Calcium Hydroxide Consumption Research Articles

Pozzolanic Reactivity of High-Alkali Supplementary Cementitious Materials and Its Impact on Mitigation of Alkali-Silica Reaction

The growing scarcity of conventional supplementary cementitious materials (SCMs) such as Class F, Class C fly ashes, and slag has necessitated exploring alternative SCMs previously considered suboptimal. In particular, high-alkali SCMs are often avoided because of the potential concern that their alkali content could release into the concrete pore solution, thus exacerbating the potential for alkali-silica reaction (ASR). However, preliminary research indicates that not all high-alkali SCMs are deleterious, and some can effectively suppress the ASR expansive reaction when used in sufficient dosage levels. This study evaluates the feasibility of using high-alkali SCMs, such as high-alkali natural pozzolans and reclaimed fly ashes, focusing on their pozzolanic reactivity and the correlation between the reactivity and their ASR mitigation performance. The pozzolanic reactivity of the SCMs was evaluated by the R3 test per ASTM C1897 and strength activity index test per ASTM C311. Thermogravimetric analysis was used to determine the calcium hydroxide consumption by the SCMs. ASR mitigation performance of SCMs was evaluated in accordance with American Association of State Highway and Transportation Officials (AASHTO) T380 miniature concrete prism test. Additionally, pore solution expression and analysis of paste specimens were conducted to determine the correlation between the total alkali and the released alkali levels into the pore solution. Based on the results of this study, all SCMs indicated high pozzolanic reactivity; however, individual performance varied by test method. Ultimately, the high-alkali SCMs, particularly natural pozzolans, did not appear to release any significant levels of alkalis into the pore solution readily and, therefore, show potential for ASR mitigation when used in sufficient dosage levels.

Thermodynamic modelling of alkali-silica reactions in blended cements

Experiments and field observations have shown that supplementary cementitious materials (SCMs) mitigate the risk of alkali-silica reaction (ASR) in Portland cement blends, but this is difficult to describe thermodynamically due to the lack of appropriate thermodynamic database information, and challenges in defining the kinetic factors. This paper examines how four well-known SCMs (namely ground granulated blast furnace slag, silica fume, metakaolin, and fly ash) mitigate ASR, by using a multi-stage simulation and a newly extended thermodynamic database. Calcium hydroxide consumption by SCM hydration, dilution of available alkalis, and alkali binding in calcium-alkali-aluminosilicate hydrate solid solutions, play pivotal roles in understanding the influence of SCMs in ASRs. Blending aluminium-rich SCMs in ASR-prone systems appears to act by preventing the dissolution of reactive silica in aggregates rather than by suppressing the formation of ASR products. This paper provides new thermodynamic insights into the mechanisms by which SCMs enhance concrete durability against ASR.

Investigating the Hydration, Mechanical Properties, and Pozzolanic Activity of Cement Paste Containing Co-Combustion Fly Ash

The heat of hydration, mechanical properties, pozzolanic activity, and microscopic characteristics of cement pastes incorporating co-combusted fly ash (CCFA) were investigated, and the disparities between the CCFA/cement system and the coal fly ash (CFA) binding system were also compared. The results indicate a decrease in the heat of hydration for both CFA and CCFA samples, with a more pronounced trend observed as the fly ash content increased from 10% to 30%. The distinction in the early hydration between CFA and CCFA samples primarily manifested in the rate of heat release, potentially correlated with variations in the active Al2O3 content in the fly ash. Neither CFA nor CCFA samples exhibited significant cementitious activity at 3 days, functioning solely as inert fillers in the cement paste. By 3 and 28 days, the mechanical properties of both CFA and CCFA samples were inferior to those of pure cement paste. However, by 180 days of hydration, the compressive strength of CCFA-blended mortar notably increased, with the highest strength observed in the 10% CCFA-blended sample. Both CFA and CCFA samples produced the secondary hydration product C-A-S-H and demonstrated comparable consumption of calcium hydroxide (CH). These findings underscore the potential of CCFA as a supplementary cementitious material (SCM) and lay a foundation for its widespread adoption.

