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Four groups of non-traditional and natural pozzolan (NNP) based (aluminosilicate-based) materials, calcined clays, ground bottom ashes, volcanic ashes, and fluidized bed combustion ashes, were alkali activated using a hybrid solution of sodium silicate and sodium hydroxide. The microhardness and reduced modulus of the phases present in eleven alkali-activated pastes were measured using nanoindentation. At least 800 data points were collected from 28-day old ambiently cured paste samples. Unsupervised machine learning techniques i.e., k-means clustering and Gaussian mixture modeling (GMM) were used for the classification of the micromechanical properties supplemented by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The clustering of micromechanical and EDS data revealed five distinct phases in these hardened pastes. The alkali-activated gels identified had reduced modulus values ranging from 12 to 34 GPa. The remnant phases were partly reacted grains and unreacted minerals exhibiting reduced modulus values ranging from 35 to 105 GPa. Micromechanical property contour plots and SEM image pixel count plots were used to validate the phase assemblage. Additionally, the identification capabilities and advantages of the method are assessed by direct comparison with experimental results for different alkali-activated gels. • Micromechanical properties of 11 alkali-activated non-traditional and natural pozzolans were investigated. • The hardness and reduced modulus of pastes were measured based on 800 nanoindents for each material. • Gaussian mixture modeling and k-means clustering classified the micromechanical properties into 5 distinct phases. • Microchemical properties of the phases were determined using energy dispersive X-ray spectroscopy. • The identification capabilities of the proposed method were assessed via direct comparison with experimental results. [ABSTRACT FROM AUTHOR]

Highlights • CSC can store up to 18% of CO 2 (by wt) in the form of CaCO 3 during carbonation. • Reaction rate of CSC was highest for water to cement ratio of 0.4. • Free moisture is essential for the carbonation reaction to progress. • The activation energy for carbonation of CSC was in the range of 44 to 57 kJ/mol. Abstract CO 2 cured calcium silicate cement (CSC) can substantially improve the sustainability of concrete-like materials when used as an alternative to ordinary portland cement (OPC). CSC is produced from the same raw materials as the OPC but at kiln temperatures around 250 °C lower than those required for the production of the OPC. This paper presents a summary of the results from a comprehensive study on the evolution of microscopic phases, reaction kinetics, and strength development in CSC paste and mortar samples during carbonation. The primary carbonation products of CSC are calcium carbonate and Ca-modified silica gel. All three polymorphs of crystalline calcium carbonate (i.e. calcite, aragonite, and vaterite) were found to be present in carbonated CSC paste, calcite being the most abundant polymorph. Influences of water to cement ratio (w/c, by weight), temperature, relative humidity, and CO 2 concentration on the carbonation rate constants of CSC have been studied here. The carbonation rate constants of CSC were found to be the highest for w/c of 0.4 in a 99.9% CO 2 environment. The carbonation activation energies of these systems varied from about 44 kJ/mol to about 57 kJ/mol, depending on the carbonation curing conditions (i.e., w/c ratio, CO 2 concentration, etc.). The CSC mortar samples achieved compressive strength as high as 40 MPa after only 3 days of carbonation. [ABSTRACT FROM AUTHOR]

[Display omitted] • NNPs activated with Ms of 1.25-1.5 and Na 2 O% of 9.25-13% can result in compressive strengths greater than 25 MPa. • The best compressive strength-giving mixtures had a workability of at least 50%. • Calcium hydroxide is a compressive strength enhancer as well as a set controller in NNP geopolymer systems. • Shrinkage-reducing admixtures (SRA) can be used as set retarders in NNP geopolymer systems. • The contour plots and multiple linear regression models are applicable in predicting the compressive strength of NNP geopolymers. The recent shortage of traditional fly ash and slag, commonly used precursors in alkali-activated binders, prompted a rigorous search for alternative materials. This study evaluated four groups of currently underused supplementary cementitious materials (SCMs) as precursors for producing alkali-activated mortars. The materials evaluated include three calcined clays, three ground bottom ashes, three volcanic ashes, and two fluidized bed combustion ashes, collectively named non-traditional and natural pozzolans (NNPs). Firstly, the compositions of the NNPs were optimized using trial-and-error methods to minimize the concentration of the activating solution. Then, the two solution parameters i.e., silica modulus and Na 2 O% were optimized for compressive strength using a central composite design. The compressive strength (ASTM C109), the workability (ASTM C1437), and the setting time (ASTM C191) of the mortar were determined. The results show that all 11 materials could produce mortar mixtures with compressive strength in the 20–40 MPa range at 28 days, satisfactory workability of at least 50 % (flow value), and at least 30 min of initial setting time without heat curing and without blending with conventional SCMs. Most of the materials were activated with lower solution parameters than those used in the literature. The binder optimization suggested Si/Al range 1.0–3.0 and a minimum Ca/Al of 0.25 as favorable molar ratios for potentially reactive aluminosilicate precursors. Multiple linear regression models validated by higher R2 and minimum RMSE and MAE could accurately predict the compressive strength of NNP mortar. Furthermore, calcium hydroxide and shrinkage-reducing admixtures were identified as set regulators in alkali-activated NNP mortars. [ABSTRACT FROM AUTHOR]

