Calcium Hydroxide Crystals Research Articles

Target dispersion of pozzolanic materials nearby hydration-released calcium hydrates to improve the pozzolanic reaction degree

The random distribution of pozzolanic materials (PM) using direct mixing strategy reduces their contact possibility with hydration-released calcium hydroxide (CH), and limits the corresponding reinforcing efficiency to cement-based materials via pozzolanic reaction. In this study, a target dispersion strategy of PM nearby CH was proposed using super absorbent polymers (SAPs) as the carrier. On the one hand, nano silica (NS) and calcined clay (CC), two typical PM, can be encapsulated into SAPs through the hydrogen bonding interaction; on the other hand, the region around SAPs was enriched with more water and higher Ca2+ concentration, which triggered the in situ crystallization of CH. In this case, the contact possibility between the NS or CC (NS/CC) and hydration-released CH was improved by the target dispersion strategy, and resulted in a 60 %–253 % increase in pozzolanic reaction degree (PRD) of NS/CC. Furthermore, the reinforcing effect of NS/CC on compressive strength of cement paste was enhanced by 25 % through this strategy. Therefore, the current study introduces a novel method for increasing the pozzolanic reinforcing efficiency of cement-based materials by turning the random to target dispersion of PM nearby CH.

The effect of freeze-thaw cycles and sulfuric acid attack separately on the compressive strength and microstructure of 3D-printed air-entrained concrete

3D-printed concrete (3DPC) is a promising technique for constructing structures and intricate designs. To ensure the successful performance of these structures throughout their service life, it is essential to understand their long-term properties and durability. While the use of air-entraining additives can enhance the durability of concrete against freeze-thaw cycles, investigating their impact on other properties is crucial. This study examines the effect of freeze-thaw cycles and sulfuric acid attack on the durability of 3D-printed concrete containing air-entraining additives. The results indicate that, in addition to improving resistance to freeze-thaw cycles, air-entraining additives also increases resistance to sulfuric acid attack. The drop in compressive strength of the samples containing air-entrained additive (0.08–0.12 % AEA) is 1.4–5.3 % less than the control samples against the freezing and thawing cycles. Examining the mechanisms of action of these invasive agents also shows this. Creating intentional voids to reduce the internal pressure of water expansion during freezing and reducing the number and width of possible microcracks are the dominant mechanisms of air-entraining additives against freeze-thaw cycles. Besides, their performance mechanisms during acid attacks are somewhat similar. Analysis of elements created in the matrix of the samples after acid attack indicates a decrease in calcium concentration and an increase in silica concentration. Among the observed reactions, the second reaction prevails, leading to the breakdown of the C-S-H structure into gypsum and silica hydroxide. This phenomenon is associated with changes in acid concentration. However, the presence of air bubbles causes surplus plate-like Calcium hydroxide crystals resulting from the elevated cement content in printed mixes to form within these bubbles, making them susceptible to damage and fracture during freeze-thaw and acid attacks.

Reducing Calcium Dissolution in Tunnel Shotcrete: The Impact of Admixtures on Shotcrete Performance.

Groundwater infiltration into tunnels causes water to percolate through the fissure channels in the initial support shotcrete. This results in the dissolution and outflow of calcium hydroxide, a key product of cement hydration. This process significantly incurs the formation of crystallization blockages in the tunnel drainage systems. Optimizing the shotcrete mixing ratio is a feasible way to mitigate these blockages. Therefore, this study conducts calcium dissolution tests to investigate the impact of six admixtures, namely, antialkali agent, nanosilica, nanosilica carbonate, fly ash, sodium methyl silicate waterproofing agents, and silane waterproofing agents, on calcium dissolution resistance. Also, mechanical and microscopic tests are carried out to examine their impact on the strength and pore structure of the shotcrete. The objective of this study is to determine the optimal admixture for enhancing the calcium dissolution resistance of shotcrete. Results indicate that the antialkali agent significantly reduces the calcium leaching content of shotcrete. When the dosage is 14%, the calcium leaching amount is reduced by 68.4% in 28 days. Followed by nanosilica and silane waterproofing agents, with optimal dosages of 12 and 0.4%, respectively, the dissolution amount of calcium ions in shotcrete was reduced by 32.87 and 26.5%, respectively. Fly ash curing for 28 days can also reduce the calcium ion dissolution of shotcrete, while nanocalcium carbonate and sodium methyl silicate have little effect on the calcium dissolution of shotcrete. The antialkali agent with a strong calcium ion dissolution effect can improve the tensile strength of shotcrete under long-term curing conditions, which can be increased by 52%, but it compromises the growth of compressive strength. Nanosilica, fly ash, and silane waterproofing agents can improve both the compressive strength and tensile strength of shotcrete under long-term curing conditions. Specifically, at 28 days of curing, the compressive strength increased by 16.83, 28.8, and 20% and the tensile strength increased by 50.24, 60, and 64.5%. In addition, the microscopy results show that the antialkali agent, nanosilica, and silane waterproofing agents promote the hydration process of cement to form ettringite with a low and stable calcium-silicon ratio and reduce calcium hydroxide crystals. Nanosilica and silane waterproofing agents optimize the pore distribution in shotcrete by increasing beneficial pores, decreasing harmful pores, and reducing total porosity.

