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The accumulation of alkali residue has led to significant contamination of soil, air, and water, posing a hazardous threat to the environment. To address this issue, a technology and method for producing alkali residue–based lightweight soil were proposed to facilitate the reuse of alkali residue, and the feasibility of this method was demonstrated. Compression tests, X-ray diffraction, X-ray fluorescence spectrometry, and scanning electron microscope (SEM) were conducted to investigate the strength characteristics and hardening mechanism of the material. The results indicate that the compressive strength of the alkali residue–based lightweight soil increases linearly with wet density, age, content of portland cement, and ground granulated blast slag. The optimal water–solid ratio was determined to be 0.6. Specimens with low and high strength exhibited plastic and brittle failures, respectively. The primary contributors to the material's compressive strength were identified as hydrated calcium silicate, ettringite, monosulfur calcium sulfoaluminate hydrate, hydrated calcium aluminate, and Friedel's salt. The 28-d compressive strength of the alkali residue–based lightweight soil, with a water–solid ratio of 0.6 and a wet density of 800.0 kg/m3 , was measured to be 1.17 MPa, meeting the requirements for most filling projects. [ABSTRACT FROM AUTHOR]

As the main hydration products, calcium-silicate-hydrate (C─ S─ H) and portlandite (CH) contribute significantly to the tensile strength of cementitious materials. However, the role of interface in the C─ S─ H/CH composites remains unexplored. This paper aims at elucidating the interfacial tensile properties of C─ S─ H/CH composites via molecular dynamics simulations with the ReaxFF forcefield. A method quantifying the thickness of the atomic-level interfacial transition zone in composites through potential energy analysis was proposed, with the value of 1.75–3.38 nm. Results showed that all models had damages at or near the interfaces. The tensile strength of composites relied highly upon material orientations. Elevated loading speed enhanced the tensile strength of composites, and their strain rate sensitivity was more significant than either C─ S─ H or CH. Failure mechanisms were revealed via chemical bond evolution. A nanoscopic tensile constitutive model was formulated based on Morse potential function. This research sheds light on the role of the interfacial area in two-phase composites at the atomic scale, which provides essential inputs for further multiscale studies. [ABSTRACT FROM AUTHOR]

Molybdenum tailings are accumulated in large quantities, occupying the land and polluting the environment. Therefore, the resource utilization of molybdenum tailings is imperative. In this paper, the influence of molybdenum tailings powder (MTs) as admixture on hydration characteristics, including hydration heat, hydration products, phase composition, and mechanical properties evolution of cement paste were investigated. Results showed that the early hydration of cement paste can be promoted when the content of MTs is not more than 15%. The compressive strength of cement paste with 15% MTs at 3, 28, and 120 days was 89.37%, 96.14%, and 91.68% of that of the portland cement (PC) group, respectively. When the content of MTs does not exceed 15%, the second exothermic peak of hydration heat of cement paste appears ahead of time and the peak value increases. With the increase of MTs content, the cumulative heat release of 72 h gradually decreased. The addition of MTs does not produce new hydration products, but introduces the mineral phase of MTs itself, and leads to the early transformation of ettringite (AFt) into monosulfoaluminate (AFm) in paste. There is still a close linear relationship between the compressive strength and cumulative heat release and chemically bound water content after addition of MTs into cement paste. It was also discovered that the pozzolanic activity of MTs in cement paste mainly occurred from 3 to 28 days. After 28 days, the particle-filling effect of MTs was dominant mainly. The research results are expected to provide basic research for the resource utilization of MTs. [ABSTRACT FROM AUTHOR]

It is important to study the steel corrosion inhibition behavior in complex chloride salt environments. This paper uses the fact that calcined layered double hydroxides (CLDHs) have a certain memory effect, which means that it can reabsorb water and anions and partially restore the lamellar structure under certain conditions. This work investigated the influence of cation types on the steel corrosion inhibition behavior of CLDH in saturated calcium hydroxide solution using equilibrium isotherms of Cl− uptake, the electrochemical method, X-ray diffraction (XRD), Fourier-transform infrared (FTIR), and scanning electron microscopy (SEM) techniques. The results show that the corrosion inhibition of the CLDH is affected by the cation types in the following order: Ca2+>Mg2+>K+≈Na+·Na+ and K+ increase the pH of the solution and decrease the corrosion inhibition of CLDH. The addition of Mg2+ can promote the reconstruction of Mg-Al-Zn-Cl hydrotalcite. The addition of Ca2+ can promote the formation of new Ca-Al layered double hydroxide (Ca-Al-LDH) and improve the corrosion inhibition. [ABSTRACT FROM AUTHOR]

The incompatibility between the superplasticizer (SP) and cement can cause problems with the cementitious materials' fresh and hardened qualities, such as unstable workability, uncontrollable setting times, excessive air entrainment, and so on. In this paper, five various types of SPs were prepared. Their interaction with pure tricalcium aluminate (C3A) and its hydration products were investigated and their effect on the generation of Ettringite (AFt) and single-sulfur calcium sulphoaluminate hydrate (AFm) in paste and their compatibility with cement were also studied. The results demonstrate that after 15 min of cement hydration, the AFt and AFm content of mixture with polycarboxylate SP and sulfonated acetone-formaldehyde superplasticizer (PCA-SAF) is 124.6% and 24.8% higher than that of SAF, respectively, which decreases the consumption of SPs molecules and improves the compatibility of SAF with cement. Compared with polynaphthalene sulfonate superplasticizer (PNS), PCA-PNS dramatically enhanced the SP's compatibility with cement. Accelerating the production of AFt and AFm during the early stages of cement hydration may be one method to increase SP–cement compatibility. [ABSTRACT FROM AUTHOR]