Natural carbonation of portland cement with synthetic zeolite Y as a supplementary cementitious material

Risks associated with carbonation are a key limitation to greater replacement levels of ordinary portland cement (OPC) by supplementary cementitious materials (SCMs). The addition of pozzolanic SCMs in OPC alters the hydrate assemblage by forming phases like calcium-(alumina)-silicate-hydrate (C-(A)-S-H). The objective of the present study was to elucidate how such changes in hydrate assemblage influence the chemical mechanisms of carbonation in a realistic OPC system. Here, we show that synthetic zeolite Y (faujasite) is a highly reactive pozzolan in OPC that reduces the calcium content of hydration products via prompt consumption of calcium hydroxide from the evolving phase assemblage prior to CO2 exposure. Suppression of portlandite at moderate to high zeolite Y content led to a more damaging mechanism of carbonation by disrupting the formation of a passivating carbonate layer. Without this layer, carbonation depth and CO2 uptake are increased. Binders containing 12–18% zeolite Y by volume consumed all the calcium hydroxide from OPC during hydration and reduced the Ca/(Si+Al) ratio of the amorphous products to near 0.67. In these cases, higher carbonation depths were observed after exposure to ambient air with decalcification of C-(A)-S-H as the main source of CO2 buffering. Binders with either 0% or 4% zeolite Y contained calcium hydroxide in the hydrated microstructure, had higher Ca/(Si+Al) ratios, and formed a calcite-rich passivation layer that halted deep carbonation. Although the carbonated layer in the samples with 12% and 18% zeolite Y contained 70% and 76% less calcite than the OPC respectively, their higher carbonation depths resulted in total CO2 uptakes that were 12x greater than the OPC sample. Passivation layer formation in samples with calcium hydroxide explains this finding and was further supported by thermodynamic modeling. High Si/Al zeolite additives to OPC should be balanced with the calcium content for optimal carbonation resistance.

Mitigation mechanisms of alkali silica reaction through the incorporation of colloidal nanoSiO2 in accelerated mortar bar testing

Alkali Silica Reactivity (ASR) is a prominent issue concerning the durability of concrete structures. To mitigate this deleterious reaction, a widely accepted practice involves replacing significant cement portions with supplementary cementitious materials (SCMs). Given the scarcity of high-quality SCMs, nanoSiO2 exhibits considerable promise, given its minimal dosage requirement for ASR mitigation, whether applied independently or in conjunction with other SCMs. This research investigates the efficacy and underlying mechanisms of colloidal nanoSiO2 (CNS) in accelerated mortar bar tests according to the ASTM C1260 method. The experimental methodology incorporated two distinct forms of CNSes, CNS1 and CNS2, with average particle sizes of 14 and 67 nanometers, respectively. The outcomes are subsequently compared to the ASR performance of a sample containing silica fume (SF). The results demonstrate that the CNS size is pivotal in governing the pozzolanic reaction rate. The CNS2 and SF exhibited analogous pozzolanic reactions, displaying superior effectiveness in promoting pozzolanic activity compared to the CNS1. At a 4.75% cement replacement level, the CNS2 achieved a 78% reduction in mortar expansion over the control sample. The CNS1, when utilized at 4.75% and 7% cement replacement levels, yielded expansion reductions of 38% and 43%, respectively. It was found that the inclusion of the CNS can increase the porosity of mortar and contribute to silica dissolution from the reactive aggregates. Meanwhile, the addition of CNS improved the bulk resistivity of mortar by 200%-600% and flexural strength by 25%-62% based on the replacement levels and the CNS types. Moreover, improvements in calcium hydroxide consumption and alkali binding in the ASR gel were observed during the test, indicating the underlying mechanisms for expansion mitigation. These research outcomes can accelerate the CNS utilization to mitigate the ASR in practical field applications.