Abstract This paper presents a study on the carbonation activated binders prepared from pure calcium silicate phases, which included tricalcium silicate (3CaO.SiO 2 , [C 3 S]), β-dicalcium silicate (β-2CaO.SiO 2 , [β-C 2 S]), γ-dicalcium silicate (γ-2CaO.SiO 2 , [γ-C 2 S]), tricalcium disilicate (rankinite, 3CaO.2SiO 2 , [C 3 S 2 ]), and monocalcium silicate (wollastonite, CaO.SiO 2 , [CS]). The overall study consisted of three experimental parts, with individual focus on the following issues: (i) reaction kinetics, (ii) mechanical performance at the microscale, and (iii) mechanical performance at the macroscale. Carbonation of calcium silicate phases was found to occur in two distinct stages, namely: phase boundary controlled stage and product layer diffusion controlled stage. Theoretical solid-state reaction approach, including contracting volume model and Jander's equations were used to determine the carbonation rate constants for the calcium silicate phases. Phase boundary controlled stage was found to be dominantly dependent on the type of the starting calcium silicate phases. On the other hand, during the diffusion controlled stage the reaction rate constants were found to depend on the type of carbonation products (in this case Ca-modified silica gel and calcium carbonate). The mechanical properties of the individual microscopic phases were evaluated using nanoindentation test whereas the overall strength of the carbonated paste was evaluated using macroscale three-point bending test. Correlations between the mechanical performances and microstructural characteristics revealed the performance controlling factors of the carbonation activated binders. The higher bound water contents of the carbonated matrix tend to increase the short-term (up to 3 h) creep deformation of the matrix when subjected to constant stress. The presence of a higher proportion of poorly-crystalline forms of calcium carbonates (i.e., vaterite and amorphous calcium carbonate) were observed to increase the flexural strength but decrease the elastic modulus of the carbonated matrix. [ABSTRACT FROM AUTHOR]

This article features an investigation of the accelerated carbonation products of four calcium silicate phases, namely monoclinic-tricalcium silicate (3CaO·SiO 2 or C 3 S), γ-dicalcium silicate (2CaO·SiO 2 or γ-C 2 S), tricalcium disilicate (3CaO·2SiO 2 or C 3 S 2 , rankinite), and monocalcium silicate (CaO·SiO 2 or CS, wollastonite). During the carbonation reaction, the calcium silicate minerals form calcium carbonate and Ca-modified silica gel. These reaction products were examined using thermogravimetric analysis (TGA), X-ray powder diffraction (XRD), 29 Si magic angle spining (MAS) nuclear magnetic resonance (NMR), 13 C { 1 H} cross polarized (CP)/MAS NMR, and dynamic vapor sorption (DVS) techniques. Along with the crystalline forms of CaCO 3 (i.e., calcite, vaterite, and aragonite), amorphous calcium carbonate (ACC) was also found to be present in the carbonated calcium silicate systems. Based on the experimental results, it is proposed that the ACC particles were stabilized by the deposition of silica layers on their surfaces. Presence of ACC also affected the pore size distribution of the matrixes, which eventually influenced the diffusion based carbonation rate of the matrixes. Moreover, the degree of carbonation of CS was found to be most effective compared to other calcium silicates in terms of the percentage of the phase reacted for the experimental conditions used in this study. [ABSTRACT FROM AUTHOR]

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 describes the multiscale characterization of the carbonated wollastonite paste using X-ray diffraction (XRD), scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS), and statistical nanoindentation (SNI, also known as ‘grid indentation’) methods as well as micromechanical homogenization models. Wollastonite (CaSiO 3 ) fibers are commonly used as filler in ceramics or plastics. However, wollastonite can also be regarded as non-hydraulic binder material since upon carbonation it forms a heterogeneous matrix with mechanical properties similar to those of the conventional hydrated cement pastes. Carbonation reaction of wollastonite results in the formation of two main products: calcium carbonate (CaCO 3 ) and amorphous silica gel (SiO 2 ). The SEM/EDS microanalysis performed on this system revealed that the average calcium to silica (Ca/Si) atomic ratio of the silica gel phase was around 0.40. Three individual carbonated wollastonite paste samples, each representing a different degree of carbonation were selected for nanoindentation tests. The obtained elastic moduli for silica gel, calcium carbonate, and unreacted wollastonite grains were, respectively, 41.7 GPa, 67.3 GPa, and 134.7 GPa. The micromechanical homogenization models were then utilized to predict the effective (also referred to as ‘homogenized’) elastic moduli of the carbonated wollastonite paste. The predicted values of the effective elastic moduli of carbonated wollastonite pastes were found to be in the range of corresponding values for hydrated high to ultra-high performance cement pastes. Additionally, the values of the effective elastic moduli of the carbonated wollastonite pastes were observed to increase with the increase in the degree of carbonation. [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]