The use of locally available industrial by-products as supplementary cementitious materials and mineral fillers is vital for reducing the embodied carbon of modern concretes. Spent pot lining (SPL), a by-product of the aluminum industry, is massively produced worldwide. SPL treated with the low caustic leaching and liming process (LCLL-ash) is no more hazardous and can be used as cementitious material. This study aims to better understand the microstructure changes of cement paste incorporating aluminum smelter wastes, such as LCLL-ash and synthetic anhydrite. Ground LCLL-ash was used to partially replace cement in cement pastes with a constant water-to-binder ratio of 0.35. A small amount of anhydrite was added to some mixes. This study investigated chemo-micromechanical properties of cement paste systems through multiple techniques, including X-ray diffraction, thermogravimetric analysis, scanning electron microscopy, wavelength-dispersive spectroscopy, and microindentation test. The results showed that the reactive alumina from LCLL-ash modified the hydrated phases with the presence of the carbo-aluminate phases. The pastes containing LCLL-ash exhibited a higher CH, and C-S-H contents refer to the reference, suggesting that LCLL-ash has a slight nucleation effect. Moreover, LCLL cement paste showed an increase in the Ca/Si ratio of the C-S-H phase intermix. Finally, microindentation results revealed that adding anhydrite with 10% LCLL-ash enhanced the mechanical property of the cement paste at 28 days. [ABSTRACT FROM AUTHOR]

In this study, variation in microstructure, diffusion of chloride (Cl−) ions, and binding of chloride in concrete exposed to solutions containing MgSO4 (MS) in NaCl (NC) under the wetting-drying cycle are analyzed and reported. The results reveal that the amount of compounds such as ettringite (E), calcium hydroxide (CH), gypsum (G), calcite (CC), and calcium chloroaluminate (CCA) varies with the depth of concrete from the exposure face. Among exposure solutions, the amount of E was higher in NC + MS solutions compared with NC solutions and with increased MS content. The amount of CCA was higher in NC solutions compared with NC + MS solutions with an increase in NC content. The amount of CH was higher in concrete subjected to NC + MS solutions than to NC solutions and decreased with an increase in NC content. The amount of CCA was mostly higher in ordinary portland cement (OPC) with 20% fly ash concrete (O20FA-C) than in OPC concrete (OPC-C) followed by portland pozzolana cement (PPC) concrete (PPC-C). The amount of E was higher in O20FA-C than in PPC-C followed by OPC-C. Free chloride content (Cf) and apparent chloride diffusion coefficient (D) were reduced in the presence of MgSO4 mainly due to the effect of filling of pores with E. PPC-C exhibited better performance against ingress of chloride (Cl−) ions than that of O20FA-C followed by OPC-C. MS in exposure solutions reduced chloride binding. The concrete made with OPC exhibited higher chloride binding than O20FA followed by PPC. [ABSTRACT FROM AUTHOR]

The main objective of this study is to evaluate and optimize the crack healing efficiency of hydrogel-encapsulated bacteria in concrete applications. To achieve this objective, Bacillus pseudofirmus was evaluated as the bacteria strain, which was combined with three mineral precursors corresponding to magnesium acetate, calcium lactate, and sodium lactate at two concentrations (67.76 and 75 mM/L). For each combination, three sets of mortar cubes, along with three sets of mortar beams, were prepared. The mechanical properties of these specimens were characterized by compressive and flexural strength tests. Once the beams were cracked, they were subjected to 28 days of wet/dry cycles in which crack width was monitored. Once the wet/dry cycles were completed, the specimens were retested to determine the strength recovery. In the compressive strength and self-healing efficiency tests, mortar samples prepared with calcium lactate at a concentration of 75 mM/L along with bacteria and yeast extract showed the best performance. With respect to flexural strength recovery, no significant difference was noted among the specimens. Microscopic evaluation revealed a high concentration of calcium-rich crystals (i.e., calcium carbonate or calcium hydroxide crystals) indicative of bacterial activities. A scale-up concrete study was performed based on the best-performing mortar samples and then compared to plain concrete. Results from the concrete analysis indicated that the addition of calcium lactate improved concrete's flexural strength. However, the plain concrete control specimen displayed superior healing efficiency after being subjected to wet/dry cycles. [ABSTRACT FROM AUTHOR]

The suitability of the rapid, relevant, and reliable (R3) heat of hydration and bound water tests for measuring the reactivity of various supplementary cementitious materials (SCMs) and their blends with calcium hydroxide (CH) was investigated in this study. These tests have been recently standardized in current standards. The test was carried out on pastes of fly ash (FA), slag (S), kaolinitic calcined clay (K), limestone (LS), and their blends. A detailed analysis of the heat of hydration and bound water measurements specified according to the R3 test was carried out. A good correlation was found between the two types of test results and also between the reactivity test results versus strength activity index measured using cement mortar compressive strength. The formation of hydration products in the R3 test paste specimens was also studied using X-ray diffraction (XRD). The hydration products for SCMs formed in the test were similar to those reported in cement systems. It was seen that the interactions in the blended SCMs with CH were also similar to those reported in cement pastes. Further, the microstructure of the specimen was studied using scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) analysis, and the overall hydration of various SCMs was studied. The influence of the proportions of the SCMs in the blended SCMs and that of fineness were also investigated. The applicability and limitations of this test method for SCMs and their blends were discussed. [ABSTRACT FROM AUTHOR]