Synergic effect of metal-organic frameworks and process parameters on the properties of concrete subjected to accelerated carbonation

This study aims to develop a novel concrete that integrates a metal-organic frameworks (MOF) and accelerated carbonation curing to offset the emissions attributed to cement production. A MOF, NH2-MIL-125(Ti), was synthesized to sequester CO2 in concrete through accelerated carbonation curing. Its impact on the concrete properties was assessed while examining various process parameters, including MOF content, initial curing duration, carbonation curing duration, and carbonation curing pressure. The parameters were evaluated through CO2 uptake, phenolphthalein indicator solution, compressive strength, water absorption, and volume of permeable voids. The microstructure of carbonated MOF-incorporating concrete was characterized using powder X-ray diffraction analysis, scanning electron microscopy, and Fourier transform infrared spectroscopy. Experimental results showed that the addition of MOF promoted a higher degree of carbonation of cement, especially with a longer initial curing duration and higher pressure. Incorporating up to 6% MOF, by cement mass, in concrete cured for 20 h in open air, followed by 20 h of carbonation curing at a pressure of 1 bar, led to a CO2 uptake of 19%, carbonation depth of 11 mm, and 28-day strength of 46 MPa, water absorption of 4%, and permeable pore voids volume of 11%. Exceeding 6% MOF addition did not improve the uptake or performance. Microstructure characterization highlighted the formation of calcite, aragonite, and calcium silicate hydrate and the consumption of calcium hydroxide. The MOF-incorporating concrete can be used in construction applications to mitigate CO2 emissions while maintaining concrete performance.

Application of sulfate ion fixation in internal sulfate attack: The gel containing barium salt

Sulfate attack is one of the most important factors affecting the durability of concrete, and the corrosion sources of internal sulfate attack (ISA) mostly come from the raw materials of concrete. Adding corrosion resistance admixtures to concrete is one of the most effective ways to prevent internal sulfate attack. By wrapping barium salt with SA gel, the modified barium chloride admixture (MB) is obtained which can exist stably in concrete and release barium ions to form barium sulfate in a sulfate environment. The modified barium salt has little effect on the cement workability and hydration process and even can improve the mechanical properties slightly. When the MB content reaches 6% (MB-6), the strength improvement effect is most obvious. Compared with the P-0, the 7d compressive strength of MB-6 is increased by 9.14%, and the 28d compressive strength is increased by 5.77%. It can be seen by SEM that the amount and crystal size of ettringite in the cement mixed with admixtures are reduced. The corrosion resistance of the matrix can be further improved by using a modified barium chloride-silica fume composite admixture (MB-F). After 90 days of internal sulfate attack, the strength of the MB-F even improves slightly. When the modified barium chloride to silica fume ratio is 4:6, the cement-based material achieved the best resistance to internal sulfate attack. In addition to MB, which can make sulfate ions fixation and inhibit the formation of ettringite, silica fume can also consume calcium hydroxide and generate more C-S-H gel, which compacts the weak part of the matrix and greatly improves the resistance to internal sulfate attack of cement-based materials. It provides the possibility for the application of modified barium salt in external sulfate attack.

Assessment of Pozzolanic Reactivity for Calcined Spent Bleaching Earth Ash (SBEA) using Strength Activity Index (SAI), Frattini Test and X-Ray Diffraction (XRD)

This research briefly investigates and compares three different and distinct methods of assessing the pozzolanic reactivity of a cementitious replacement material, Spent Bleaching Earth Ash (SBEA). SBEA is waste by-product of the edible oils refining industry and has some pozzolanic properties that could possibly be enhanced through calcination. Many methods have been developed to assess the reactivity of pozzolans and three of the most common ones were employed in this research; namely the Strength Activity Index (SAI), Frattini Test and X-Ray Diffraction (XRD) methods. These three methods were chosen as they measured reactivity based on fundamentally different methods; the SAI method relies on the mechanical strength of mortar cubes, XRD based it on the morphology of the material whilst the Frattini test looks into consumption of artificially-introduced calcium hydroxide by the pozzolan. This investigation allows correlation to be drawn between the three methods and at the same time, the effectiveness of calcination on SBEA can also be evaluated. The calcined SBEA was obtained through a 700°C heating process in the furnace at the for 4 hours. Both the SAI and Frattini tests were in good agreement and showed that the calcined SBEA consistently under-performed compared to uncalcined SBEA at both testing ages. This was, however, disputed by the XRD result which showed that SBEA benefitted from the calcination process as it lost over 7 % in crystallinity, hence making it more reactive. This outcome tells us that the selection of testing methods for pozzolanic reactivity must be done carefully and ideally several methods be used concurrently in order to correlate the results.