Low alkaline vegetation concrete with silica fume and nano-fly ash composites to improve the planting properties and soil ecology

Abstract Slope protection and erosion management are severely hampered by the rapid infrastructure development in mountainous valleys, especially during the monsoon season. While conventional approaches like vegetation, porous concrete, and inorganic procedures have been used, stronger and more ecologically friendly alternatives are still needed. A new kind of concrete called vegetation concrete (VC) allows roots to grow through the concrete frame by combining plant integration with porous concrete. This creative method might be used for environmentally friendly building and planting. The alkalinity of VC significantly impacts its planting capabilities and soil nutrient levels, making it crucial to reduce VC alkalinity. In this study, silica fume (SF) and fly ash (FA) were combined to create low-alkaline VC. The effects of SF and FA on VC’s alkalinity, porosity, compressive strength, and planting characteristics were examined. The study also investigated VC’s influence on soil fertility and its impact on soil nutrients. Test results revealed that SF and FA reduced the pH of the VC by reducing calcium hydroxide (CH) crystals. While SF had a lower basicity coefficient (M) than FA, it had a more significant effect on lowering VC alkalinity. The compressive strength decreased with FA but increased with SF, despite SF having a smaller cement component in VC–SF mixes. This suggests that blending VC with SF and FA is feasible, with the SF dosage exceeding the FA dosage for reduced alkalinity and increased strength. Lowering VC alkalinity through SF and FA increased soil nutrients, including hydrolyzable nitrogen (AH-N), extractable phosphorus (P), and potassium (K). It also improved planting properties like root development, stem height, and leaf relative water content. Using VC for soil stabilization did not reduce soil fertility but instead increased the available phosphorus and alkali-hydrolyzable nitrogen in the soil by 32.81 and 52.92%, respectively. The findings of this study open up new avenues for investigation into this technology and have important ramifications for the use of VC technology, particularly in Indian contexts.

Preparation and Performance Study of High-Strength and Corrosion-Resistant Cement-Based Materials Applied in Coastal Acid Rain Areas.

Investigations regarding the preparation and durability of cement-based materials applied in specific coastal acid rain environments are scarce, particularly those involving the addition of four auxiliary cementitious materials (ACMs) to cement for modification. To improve the durability of concrete structures in coastal acid rain areas, a systematic study was conducted regarding the preparation of high-strength and corrosion-resistant cement-based materials using ACM systems composed of fly ash (FA), granulated blast furnace slag (GBFS), silica fume (SF), and desulfurization gypsum (DG) instead of partial cement. Through an orthogonal experimental design, the effect of the water-binder ratio, cementitious ratio, and replacement cement ratio on the compressive strength, corrosion resistance coefficient, and chloride ion permeability coefficient of the materials were analyzed and the mix proportions of the materials were evaluated and optimized using the comprehensive scoring method. The results show that implementing a FA:GBFS:SF:DG ratio of 2:6:1:1 to replace 60% of cement allows the consumption of calcium hydroxide crystals generated through cement hydration, promotes the formation of ettringite, optimizes the pore structures of cementitious materials, and improves the compressive strength, acid corrosion resistance, and chloride ion permeability of the materials. This study provides a reference for selecting concrete materials for buildings in coastal acid rain environments.