The drying shrinkage of alkali-activated concrete (AAC) is a very complex process, and it warrants long-term testing. The present study was focused on the development of drying shrinkage prediction models for ambient-cured AAC. AAC with four distinct precursor combinations for experimental validation and investigation of the influence of mix proportions were used. The applied fly ash (FA) to ground granulated blast-furnace slag (GGBFS) ratio was varied as 100:0, 70:30, 60:40, and 50:50. Drying shrinkage strains were determined for hardened paste, mortar, and concrete specimens. The observed drying shrinkage behavior was correlated with the mineralogical, chemical, and morphological characteristics of corresponding paste specimens of alkali-activated binder (AAB). These characteristics were evaluated using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS), respectively. The results showed that the drying shrinkage of hardened AAB paste and AAB mortar increases by 26%–32% and 13%–28% with GGBFS content, respectively. The qualitative and quantitative microstructural analysis suggested that this behavior could be ascribed to the formation of a calcium aluminosilicate hydrate matrix. Moreover, FA-based AAC exhibited maximum shrinkage, probably due to its comparatively high overall porosity with the addition of both coarse and fine aggregates. Using the experimental data, stepwise linear and multivariate nonlinear regression models were developed by modifying existing portland cement concrete (PCC) models to predict the drying shrinkage strains of AAC. Recommended linear and nonlinear regression models were selected based on the least deviation from the experimental value. The modified GL2000 model was found to be the best fit for shrinkage predictions in AAC because of its simplicity and comparable precision to experimental findings. [ABSTRACT FROM AUTHOR]

Biobased green plasticizers are preferred in the production of plastics in light of the environmental and human health issues posed by traditional plasticizers. Epoxidized soybean oil is an environmentally friendly admixture that is a significant alternative for biobased polymer concrete rather than petroleum-derived products such as polyethylene terephthalate, rubber, and polyvinylchloride. This study aims to determine the effect of epoxidized soybean oil additives on cement paste properties, and to compare the effects of epoxidized soybean oil and dioctyl terephthalate on cement paste properties. With the addition of 10% epoxidized soybean oil to cement paste, electrical resistance was 1.91 times higher, and thermal resistance was 1.70 times higher, than the control sample, while water absorption and porosity decreased by 65%±4.0%. Compared to dioctyl terephthalate-impregnated cement pastes, epoxidized soybean oil cement pastes have higher values for calcium hydroxide content and ultrasonic pulse velocity (91%±0.3% and 182%±2.0% , respectively), and lower values for porosity and thermal conductivity (30%±3.0% and 16%±0.1% , respectively). Moreover, epoxidized soybean oil can form three times more hydrogen bonds than dioctyl terephthalate, which allows for the formation of less porous and more compact cement pastes. [ABSTRACT FROM AUTHOR]

Using new techniques based on atomic force microscopy (AFM) and scanning electron microscopy (SEM), the stiffness, thickness, and microstructures of the shell of air bubbles extracted from air-entrained cement paste were examined, and coalescence resistances of pairs of bubbles were measured. The surface tension of air-entraining admixture (AEA) solutions in water and lime water also was determined. The results showed that air bubbles associated with synthetic AEAs had thinner and more-flexible shells than those with neutralized vinsol resin (NVR). The stiffness and thickness of the bubble shells were shown to be correlated with the differences in the stability of air bubbles in concrete among the same AEAs reported in a previous study. A mechanism leading to the differences in bubble shell properties between the AEAs is proposed based on the results of the surface tension testing and the particle-stabilized bubble theory, but further work remains to verify and refine this potential mechanism. [ABSTRACT FROM AUTHOR]

Calcium sulfoaluminate (CSA) cement showed a good binding capacity on the external Cl− transported into concretes from its surrounding environment. However, it is unclear whether the same binding mechanism can still apply if the Cl− ions were introduced during a concrete mixing stage when marine resources, without being properly processed, are used as fine aggregates. Experimental work on CSA mixes, with different water/powder, gypsum content, and existences of premixed Cl− , has been conducted and showed that the highest binding capacity on premixed Cl− was obtained in the mixes with a higher water/powder and without gypsum. Mineralogy and Cl− binding results of the tested CSA mixes proved that except for the calcium monosulfoaluminate hydrate (Ms), which has been well established with a good Cl− binding capacity, the CAH10 produced should also contribute to the binding on premixed Cl−. The premixed Cl− in the CSA cement led to a conversion of Ms and CAH10 to Friedel's salt and, meanwhile, shortened the setting time, altered the heat release process, prohibited the generation of microcracks, and resulted in a more densified microstructure. [ABSTRACT FROM AUTHOR]