Investigating the governing factors influencing the pozzolanic activity through a database approach for the development of sustainable cementitious materials

Pozzolans, known to possess high pozzolanic activity, enhances the long-term engineering properties of concrete due to the consumption of calcium hydroxide and the consequent formation of the calcium-silicate-hydrate gels within the cementitious matrix. Although the key factors that affect the pozzolanic activity such as the chemical composition, amorphousness, and fineness are commonly addressed in literature, there is a growing need to further gain an insight into the factors that govern this activity in a more comprehensive approach. The aim of this empirical study is to develop concrete models comprising optimal replacement of pozzolans based on the governing factors affecting the activity through the database approach. The database, consisting of 631 number of data points harvested from the literature, is established to determine the optimum replacement levels of the designated pozzolans in concrete. The governing factors therefore played a key role in establishing the boundary conditions that enabled the potential concrete models to be generated particularly for the sustainability assessment of concrete incorporating pozzolans. The study shows that the optimum replacement levels in concrete mixtures are 15–50% for GGBS, 10–35% for fly ash, and 5–15% for silica fume. The study furthermore demonstrated that the utilisation of these substitutions leaded a considerable reduction in carbon emissions that ranged from 13% to 43% for GGBS, 9–31% for fly ash, and 4–13% for silica fume. The study significantly contributes to the generation of greener construction materials, and offers a cleaner disposal route for the pozzolans principally compared to the traditional waste management alternatives.

Performance of sustainable ternary blended cement containing municipal solid waste incineration fly ash coupled with slag, coal fly ash or metakaolin

Developing low-clinker cements can effectively reduce carbon footprint of the cement industry. Additionally, more sustainable methods over landfilling are needed to tackle the increasing annual generation of municipal solid waste incineration fly ash (IFA). In this study, carbonated-washed IFA coupled with slag (SG), coal fly ash (CFA) or metakaolin (MK) were used to substitute 60 % of Portland cement to form ternary blended cement. Performance of the blended cements was investigated and compared with pure Portland cement in terms of hydration kinetics, binding capacity, hydration mechanisms, pore characteristics, microstructure, and leaching toxicity. Results indicated that in the blended cement systems, ettringite was stabilized by the formation of hemicarboaluminate as a result of the reaction between calcium carbonate in the IFA and aluminate in SG, CFA and MK. The SG displayed the best synergy with IFA, refining pore structure through pozzolanic reaction and increased formation of ettringite and hemicarboaluminate. For an optimal 40 % SG+20 % IFA blended cement, the mortar compressive strength reached 90 % of that for pure Portland cement at 90 days. Although CFA had abundant aluminate, its low reactivity restrained the performance of the blended cement, resulting in poorer pore structure and lower compressive strength. The MK had abundant reactive silicate and aluminate, but the rapid consumption of calcium hydroxide by the pozzolanic reaction and hemicarboaluminate formation restrained further reaction, limiting the performance of the blended cement at the later stage. Besides, the heavy metal leachability and dioxin level in the produced mortar satisfy the prerequisites to be categorized as nonhazardous according to HJ 1134. This study demonstrated the promising potential of the blended cements to simultaneously divert IFA from landfills and reduce the clinker content of cement.

Smart sustainable concrete: enhancing the strength and durability with nano silica

A detailed investigation was conducted to analyse the mechanical and durability features of a mixture of binary cement concrete modified with nanomaterials. In the context of the concrete matrix, the substitution of fractional cement content was carried out using Nano silica (NS) at concentrations of 1%, 2%, and 3%. Four distinct cementitious blends were subjected to a comprehensive match of tests, which encompassed compressive strength, flexural strength, split tensile strength, static modulus of elasticity, bulk density, water absorption, permeability, carbonation resistance, acid attack resistance, and rapid chloride penetration. The compositions of the mixes were investigated through the use of various microstructural analysis techniques, including scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (SEM-EDS), thermogravimetric analysis (TGA), X-ray diffraction (XRD), and Fourier-transform infrared spectroscopy (FTIR). The research revealed significant improvements in the mechanical and durability characteristics of the material. An increment in the mechanical and durability properties of mixtures were seen due to inclusion of marble power and Nano silica due to enhanced pozzolanic activities of composite and its filling effect. It is worth mentioning that Nano silica has shown the potential to mitigate the degradation caused by exposure to sulfuric acid. The SEM-EDX analysis demonstrated a decrease in the Ca/Si ratio when compared to the reference combination, suggesting an increase in the consumption of calcium hydroxide (CH) and the creation of a more compact calcium-silicate-hydrate (C-S-H) gel. The X-ray diffraction (XRD) findings indicate that NS has the ability to act as an accelerator for pozzolanic processes. This is achieved by consuming calcium hydroxide (CH) and promoting the creation of extra calcium silicate hydrate (C-S-H), which ultimately enhances the overall performance of the concrete mixture.

