstrated by BSE images. Aggregate size appears to have an important influence on the ITZ characteristics. Reducing the aggregate size tends to reduce the ITZ porosity. The evolution of the ITZ microstructure relative to that of the bulk paste appears to depend on the initial content of the unhydrated cement grains (UH). The results suggest that the presence of a relatively low amount of UH in the ITZ at early age may cause the porosity of the ITZ, relative to that of the bulk paste, to increase with time. The presence of relatively large amount of UH in the ITZ at early ages may cause its porosity, relative to that of the bulk paste, to decrease with time. [Copyright &y& Elsevier]

The first paper in this two-part series [Cem. Concr. Res. 32 (2002) 915] summarized the experimental results from a comprehensive research study on sulfate attack. The current paper utilizes these results to develop models for the mechanism of attack resulting from sodium and magnesium sulfate solutions. Implications of changing the binder constituents or the experimental variables, such as concentration and temperature of the solution on the proposed mechanism, are also discussed. The potential of these mechanistic models for use in service life prediction models has also been identified.According to the proposed mechanism, the attack due to sodium sulfate solution progresses in stages. The expansion of an outer skin of the specimen leads to the formation of cracks in the interior region, which is chemically unaltered. With continued immersion, the surface skin disintegrates, and the sulfate solution is able to react with the hydration products in the cracked interior zone leading to the deposition of attack products in this zone. Now, this zone becomes the expanding zone, leading to further cracking of the interior of the mortar.In the case of magnesium sulfate solution, a layer of brucite (magnesium hydroxide) forms on the surface of the mortar specimen. The penetration of the sulfate solution then occurs by diffusion across this surface layer. As the attack progresses, the formation of attack products such as gypsum and ettringite in the paste under the surface leads to expansion and strength loss. The expansion also causes cracking in the surface brucite layer, and this leaves the mortar susceptible to direct attack by the magnesium sulfate solution. Conditions favorable for the decalcification of calcium silicate hydrate (C-S-H) are thus created, and the ultimate destruction of the mortar occurs as a result of the conversion of C-S-H to the noncementitious magnesium silicate hydrate (M-S-H). [Copyright &y& Elsevier]

Sodium sulfate attack was studied on C3S mortars, along with ASTM Type I Portland cement (PC) mortars, in an attempt to independently evaluate the effect of gypsum formation on the performance. The quantity of gypsum and ettringite, as measured by differential scanning calorimetry (DSC), increased with the time of immersion in the sulfate solution. An increase in length of the mortar specimens was also registered along with the increase in the quantity of gypsum. This result suggests that the formation of gypsum could be expansive. Indeed, considerable expansion, although delayed compared to PC mortars, was observed in the C3S mortars. Thus, it can be concluded that the expansion of the PC mortars occurred due to the combined effect of gypsum and ettringite formation, while the expansion of C3S mortars occurred as a result of gypsum formation.Thaumasite formation as small inclusions was also detected in both the C3S and the PC mortars, especially in regions of high gypsum deposition. The formation of thaumasite, despite the absence of carbonate bearing minerals and low temperatures, could be because of the carbonation of the surface zones of the mortars. However, it would be speculative to attribute any expansion to the formation of thaumasite, since it was detected only in minute amounts in the microstructural investigation. [Copyright &y& Elsevier]

This paper reports the results of an investigation on the effects of sodium and magnesium sulfate solutions on expansion and microstructure of different types of Portland cement mortars. The effects of using various sulfate concentrations and of using different temperatures are also reported. The results suggest that the expansion of mortars in sodium sulfate solution follows a two-stage process. In the initial stage, Stage 1, there is little expansion. This is followed by a sudden and rapid increase in the expansion in Stage 2. Microstructural studies suggest that the onset of expansion in Stage 2 corresponds to the appearance of cracks in the chemically unaltered interior of the mortar. Beyond this point, the expansion proceeds at an almost constant rate until the complete deterioration of the mortar specimen. In the case of magnesium sulfate attack, expansion occurs at a continually increasing rate. Microstructural studies suggest that a layer of brucite (magnesium hydroxide) on the surface forms almost immediately after the introduction of the specimens into the solution. The attack is then governed by the steady diffusion of sulfate ions across the brucite surface barrier. The ultimate failure of the specimen occurs as a result of the decalcification of the calcium silicate hydrate (C-S-H), and its conversion to magnesium silicate hydrate (M-S-H), after prolonged exposure to the solution. The effects of using various admixtures, and of changing the experimental variables such as the temperature and concentration of the solution, are also summarized in this paper. Models for the mechanism of the attack resulting from sodium and magnesium sulfate solutions will be presented in Part 2. [Copyright &y& Elsevier]

Simple chemistry-based empirical models have been developed to assess the role of temperature and concentration of the sulfate solution in the process of expansion of cement mortars that are subjected to external sulfate attack. ASTM Type I PC mortars, prepared according to ASTM C-109, were immersed in sodium and magnesium sulfate solutions at five different concentrations and four different temperatures. For both solutions, the trends in the measured expansion suggested the use of a simple rate law to analyze the effect of concentration. For the effect of temperature, an Arrhenius relationship was developed to determine the activation energy required to initiate expansion in sodium sulfate solution. Regression-based statistical models were found to be sufficient to explain the effect of temperature of magnesium sulfate solution on the expansion. Implications of using these models for developing potential test methods, as well as to enable interpretation of data from nonstandard test methods, are discussed. [Copyright &y& Elsevier]

This article presents an investigation on the application of amino acids to control the CaCO 3 crystallization in carbonation cured wollastonite composites. It was observed that wollastonite carbonated without any amino acid formed calcite as the primary polymorph of CaCO 3. In contrast, the use of amino acids as admixtures resulted in the formation of stable amorphous calcium carbonate (ACC), vaterite, and aragonite during the carbonation of wollastonite. The carbonated composites produced with amino acids were observed to have a lower critical pore size, but a higher total porosity, compared to the control batch. Additionally, the utilization of amino acids was observed to increase the flexural strength and compressive strength of the composites up to 106% and 48%, respectively, compared to the control batch. Such performance enhancement of the carbonated composites in the presence of amino acids was attributed to the reduced critical pore size and the formation of organic-inorganic hybrid phases in the matrix. [ABSTRACT FROM AUTHOR]

Microstructural phases and mechanical properties of lamellar 3D-printed and cast hardened cement paste (hcp) elements were investigated using a lab-based X-ray microscope at two levels of magnification (0.4× and 4×). K-means clustering was used for quantitative image analysis. The entire volume of intact 3-days-old 3D-printed and cast hcp elements was characterized at 0.4× magnification. Three microstructural features (macro-pores, micro-channels, and interfacial micro-pores) were found to reside in three distinct pore size domains. The largest pores of the 3D-printed element were larger than the largest pores of the reference cast hcp element. Moreover, the smallest pore sizes of the 3D-printed element were found to be smaller than those present in the cast counterparts. Micro-channels were found to be connected to one another through the micro-pores present at interfacial regions, indicating the presence of a uniquely patterned and interconnected pore network. The role of locally weak and porous interfaces on mechanical response and fracture properties is discussed. • Processing alters the microstructure in 3D-printed lamellar cubic element. • A unique patterned and connected pore microstructure network exists. • Micro-channels are connected through micro-pores present at interfaces regions. • Three distinct pore volume domains exists in hardened microstructure. • Volume and frequency of pore, hydrated, and unhydrated clusters are dissimilar. [ABSTRACT FROM AUTHOR]