Laboratory tests demonstrated that biochar filler added to portland cement stabilized peat results in an increase of unconfined compressive strength comparable with that of a sand filler. The strength increase is significantly higher when biochar is ground to a size below 75 μm. This study investigated the changes in mineralogy, texture, and microstructure during the early hydration of cement mixed with peat and biochar filler to identify the mechanisms responsible for the strength increase. The results show that the biochar surface catalyzes nucleation of hydration products. Labile carbon in biochar promotes carbonation, with precipitation of calcite within its cells and on its surface, as well as formation of hemi and monocarboaluminate, two stable calcium aluminate hydrate (AFm) phases. For larger fragments of biochar, early hydration products do not reach the inner cells. Instead, the fine fragments tend to be fully covered, leading to a more homogeneous spatial distribution of cement and voids. [ABSTRACT FROM AUTHOR]

To develop high-performance microfine cementitious grout (HPMCG), the matrix [microfine Portland cement clinker (MPCC) + flue gas desulfurizing gypsum (MFGDG) + calcium carbonate (MCC)], microfine fly ash (MFA), silica fume (SF), and blast-furnace slag (MBFS) were selected. The viscoelasticity, fresh-state properties, mechanical performance, antipermeability, hydration mineral, and microstructure were investigated systematically. The 0%–3.0% naphthalene-based superplasticizer (N) and 1.0%–2.6% composite activator (CA) were applied, and the ratio of the water to binder (w/b) was selected as 0.8–3.0 by weight. The optimum component of HPMCG was acquired as the amount of MFA was 40% relative to that of the matrix, and the additional contents of SF and MBFS were 10% and 30%. The amounts of N and CA were suggested as 1.5%–2.0% and 2.4%, respectively. After optimization, the maximal flexural strength (FS) and unconfined compressive strength (UCS) of the HPMCG (91-day) can achieve 7.29 and 31.65 MPa; nearly all of the calcium hydroxide reacted, and more gels were generated. More generated gels interconnected with each other to form dense network structures, there were few microvoids and pores, and the entirety of the microstructure was enhanced obviously. The optimized HPMCG surpassed the expectation. The optimized HPMCG has superiorities, such as excellent viscoelasticity, good fluidity, high stability or stone rate, high mechanical strength, good antipermeability, an advantageous mineral component, and a microstructure. In civil engineering, they can satisfy high standard demands of construction and repair grouting practices. [ABSTRACT FROM AUTHOR]

Organic acids, such as acetic and lactic acids, are prevalent in agricultural and food effluents. They pose a considerable pollution threat and must be collected and stored safely before treatment and release. They cause significant damage to cementitious materials, reducing the service life of structures. In this study, the resistance of alkali-activated fly ash and slag-blended binders to organic acids was studied, and a comparison with ordinary portland cement binders was carried out. The findings demonstrate that alkali-activated binders with increased fly ash content have marginally better resistance to acetic acid, but mixes with increased slag content have better resistance to lactic acid. This is due to the solubility of the calcium and aluminum salts of acetic and lactic acids. Overall, the performance of the alkali-activated binders was better than that of the ordinary portland cement binder, with a lower mass and strength losses observed. This was attributed to their lower calcium content with less vulnerable phases, such as calcium hydroxide and ettringite. Instead, the calcium-silicate-hydrate (C-S-H) type gels in alkali-activated binders suffered decalcification and dealumination but left behind a silicon-rich gel, which helped to resist further acid attack. [ABSTRACT FROM AUTHOR]

Geopolymers and alkali-activated materials (AAM) have emerged as a promising sustainable alternative to ordinary Portland cement (OPC). Although much effort has recently been devoted to a wide range of research on the self-healing of cracks in OPC-based composites, little is known about the self-healing potential of AAMs. Therefore, this study explores the crack self-healing capability of NaOH-activated slag composites using a portfolio of testing methods, including electrical conductivity, mercury intrusion porosimetry, inductively coupled plasma optical emission spectroscopy, scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy, and X-ray microcomputed tomography. Experimental results indicate that alkali-activated slag-based composites incorporating calcium hydroxide achieved higher levels of self-healing than control specimens without calcium hydroxide. X-ray micro-computed tomography coupled with three-dimensional image analysis demonstrated that the observed self-healing was a surface mechanism that only occurred at surface cracks. Calcium carbonate was found to be the main self-healing product in all test specimens. Leaching experimental results indicated that the concentration of Ca2+ ions in the AAM matrix plays a critical role in calcium carbonate precipitation and, thus, in the self-healing potential. [ABSTRACT FROM AUTHOR]

By combining the nearest surface distribution function and hydration kinetic equation, analytical equations are proposed to determine the pore phase and solid phase evolution in the interfacial transition zone (ITZ). In addition, the numerical models for concrete with a single flat aggregate are constructed based on the CEMHYD3D hydration model. The results show that the degree of hydration is the main factor affecting the ratio of the average porosity to the matrix porosity in the ITZ. With an increase in the aggregate volume fraction and interface thickness, the volume fraction of the ITZ first increases and then decreases. The degree of overlap increases linearly with the interface thickness and the aggregate volume fraction. As the distance from the aggregate surface increases, high-density C-S-H increases, while low-density C-S-H and calcium hydroxide phases decrease. Finally, the predicted results using the analytical equations and numerical simulation are verified by testing via back-scattered electron microscopy, which indicates that the experimental results are more consistent with the predicted results of the analytical model than with those of the numerical model. [ABSTRACT FROM AUTHOR]