Effects of the heterogeneous hydration of alkaline-earth aluminosilicate glasses on phase evolution in hardened blended cement paste

Supplementary cementitious materials (SCMs) affect the hydration process and long-term properties of cementitious systems. Since a large percentage of SCMs belong to alkaline-earth aluminosilicate glasses that possess a phase-separated structure, further research into their hydration characteristics is essential. This study investigated the effects of the hydration characteristics of synthetic glasses with various phase-separated structure on the phase evolution of cementitious systems by combining simulations with experiments. The results showed that with the increasing degree of phase separation, the hydration of synthetic glass transitions from homogeneous to heterogeneous hydration. The phase evolution of hardened paste, particularly for calcium hydroxide, is influenced by the difference in hydration rates among each separated glassy phase. Compared to homogeneous hydration, the rapid hydration of the Ca-rich phase reduced the consumption of calcium hydroxide, while the rapid hydration of the Si-rich phase accelerated calcium hydroxide consumption. For glasses exhibiting phase separation characteristics, the consistency between thermodynamic simulations employing a heterogeneous hydration model and experimental results exhibits a notable enhancement.

4-year monitoring of degradation mechanisms of seawater sea-sand concrete exposed to tidal conditions: development of chemical composition and micro-performance

In tidal zones, the transfer of conductive ions accelerates concrete deterioration due to dry-wet cycles, temperature, and humidity gradients over time. Thus, this study investigates the effects of ground granulated blast furnace slag (GGBS), silica fume (SF), and metakaolin (MK) on the deterioration rate of seawater Sea-sand concrete (SWSSC) exposed to tidal conditions for four years caused by carbonation, sulphate and chloride penetration. Results showed continuous exposure to MgSO4, reduced Fe content and converted AFm into AFt, contributing to an increase in total SWSSC porosity of 12.9% while reduced to 10.2%, 7.4% and 7.3% with the optimal addition of GGBS, SF and MK. In combined sulphate and chloride-rich conditions, enhanced gel phase physical properties were more influential than AFm phases in increasing chloride binding capacity. SF suppressed AFt expansion, formed dense C−S−H with optimal Ca/Si ratio and physically increased the chloride binding capacity of SWSSC. Similarly, despite its high alumina content, MK restricted chloride mobility mainly through C−S−H and, subsequently, AFm phases. Additionally, through its initial acceleration of calcium-hydroxide consumption, SF reduced alkalinity and minimized the formation of CaCO3, outperforming MK in mitigating carbonation. Higher CaO and MgO in GGBS-mixed samples led to dolomite formation in severely carbonated zones, causing long-term mechanical strength reduction. In summary, SF significantly enhanced SWSSC durability and served as an effective additive in tidal-exposed SWSSC.

Comprehensive Review of Direct and Indirect Pozzolanic Reactivity Testing Methods

The production of portland cement is among the major contributors to greenhouse gas emissions that adversely affect the environment. Identifying sustainable materials to partially replace portland cement in concrete, such as pozzolanic materials, is crucial in addressing this issue. These materials mainly consist of silica and alumina that react with the available calcium hydroxide to form strength-bearing phases such as calcium silicate hydrates. Understanding the degree of pozzolanic reactivity of materials using efficient reactivity test methods is an important consideration. The paper thoroughly reviews the available literature related to direct and indirect pozzolanic reactivity test methods that have been utilized over the years. Direct methods quantify the amount of consumed calcium hydroxide, whereas indirect methods assess changes in the physical properties of the specimen due to pozzolanic reactions. The aim of this paper is to identify affordable, time-saving, and effective direct and indirect methods. Based on this study, the Frattini, electrical conductivity, and pH tests are considered the most time-efficient methods to assess pozzolanic materials. Electrical conductivity and pH tests are also easy to perform. In contrast, other methods are more time-consuming.