The Impact of NaOH on the Micro-Mechanical Properties of the Interface Transition Zone in Low-Carbon Concrete.

To tackle carbon emissions from cement production and address the decline in concrete's mechanical properties due to the substitution of cement with solid waste (glass powder) and natural mineral admixture (zeolite powder) materials, we employed glass powder and zeolite powder to create composite cementitious materials. These materials underwent alkali activation treatment with a 4% NaOH dosage, replacing 50% of cement to produce low-carbon concrete. Nanoindentation tests and mercury intrusion porosimetry (MIP) were employed to uncover the micro-mechanical properties and influencing mechanisms of alkali-activated low-carbon concrete. The results indicate a notable enhancement in the indentation modulus (19.9%) and hardness (25.9%) of alkali-activated low-carbon concrete compared to non-activated concrete. Simultaneously, the interfacial transition zone thickness decreased by 10 µm. The addition of NaOH led to a reduced volume fraction of pores (diameter >100 nm) and an increased fraction of pores (diameter < 100 nm), thereby reducing porosity by 2.6%, optimizing the pore structure of low-carbon concrete. The indentation modulus, hardness and volume fraction of the hydrated phase derived from Gaussian fitting analysis of the nanoindentation statistics showed that NaOH significantly improved the modulus and hardness of the hydration products of low-carbon concrete. This activation resulted in decreased LDC-S-H gel (low-density hydrated calcium silicate Ca5Si6O16(OH)·4H2O) and pore content, while the HD C-S-H gel (high-density hydrated calcium silicate Ca5Si6O16(OH)·4H2O) and CH (calcium hydroxide crystals Ca(OH)2) content increased by 13.91% and 23.46%, respectively. Consequently, NaOH influenced the micro-mechanical properties of low-carbon concrete by generating more high-density hydration products, reducing pore content, enhancing the pore indentation modulus and hardness, and shortening the interfacial transition zone. This study offers novel insights into reducing carbon emissions and promoting the use of solid waste (glass powder) and natural mineral admixture (zeolite powder) materials in concrete, contributing to the advancement of sustainable construction practices.

Thickening progression mechanism of silica fume – oil well cement composite system at high temperatures

This work studied the thickening progression mechanism of the silica fume – oil well cement composite system at high temperatures (110–180 °C) in order to provide a theoretical guidance for the rational application of silica fume in the cementing engineering. Results showed that silica fume seldom affected the thickening progression of oil well cement slurry at 110–120 °C, but when temperature reached above 130 °C, it would aggravate the bulging degree of thickening curves and significantly extend the thickening time, meanwhile causing the abnormal “temperature-based thickening time reversal” and “dosage-based thickening time reversal” phenomena in the range of 130–160 °C and 170–180 °C respectively. At 130–160 °C, the thickening time of oil well cement slurry was mainly associated with the generation rate of calcium hydroxide (CH) crystal. The introduced silica fume would be attracted to the cement minerals’ surface that were hydrating to produce CH and agglomerate together to form an “adsorptive barrier” to hinder further hydration of the inner cement minerals. This “adsorptive barrier” effect strengthened with the rising temperature which extended the thickening time and caused the occurrence of the “temperature-based thickening time reversal” phenomenon. At 170–180 °C, the pozzolanic activity of silica fume significantly enhanced and considerable amount of C−S−H was generated, thus the “temperature-based thickening time reversal” vanished and the “dosage-based thickening time reversal” was presented.

Effect of curing regimes on the rheological, mechanical, hydration and microstructure of ultra-high-performance concrete (UHPC) using indigenous resources: progress towards sustainable construction practices