Alternative materials are becoming the substances of choice for designers over conventional materials due to cost and availability constraints. The sustainability and maintenance of the supply of conventional construction materials often becomes a limiting factor. This calls for the utilization of industrial refuse and agricultural by-products. Recycling of this waste can be an effective tool to decrease the levels of pollution and enhance the economy. Different materials like rice husk ash (RHA), fly ash (FA), silica fume (SF), and slag can be substituted in cement owing to their pozzolainic behavior. However, their use may make the concrete more prone to carbonation than ordinary portland cement (OPC) concrete. This research is aimed at knowing the influence of RHA on carbonation at distinct conditions. Concrete samples with varied fractions of RHA (0%–20%) with interval of 5% under three different relative humidity levels (30%, 45%, and 60%) were made to undergo accelerated carbonation tests at 5% CO2 content after being cured for 7, 14, and 28 days. It was found that there was no carbonation observed in relative humidity level of 30%. However, the depth of carbonation was directly proportional to the RHA replacement level for humidity levels of 45% and 60% and the increase was nearly the same corresponding to 15% and 20% replacement for curing age of 28 days. Hence, without compromising the carbonation resistance, greater RHA fractions can be used with a longer curing period. Besides, the results also revealed that the depth of carbonation decreased with an increase in relative humidity levels from 45% to 60%. The effect of compressive strength on carbonation depth of RHA concrete was also studied and it was found that the carbonation depth decreased as the compressive strength of concrete increased. [ABSTRACT FROM AUTHOR]

The reaction of blast furnace slag in sodium hydroxide (NaOH) solutions of different molarities is evaluated. The compressive strength of the activated slag does not scale with the molarity of NaOH. The primary reaction product in the activated slag is identified with calcium aluminosilicate hydrate [C(A)SH]. While the early reactivity of slag is enhanced at higher alkalinity, and the dissolution of slag increases with the molarity of NaOH, the quantity of C(A)SH in the hydrating system does not scale with the molarity of NaOH in the activated slag. From X-ray diffraction (XRD) analysis, an additional water-soluble, sodium-based amorphous product is identified in the reaction products. The water-soluble product, which does not contribute to strength, increases proportionately with the initial Na content in the solution. At higher molarity, there is a larger proportion of the water-soluble product relative to C(A)SH in the reaction products. The Ca/Si ratio and Al/Si ratios in the C(A)SH gel are relatively invariant of the NaOH molarity in the activating solution. The compressive strength gain in the alkali-activated slag is determined by the quantities of C(A)SH and the intrinsic sodium-filled water-soluble product. [ABSTRACT FROM AUTHOR]

This paper presents the results of an experimental investigation carried out to determine the consolidating effect of ureolytic and nonureolytic Bacillus sp. on desert sand. Bacteria-infused desert-sand bricks were prepared using various binders (cement, lime, fly ash), and their engineering properties were evaluated. The results showed an increase of ~19% in compressive strength and water absorption value of 17% in bacteria treated desert sand-cement bricks relative to the same properties in control. In treated bricks field emission scanning electron microscopy (FESEM) analysis revealed thick biodepositions at points of particle to particle contact of desert sand, creating a densified microstructure compared to untreated bricks. X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR) analysis of bacteria-treated desert sand–cement bricks indicated the enhanced formation of other hydration products in addition to calcite, and thermogravimetric (TG) analysis validated the formation of additional C─ S─ H (approximately 12%) and calcium hydroxide (approximately 40%). This study shows that nonureolytic bacteria–infused desert sand–cement bricks can achieve an unfired compressive strength ≥ 5 MPa and, therefore, may serve as a sustainable alternative to other conventionally available bricks. [ABSTRACT FROM AUTHOR]

Fly ash, a supplementary cementitious material, is normally used in conjunction with portland cement to improve the durability properties of concrete in the field, such as sea-crossing bridges, high-speed railways, and mass concrete in dams. Currently, these concrete structures are frequently exposed to sulfate environments. The object of this paper is to investigate the relationship among content of calcium hydroxide (CH), formation of gypsum, and compressive strength, as well as the relationship among the formation of ettringite, pore structure, and expansion properties in blend cement paste at long-term sulfate attack, up to 1,110 days. Mercury intrusion porosimetry (MIP) and thermo gravimetric (TG) analysis were used to evaluate the pore structure and content of CH, respectively. The results show that there is a significant negative relationship between CH content and the coefficient of compressive strength during sulfate attack as more gypsum formation is contributed for the loss of strength. Compared to the reference sample, the addition of 20% and 40% fly ash decreased the CH content by 16.7% and 25.1% through the pozzolanic reaction, which reduced the formation of gypsum and prevented the decalcification of calcium silicate hydrate (C-S-H). Crystallization pressure, due to the formation of ettringite in a limited space, is discussed to explain the cracking of cement-based materials. Several harmful pores (>0.1 µm) were formed in the reference sample due to crystal pressure with the formation of ettringite, which caused the expansion and exacerbated the diffusion of sulfate ions. The pozzolanic reaction of fly ash also improved the pore structures and reduced the degree of supersaturation of ettringite by preventing the ingress of external sulfate ions. [ABSTRACT FROM AUTHOR]