Enhancing physiochemical properties and reactivity of landfilled fly ash through thermo-mechanical beneficiation

This study investigates the impact of thermo-mechanical beneficiation techniques on the physiochemical characteristics and reactivity of landfilled fly ashes (LFAs) obtained from power plants in Wyoming and Colorado, USA. Thermo-mechanical beneficiation significantly improved the physiochemical properties of LFAs, leading to a decrease in loss in ignition (LOI) and an increase in fineness. The results revealed improvements in the strength activity index of all LFAs after beneficiation, surpassing the minimum requirement of 75% at 28 days. Beneficiation also enhanced heat release and calcium hydroxide consumption, indicating improved pozzolanic reactivity. The findings demonstrated that as beneficiation temperatures increased, there was a significant reduction in LOI, with levels falling below 3% at temperatures above 450 °C. The relationship between LOI and heat release indicated that reducing LOI could enhance the reactivity of fly ash. However, there was an inflection point at certain LOI thresholds, suggesting that further reductions in LOI may not significantly impact hydration kinetics. The results also showed that the specific surface area of LFA was correlated with heat release and calcium hydroxide consumption, emphasizing the importance of increased fineness for enhanced reactivity. The study also found associations between calcium hydroxide consumption, heat release, and early- and later-age strength activity indices. Higher calcium hydroxide consumption corresponded to increased heat release and reactivity, which in turn contributed to greater strength development over time, particularly at later.stages.

Effect of modified nano-SiO2 on properties and microstructure of the high-performance cementitious composites

The effect of modified nano-SiO2 (MNS) on the performance of high-performance cementitious composites (HPCC) was investigated and the underlying mechanism was revealed by various microscopic analysis and testing methods. Results indicated that the fluidity of HPCC initially increased and then decreased with the increase of MNS content. Meanwhile, The HPCC with 0.8% MNS exhibited a 25% increase in compressive strength at the curing age of 3 days, while the HPCC with 0.8% nano-SiO2 showed a 10% increase. Compared with the reference mixture, the durability and autogenous shrinkage of HPCC with MNS were significantly improved, the chloride diffusion coefficient of HPCC with 0.8% MNS decreased by 25.9%. Furthermore, the quantitative analysis results of hydration process and hydration products show that the inclusion of nano-materials accelerated the hydration rate of cementitious composites, resulting in increased cumulative heat release. HPCC with MNS exhibited a higher generation of amorphous phases and greater consumption of calcium hydroxide compared with reference mixture. Moreover, the addition of MNS effectively refined the pore size of HPCC and converted some harmful pores into less harmful or harmless pores, enhancing the microstructure of HPCC with MNS and improving the interface transition zone. This optimization was mainly attributed to the effect of filling, nucleation of crystal and pozzolanic reactive of nano-materials, which further benefit from the good dispersion of MNS, resulting in excellent mechanical properties and durability of HPCC.

Investigation of performance and mechanism of zinc removal from polluted water by concrete fines derived from aggregate recycling: From problematic byproducts to effective adsorbent

Concrete fines are byproducts produced from aggregate recycling. Because of their properties, they cannot be directly recycled for use in concrete manufacturing, which is problematic to move the cement and concrete industry toward sustainable development goals and reduce its environmental impact. Taking advantage of concrete fines unique properties was regarded as a possible research direction. The hydrated cement fraction was expected to provide alkalinity to neutralize the acidic solutions, while calcium related compounds were expected to provide the function of heavy metals removal. In this research, concrete fines were used to remove Zn from acid mine drainage as an active treatment. The removal performance was comprehensively investigated. The maximum capacity of Zn-adsorption is 111.9 mg/g, and almost 100% Zn can be removed for an initial Zn concentration of 20 mg/L. The dominant reaction mechanism of Zn adsorption to concrete fines was determined to be ion-exchange reaction with surface complexation and precipitation. The Zn2+ ions in solution can exchange with the Ca2+ ions in calcium silicates and calcium silicate hydrates in concrete fines and replace the protons released by ionization of the silanol group for complexation, and thus Zn removal is not limited to an alkaline environment or high initial Zn concentration. The acidity was alkalized by hydration reaction, mainly consuming calcium hydroxide. Based on these mechanisms, concrete fines are effective adsorbent to remove Zn without the need for the synergistic reactions of other metals and for making the aqueous solution strongly alkaline, even in the strongly acidic environments and in effluents with high Zn concentration. Through evaluation and comparison of Zn adsorption capacity with other materials, concrete fines were regarded as promising alternative adsorbent for Zn removal.