Ability to extrude and to achieve shape stability of layer-wise additively manufactured cement-based elements at large scale depends upon the early-age rheological properties (shear moduli, yield stress, viscosity) of the deposited materials. Upon successful extrusion, buildability challenges can manifest themselves through two common failure mechanisms: yielding of lower layers or buckling of the element. However, it is yet unclear which of the various rheological properties control the early-age materials' deformation, specifically buckling, during the printing processes and thus influence the resulting buildability of the elements. This paper focuses on how buildability is dependent upon rheological properties as well as on predicting the buildability using a buckling theoretical framework. Specifically, the relationship between early-age rheological properties of various cement pastes and the buildability of hollow cylinders with failure dominated by buckling mechanism was investigated. It was found that certain types of shear moduli of the fresh pastes (G, G*, and G′) obtained from oscillatory shear stress sweep tests, performed within the first 30 min of hydration, correlate better with the buildability of hollow elements than some other rheological properties (loss modulus G″ , yield stress σ yield , yield strain γ yield , and complex viscosity η*). Measured values of shear modulus (G) were used to calculate elastic modulus (E) of the pastes using the assumed value of 0.5 for Poisson's ratio (ν) of fresh cement paste. Euler's buckling theory was used to predict buildability (height of the element) of hollow cylinders. It was found that Euler's theory overestimates the buildability by 93%–194%, mainly due to assumption of ideal geometry (i.e., absence initial or printing imperfection), rate-independent behavior and linear elasticity. [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: This research was primarily conducted to verify the presence of synergistic effects in ternary cementitious systems containing portland cement (OPC), class C fly ash (FA) and silica fume (SF). A subsequent objective of the study was to quantify the magnitude of the synergy and to determine its source. For a ternary mixture containing 20% FA and 5% SF by mass, the synergistic effect was observed mostly at later ages (7days onward) and it resulted in an increased compressive strength and resistance to chloride ion penetration as well as a reduced rate of water absorption (sorptivity) compared to predictions based on individual effects of FA and SF in respective binary systems. The observed synergy was attributed to both chemical and physical effects. The chemical effect manifested itself in the form of an increased amount of hydration products. The physical effect associated with packing density was, somewhat contrary to general belief, not due to an optimized particle size distribution of the binder components of the ternary cementitious system. Instead, it was the result of smaller initial inter-particle spacing caused by lower specific gravities of both FA and SF which, in turn, led to a lower volumetric w/cm. If the mixture design was adjusted to account for these differences, the physical effect would be diminished. [Copyright &y& Elsevier]

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]

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]

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]

This paper presents the mechanical properties of composite beams additively manufactured by 3D-printing of layers of mortar on top of custom-designed (also 3D-printed) steel plates, which served as external flexural reinforcement. The mechanical performance of the composite beams was evaluated using three-point bending test. The results were compared with the data obtained from two types of reference specimens: 3D-printed beams with no external reinforcing plate, and composite beams with mortar cast (rather than 3D printed) on the top of the steel plate. Four different architectures (also referred to as configurations) were created by either 3D-printing (using two different filament orientations), casting, or a combination of both processes (referred to as a hybrid configuration). This configuration consisted of 3D-printed external walls (outline) of the beam and cast interior (infill). The effects of these architectures were investigated using both unreinforced and composite elements, in order to identify the contributions of the orientation of filaments and associated interfaces on the initiation and propagation of the cracks. Analysis of the initiation and propagation of the cracks was based on the data obtained from the digital image correlation (DIC) technique. Mechanical performance parameters that were investigated included the following: load-displacement responses, flexural stress-flexural strain responses, shear stress-shear strain responses, and the work of failure. The results of the test revealed that the incorporation of steel plates as an external reinforcement resulted in the transition from flexural to shear mode failure. In terms of flexural strength, all composite configurations demonstrated comparable performance independently of the architecture used while hybrid configuration outperformed cast composite counterparts by attaining significantly higher values of the work of failure. This indicates that hybrid elements developed enhanced energy dissipation characteristics compared to the other configurations. The results of specific modulus of rupture and shear strength obtained from unreinforced and reinforced hybrid elements, respectively, were comparable to those obtained from cast counterparts, indicating that the hybrid configuration may offer a viable alternative for the construction of structural concrete elements. Finally, the values of modulus of rupture and shear stress obtained for different configurations used in the study were compared to the design expressions in the ACI 318–19 code. It was found that hybrid elements as well as cast elements used in this study satisfy the minimum requirements and further studies at larger scales could confirm their use in structural applications. [ABSTRACT FROM AUTHOR]