If not properly managed, the hybrid powders produced during recycling mixed construction and demolition (C&D) materials will present a serious hazard to the environment and human health. Recent studies have found that a potential green way to reuse these powders is to add them as a cement replacement in concrete. In this study, the mechanisms of the concrete-clay brick hybrid recycled powder (CB-HRP) in cement hydration have been comprehensively investigated by utilizing isothermal calorimetric test, Raman spectroscopy measurement, and nanocharacterization including atomic force microscopy (AFM) and scanning electron microscopy (SEM) imaging analysis. Based on its behavior in heat generation, it is found that the CB-HRP serves as a filler material in concrete that exhibits a higher heat of wetting than normal portland cement. In the Raman identification, the spectrum of concrete containing the CB-HRP is compared with that of the benchmark samples. It is noticed that the calcium-silicate-hydrate (C-S-H) peak in the spectrum, which is absent in over two-thirds of the samples made of normal concrete, appears in all the samples containing the CB-HRP. In the SEMscan, the backscattered electron (BSE) technique was used to probe the distribution of different phases in concrete microstructure. Based on the contrast image, it is found that C-S-H phase and calcium hydroxide (CH) phase are precipitated along the boundary of the CB-HRP. This is further confirmed by the modulus profile and phase distribution obtained by the AFM characterization. The present investigation shows that the CB-HRP will promote the cement hydration by serving as a precipitation host. [ABSTRACT FROM AUTHOR]

A mathematical model for the transport process of migrating corrosion inhibitor (MCI) in cement-based material and a two-dimensional numerical solution method using commercially-available software are proposed. The numerical analysis results are in a good agreement with experimental data obtained from the two-dimensional absorption tests for nonreactive MCI but do not coincide with the data for reactive MCI. The reason behind may be that the latter reacted with calcium hydroxide (CH) in cement mortar and hindered the transport process. Therefore a modified transport model involving the pore-blocking effect of reactive MCI in cement-based materials is established according to the chemometrics method. The numerical and test results prove that the amendatory model is reasonable to understand the mass transfer of reactive MCI in cement mortar. Variance analyses of the factors in the model are also performed to find the key factor affecting the MCI transport process. A basic transport model and numerical analysis method of MCI in concrete is proposed. The model is corrected involving pore-blocking effect for reactive MCI based on chemical metrology. The numerical calculation results of the proposed model have a fine agreement with experimental results. [ABSTRACT FROM AUTHOR]

In the research reported in this paper, an in-situ observation on the hydration of cement pastes with various water-to-cement (w=c) ratios was implemented by Raman spectroscopy (RS). The RS was applied to study the paste continuously after cement mixing with water until the final setting was complete. Hydration products, including calcium hydroxide (CH), calcium-silicate-hydrate (C-S-H) gel, and ettringite were detected. The CH and C-S-H gel were present consistently with slight intensity changes in the RS patterns after their first detections. In all the pastes studied, the intensity of ettringite remained constant first, followed by a rapid increasing period; thereafter it began to decrease. Within the measuring duration a new peak assigned to monosulfate was detected in the pastes with w=c ratios of 0.35 and 0.40. These Raman detections were also analyzed together with the setting time and heat evolution to elucidate the early hydration mechanism. [ABSTRACT FROM AUTHOR]

The low degree of hydration of fly ash (FA) and slag (BFS) particles in high-volume FA and BFS concrete offers the possibility to activate the unreacted particles upon crack formation to close the crack. In this paper, a preliminary study is performed to evaluate the use of alkaline activators to stimulate the formation of reaction products in the crack. First, the reaction rates of crushed pastes mixed with alkaline solutions or water were monitored by calorimetry. These tests showed that alkaline activators stimulate the reactions more than water. Secondly, cracked mortar beams were stored in different alkaline solutions or water and the crack self-closing ratio was calculated after microscopic investigation at different time intervals. The results indicated that reference mixes, containing only ordinary portland cement as binder, heal very well autogenously, while a solution of Ca(OH)2+Na2CO3 seems to be the most suitable activator to stimulate crack closing of BFS concrete. Thirdly, the regain in strength due to healing of the cracks was determined. Compared to other self-healing techniques, strength regain is limited (about 10-40%). [ABSTRACT FROM AUTHOR]

Some researchers suggest the use of residual rice-husk ash (RHA) finely divided as a mineral addition to cement, despite the reduced content of amorphous silica compared to RHA produced by controlled burning. This article presents an investigation of the pozzolanic activity of residual RHA A and residual RHA B, with high and low contents of amorphous silica, respectively, showing different particle size distributions. Scanning electron microscopy (SEM), 29Si magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy, and determination of amorphous silica through the rapid analytical method were used for the microstructural characterization of the RHA samples. The RHA B with low content of amorphous silica increases the consumption of calcium hydroxide when its particle size decreases. [ABSTRACT FROM AUTHOR]

The microstructural development of lime putty is widely recognized and commonly associated with an increase in quality. Many heritage institutions encourage the systematic application of lime putties in conservation; however, all the issues associated with age are not fully understood. Changes occur during storage underwater, which traditionally took place in pits to prevent carbonation. The properties of mortars, plasters, and decorative finishes formed from lime putties are intimately linked to manufacturing parameters such as storage time. The hydration of calcium oxide was investigated using an environmental scanning electron microscope with particular attention to the nucleation of calcium hydroxide crystals on the surface of calcium oxide. Crystal modifications were monitored. The results obtained confirm and quantify the influence of slaking time in the microstructure of lime putties that is also reflected in the current referred special characteristics of the mortars produced with them, such as greater plasticity, water retention, and reactivity of mortars. The results also emphasize that the most significant microstructural changes of the lime putties start to consolidate after 8 months of slaking and appear to continue to develop, even though at a slower rate, until at least 5 years. [ABSTRACT FROM AUTHOR]