Monitoring consumption of calcium hydroxide via a new approach

In this computer-based experimental study, tracking the consumption of calcium hydroxide was carried out via a novel image processing approach done first in the literature. To do this, eight different cement mortars including four different mineral admixtures were produced and their fundamental mechanical and microstructural properties were investigated. After this step, these results were evaluated via an image processing approach. Regardless of curing age, test results show that there was a correlation between color pixels and fundamental mechanical and microstructural properties of cement mortars. Minimum and maximum differences between pixel indexes of R-G and B-G channels for the curing age of 7 days were determined as 11 and 30 while they were 9 and 22 for the curing age of 28 days. As expected, this is because CH is consumed as curing age goes by, comparatively a lighter pink/purple colour was observed in curing age of 7 days comparing to curing age of 28 days. Therefore, the advised current image processing approach made it possible to acquire a basic fundamental basis in terms of being a supplementary approach for destructive and non-destructive testing methods in terms of monitoring the consumption of CH.

The role of ultra-fine supplementary cementitious materials in the durability and microstructure of airport pavement concrete

Conventional Supplementary cementitious materials (SCMs) are indispensable constituents of high-performance concrete, despite their potential drawback of reducing the early strength of concrete. Ultra-fine supplementary cementitious materials (USCMs) have been employed to expedite the pozzolanic reaction and refine micro-scale particle gradation in airport pavement concrete. The investigation encompassed an assessment of mechanical properties, durability, phase assemblage, pore structure, morphology, and microstructure. Results showed that the incorporation of USCMs significantly enhanced the flexural and compressive strength of concrete during the early age, exhibiting respective improvements of 25.4% and 12.8%, respectively. Moreover, the freezing-thawing resistance was enhanced by a factor of 1.6, and the chloride penetration resistance was improved approximately 4 times. These enhancements can be attributed to the heightened pozzolanic and filling effects of USCMs, which resulted in the consumption of more calcium hydroxide (CH) at 7 d and facilitated the growth of additional calcium silicate hydrate (C-S-H) gel, along with lowering hydration heat and decreasing remarkably porosity. Notably, the incorporation of USCMs led to a considerable improvement in the volume fraction of harmless pores (with pore diameter ≤ 50 nm), optimizing the pore structure of the concrete and thereby augmenting its mechanical properties and durability.

Reactivity of waterglass in cementitious systems

This research investigates the chemical reactivity of waterglass, a sodium silicate (Na2O·nSiO2·yH2O). This research establishes a framework for thermodynamic modeling of waterglass systems that contain calcium hydroxide and potassium hydroxide. Conventional pozzolanic reactivity tests used for supplementary cementitious materials (e.g., fly ash) that rely on heat release and calcium hydroxide consumption cannot adequately capture the waterglass reactivity, primarily due to the high reaction rates. Capturing the heat released using isothermal calorimetry requires procedural changes in the testing protocol. Specifically, the test is modified by lowering the temperature of the test to slow the reaction rate and using internal mixing to capture the initial reaction. The heat release and calcium hydroxide consumption are used to quantify the reactivity. The theoretical relationship between heat, reactivity, and calcium hydroxide consumption is related to the molar ratio of SiO2 to Na2O, also known as the waterglass modulus (n). Thermodynamic modeling and X-ray powder diffraction results demonstrate that the mixtures react to produce Tobermorite-like calcium-silicate-hydrate (C–S–H, C/S = 1.42), which increases in amount with waterglass modulus. Finally, the developed approach demonstrates how the quantified reactivity is used in thermodynamic calculations to predict the reaction products and paste properties.