[Display omitted] • SAP added to mortars with ∼ 110% flow could be cast into well-consolidated samples. • Both SAP absorption capacity and kinetics affect percent flow of mortar. • SAP could be selected based on desired flow without sacrificing strength. • 5% extra water does not guarantee well-consolidated SAP-containing mortar. • SAP-internally cured mortars at 0.42 w/c displayed a notable absence of microcracks. A straightforward mix design method was developed for proportioning mortars containing superabsorbent polymers (SAPs). When modified by introduction of a typical amount of SAP (i.e., 0.2% by weight of cement), the 0.42 w/c ordinary Portland cement (OPC) mortars required addition of extra water and/or high-range water reducing admixture (HRWRA) to achieve a minimum target percent flow in mortar flow table tests. At high w/c (≥0.49), SAP accelerated compressive and flexural strength development. In all mortars tested, the addition of SAP either preserved or increased compressive and flexural strength values relative to SAP-free mortar with the same w/c. [ABSTRACT FROM AUTHOR]

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 describes the influence of morphologically altered cellulose fibers on the acoustic and mechanical properties of cellulose–cement composites. Three fiber morphologies were considered (macro-nodules, discrete fibers, and petite nodules). The main parameters studied include the normal incident acoustic absorption coefficient (α), specific damping capacity (ψ), loss tangent (tanδ), storage modulus (E′), and loss modulus (E′′=E′tanδ). The acoustic absorption coefficient was found to increase with an increase in fiber volume for all three fiber types investigated, though “macro-nodule” fibers were found to be the most effective. Stiffness–loss relationships are reported for these composites and the behavior of cellulose–cement composites with soft cellulose fiber inclusions was found to be similar to a Voigt (series) composite model. Low volumes of fibers had a minimal effect on the loss tangent; however the stiffness was considerably reduced. Predictive equations for loss modulus as a function of fiber volume at different moisture conditions were developed. These relations compare well with the experimental values as well as the idealized Voigt composite behavior. This suggests that there is an optimum fiber volume, which maximizes the loss modulus for saturated composites while the loss modulus is practically independent of fiber volume for dry composites. [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]

High quality pozzolans are needed to produce durable and low CO 2 concrete. While extensive literature is available on pozzolanic reactivity and reaction products of conventional fly ash and slag cement, similar information is scarce for natural and non-conventional pozzolans (NNPs), impeding their use in concrete. This study presents a comprehensive evaluation of eleven NNPs, including calcined clays, volcanic ashes, ground bottom ashes, and fluidized bed combustion (FBC) fly ashes. The chemistry, mineralogy, physical properties, and pozzolanic reactivity of these materials were evaluated and benchmarked against conventional pozzolans. Except for one case of high SO 3 , all NNPs met ASTM C618 specifications as well as RILEM limits of pozzolanic reactivity. Based on ASTM C1897 (the R3 test), calcined clays were found to be significantly more reactive than fly ash and comparable with slag and silica fume. Volcanic ashes and FBC fly ashes showed similar pozzolanic reactivity to that of Class F fly ash, while ground bottom ashes were comparable with the top 50% of Class F fly ashes. The reaction products of these NNPs were identified using QXRD and thermodynamic modeling as C-A-S-H, mono- and hemi-carboaluminate, monosulfate, and ettringite. Calcined clays formed monosulfate and little to no ettringite while other NNPs formed ettringite but no monosulfate. This was attributed to the availability and dissolution kinetics of alumina versus calcite in the R3 experiment. [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]

In this study, a series of cementitious composite with internal curing agent incorporating ranging from 0% to 30% were prepared to evaluate their autogenous healing performance. The Clinoptilolite Zeolite was utilized as an internal curing compound to replace parts of sand for storing the water inside the cement matrix. The mechanical properties were examined via compressive and splitting tensile tests. To quantify the self-healing behavior of this cementitious composites, the samples were pre-damaged using splitting tensile method and placed in the wet/dry cycle for several days. Two non-destructive testing methods, including resonant frequency and ultrasonic pulse velocity tests, were employed to assess the healing efficiency. The results revealed that the incorporation of zeolite could enhance the compressive strength and the self-healing performance of the proposed cementitious composites. However, if the replacement percentage of zeolite was higher than 15%, the healing efficiency decreased due to the "blocking effect", which was further investigated by the water absorption test and thermogravimetric analysis (TGA). [ABSTRACT FROM AUTHOR]