Some researchers suggest the use of residual rice-husk ash (RHA) finely divided as a mineral addition to cement, despite the reduced content of amorphous silica compared to RHA produced by controlled burning. This article presents an investigation of the pozzolanic activity of residual RHA A and residual RHA B, with high and low contents of amorphous silica, respectively, showing different particle size distributions. Scanning electron microscopy (SEM), 29Si magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy, and determination of amorphous silica through the rapid analytical method were used for the microstructural characterization of the RHA samples. The RHA B with low content of amorphous silica increases the consumption of calcium hydroxide when its particle size decreases. [ABSTRACT FROM AUTHOR]

Fly ash changes the microstructure of paste and can increase concrete durability. Calcium hydrated lime was added to concrete with 50% of the cement, by weight, substituted with fly ash, with the intent of replacing some of the calcium hydroxide consumed by the pozzolanic activity. Three proportions of mixes were studied: concrete with high early strength cement (HESC), concrete with 50% HESC and 50% fly ash, and concrete with 50% HESC, 50% fly ash and an additional 20% of hydrated lime. They were then tested according to ASTM C1202, but with the application of a voltage of 30 V during 35 h. Results revealed that concretes containing fly ash and hydrated lime presented a lower accumulated charge density and coefficient of chloride diffusion than the HESC concrete used as a reference. [ABSTRACT FROM AUTHOR]

The effect of calcium hydroxide on the mechanical and microstructural characteristics of geopolymer produced from alkaline activation of aluminosilicate wastes produced from demolition works were investigated. Demolition wastes are coarse aggregates, including waste concrete and demolished walls with cementing binder, both passing a sieve of 90 µm. A trial was carried out by making a mix containing demolished walls/waste concrete in the ratio of 60/40 % by weight and other mixes having a hydrated lime content of 0-25% of the dry weight as a partial replacement for demolished walls. Half of the waste concrete was substituted with metakaolin, produced by firing kaolin at 800°C for 3 h. The alkaline activators used were sodium hydroxide and sodium silicate in the ratio of 3∶3 % by weight of the dry weight. Curing of the geopolymer mixes was performed at ambient temperature (23°C) in tap water and under a mild temperature of 40°C [100% relative humidity (RH)]. Drying treatment at 80°C for 24 h was carried out on each specimen mix before subjecting it to compressive strength so as to improve the mechanical properties, and the resulting properties of dried and saturated samples were compared. Addition of the calcium compound improved the mechanical and microstructural properties of the specimens with the increase of hydrated lime up to 10 % by weight for mix specimens that were water-cured under ambient temperature, whereas the properties slightly decreased for those specimens cured in 100% RH at 40°C. [ABSTRACT FROM AUTHOR]

Fifteen hwangtoh-based alkali-activated concrete mixes were tested to explore the significance and limitations of the development of cementless concrete without carbon dioxide emissions while maintaining various beneficial effects. Hwangtoh, which is a kind of kaolin, was incorporated with inorganic materials, such as calcium hydroxide, to produce a cementless binder. The main variables investigated were the water-binder ratio and fine aggregate–total aggregate ratio to ascertain the reliable mixing design of hwangtoh-based cementless concrete. The variation of slump with elapsed time was recorded in fresh concrete specimens. Mechanical properties of hardened concrete were also measured, including compressive strength gain, splitting tensile strength, moduli of rupture and elasticity, stress-strain relationship, and bond resistance. In addition, mechanical properties of hwangtoh-based cementless concrete were compared with those of ordinary portland cement (OPC) concrete and predictions obtained from the design equations specified in American Concrete Institute 318-05 and Comite Euro-International du Beton-Federation International de la Precontrainte for OPC concrete, wherever possible. Test results show that the mechanical properties of hwangtoh-based concrete were significantly influenced by the water-binder ratio and to less extend by fine aggregate–total aggregate ratio. Based on the measured mechanical properties and code provisions, it can be proposed that the hwangtoh-based alkali-activated concrete is practically applicable as structural concrete when the water-binder ratio is less than 40%. [ABSTRACT FROM AUTHOR]

In this paper a complete characterization of lime cycle transitions is described. CaCO3 was collected from a Mexican mine and was processed to obtain Ca(OH)2 through CaO. When the calcium hydroxide interacted with the CO2 in the air forming CaCO3, the lime cycle was completed. Crystal structure and spectroscopy characterization was carried out to get the knowledge of lime cycle CaCO3→CaO→Ca(OH)2→CaCO3 deeply. The changes in the crystalline structures were described using X-ray diffraction by powder to accomplish phase identification. Scanning electron microscopy was done to examine the morphology, topology, crystal structure, and crystalline growth habit. Dynamic light scattering was used to determine the particle-size distribution of Ca(OH)2. Fourier-transform infrared was done to identify molecules and atomic bonds. Finally, inductively coupled plasma-mass spectrometry was done to get quantitative analyses. These results are a contribution to the restoration and construction industry fields, as well as a starting point to new possible applications of lime in current engineering. [ABSTRACT FROM AUTHOR]

Nine commercially available corrosion-inhibiting systems for use in concrete structures exposed to corrosive environments were evaluated on bridge barrier walls and in electrochemical cells. The results of this study are presented in two companion papers reporting on a five-year field evaluation and an accelerated laboratory investigation. The latter, presented in this paper, included the assessment of the effect of the corrosion inhibitors on the oxidation and reduction reactions by the cyclic voltammetry method, and their effectiveness in delaying or reducing corrosion by measurements of chloride thresholds and corrosion rates. The results indicated that the inorganic admixture and some organic admixtures performed very well in the saturated calcium hydroxide solution (pH of 12.6); however, their performance improvement over the control was not observed in a simulated concrete pore solution (pH of 13.5). Although tests in simulated concrete pore solutions cannot adequately simulate all the complex conditions usually found in the field, they were very useful to rapidly identify the effects of the corrosion inhibitors on the corrosion reactions, and to provide supporting information to the corresponding field evaluation. [ABSTRACT FROM AUTHOR]

The writers have carried out an investigation to develop apparent pH profiles of concretes for a variety of cement blends viz. normal portland cement containing by mass 30% pulverized fuel ash, 50% ground granulated blast-furnace slag, 10% metakaolin, and 10% microsilica, chosen to replicate common replacement levels, along with 100% normal portland cement (OPC) mix. The samples were exposed in an accelerated carbonation environment (5% CO2) for 6 weeks during which pH profiles were obtained every week as the concrete carbonated. Measurement of air permeability, carbonation depth, resistivity, and calcium hydroxide content were performed to assist in interpretation of the results. The nature of the pH profiles obtained depended on both the type of binder and the duration of exposure to the carbonation environment. Utilizing the pH profiles, a rate of carbonation was determined, which was found to depend on the type of binder. Both the rate of carbonation and the depth of carbonation after 6 weeks of exposure indicated that OPC concrete performed better than concretes containing supplementary cementitious materials. It was also determined that the gas permeability alone cannot provide an accurate indication of the likely rate of carbonation. The thermogravimetric analysis suggests the existence of a relationship between calcium hydroxide content and the apparent pH of carbonated concretes. On the basis of the results in this paper, it can be concluded that the pH profiles, using the technique described in this paper, can be used for measuring the carbonation resistance of concretes containing supplementary cementitious materials. [ABSTRACT FROM AUTHOR]

Two metakaolins, with similar mineralogical composition but which vary in their surface area (11.1 versus 25.4 m2/g), were evaluated for use as supplementary cementitious materials through measurements of workability, setting time, strength, elastic modulus, heat evolution, calcium hydroxide (CH) content, and surface area. Compressive and flexural strength of concrete were greater and increased at a faster rate when the finer metakaolin was used, as expected. The addition of metakaolin increased early age (i.e., 1–3 days) flexural strength by as much as 60%. The effect of metakaolin surface area on compressive strength was particularly evident at the lower water-to-cementitious materials ratios (w/cms) examined and generally at later ages (i.e., 7 days or later). However, although greater in the metakaolin–cement concretes than the ordinary concretes (particularly at the lowest w/cm examined, 0.40) elastic modulus measured at 28 days, was not affected by the metakaolin surface area. The greater surface area metakaolin caused a greater and more rapid heat evolution, indicating a higher reactivity and a greater rate of hydration product formation. Both metakaolins decreased CH content compared to controls, with the consumption of CH extending beyond 14 days. Surface area measurements indicated a more refined pore structure relative to controls by 28 days. These analyses illustrate the effect of metakaolin fineness on pozzolanic reactivity, associated CH consumption, and pore structure refinement, and suggest links to the observed increased mechanical properties of metakaolin–concretes. [ABSTRACT FROM AUTHOR]

The aim of this paper is to develop a model for simulating carbonation of fly ash concrete. The model describes the diffusion of gases (CO2 and water vapor) in concrete by the mass balance equations and the Fick’s law of diffusion. The time dependent hydration, and pozzolanic and carbonation reactions were considered for evaluating the amount of calcium hydroxide in concrete. The rate of carbonation reaction was formulated based on the Arrhenius’s formulation. The model was verified by using the results of relative water content, amount of calcium hydroxide, and carbonation depth tested in accelerated and real environments. The influence of changes in parameters on mix proportion and environmental condition were investigated. The verification results were found to be satisfactory in the prediction of carbonation of fly ash concrete. [ABSTRACT FROM AUTHOR]

This study deals with the carbonation of concrete incorporating ground granulated blast-furnace slag (GGBS), fly ash (FA), and silica fume (SF). It is observed that a decreased water-to-binder ratio and replacement level of GGBS, FA, or SF, or an increase in GGBS fineness and curing age in water, led to better carbonation resistance. However, compared to a plain concrete, the concrete incorporating mineral admixtures (except GGBS with higher fineness and SF) generally showed lower resistance to carbonation, possibly due to the dominating effect of the reduction in calcium hydroxide over pore refinement. Hence, adequate curing is recommended for enhancing the resistance of concrete containing GGBS, FA, and SF to carbonation. It is also found that both the carbonation coefficient and compressive strength served as good indicators for the carbonation rate of concrete with and without mineral admixtures. [ABSTRACT FROM AUTHOR]