ABSTRACT Ultra-High-Performance Concrete (UHPC) was renowned for its exceptional mechanical strength and durability. While contemporary UHPC is made using high-quality materials and equipment, its enormous use of natural resources in construction has had global ecological impacts. Employing materials that are accessible locally without specialized procedures. This study concentrates on UHPC mixtures, formulated with indigenous materials, aiming to develop an economical product without the need for high-speed mixers or special fibers. The mixtures are designed with optimal proportions of various supplementary cementitious materials (SCM’s). The study begins with an analysis of flow behaviour, followed by an evaluation of mechanical properties using non-destructive techniques (NDT) under different curing conditions. The UHPC mixtures demonstrate compressive strengths from 70–85 MPa to 110–144 MPa under standard and hot water curing. Relationships between destructive and NDT test outcomes are also determined. The hot water curing process leads to the formation of calcium-silicate-hydrate(C – S – H) gel, with calcium-hydroxide (CH) crystals observed within the amorphous C – S – H gel. The use of high-temperature curing improves mechanical characteristics by (35–40%) and densifies the microstructure. Consequently, producing UHPC using widely accessible materials without further processing and common technologies like conventional casting and standard curing would increase its cost-effectiveness and environmental sustainability.

Hydration behavior and microstructure of cement-based materials modified by Field’s metal particles

The hydration of cement-based materials, especially at early ages, directly affects the development of matrix strength and the later durability. In this paper, in order to explore the hydration behavior and microstructure of cement-based composites modified by low melting point Field's Metal Particles (FMP), inverted cone method, standard Vicatometer method, isothermal calorimetry, and compressive strength test, TGA, SEM, and XRD are used to investigate the fresh properties, heat of hydration, matrix strength, hydration products and transition zone between FMP and cement matrix. The results show that the incorporation of FMP can improve the fluidity of cementitious composites by 31.6%, due to the hydrophilicity of FMP. Meanwhile, due to the small heat capacity and large thermal conductivity of low-melting FMP, the heat stored in FMP is naturally released and used to promote hydration reaction when the hydration rate slows down. Additionally, FMP does not affect the types of hydration products in cement-based composites, but the total amount of hydration products increased at 7 d, especially calcium hydroxide (CH). Unexpectedly, FMP can reduce the compressive strength of the hardened paste, due to the large particle size of FMP than cement particles and the presence of directionally growing CH crystals in the interface transition zone between FMP and cement matrix.

A comparative study of alkanolamines and inorganic additives on the microstructure development of blast furnace slag blended cement at low curing temperature

The initial reactivity and strength development of blast furnace slag cement (BFSC) are crucial factors that increase its utilization. Therefore, it is important to select the most suitable accelerator. In this study, the impact of four accelerators, namely sodium sulfate (Na2SO4), calcium nitrite (Ca(NO2)2), triethanolamine (TEA), and diethanolisopropanolamine (DEIPA), on the hydration and strength development of BFSC at curing temperatures of 5 and 20 °C was investigated. Isothermal calorimetry, X-ray diffraction (XRD), scanning electron microscopy (SEM), and mercury intrusion porosimetry (MIP) were conducted. The results revealed that Na2SO4 effectively enhanced early age strength by accelerating the hydration of the clinker and slag. Ca(NO2)2 is sensitive to temperature changes. At 20 °C, Ca(NO2)2 promoted the reaction of the aluminates and slag in the initial stage. However, the presence of slag inhibited clinker hydration, and the addition of Ca(NO2)2 exacerbated this effect, resulting in reduced strength at 5 °C. Indeed, the use of TEA and DEIPA at 0.02 % at 5 °C is not recommended. Complex interactions between TEA and Ca2+ may inhibit the formation of calcium hydroxide crystals, further inhibiting aluminate hydration. This may lead to low strength development at low curing temperatures such as 5 °C.

Utilization of nano-ZrO2 to improve the efficiency of carbonation curing of cement-based materials

Nanomaterials are characterized by large surface areas and ultrasmall particle sizes, which affect their micromorphologies. In this study, the impact of nano-ZrO2 on the efficiency of carbonation curing was systematically investigated to compare the mechanism for nano-ZrO2 in the hydration and carbonation reactions occurring during sealed and carbonation curing and the effects on the micro- and macroscopic properties. The experimental results showed that the hydration rate and degree of carbonation increased gradually with increasing nano-ZrO2 content. The effect of nano-ZrO2 on the hydration rate was more significant in the early period (0–72 h); the impact on the carbonation curing efficiency was more significant in the later period (28 days). After 28 days of carbonation curing, the compressive strengths of samples doped with 0.5% and 1% nano-ZrO2 were higher by 7.5% and 4.9%, respectively, compared to those of the samples without nano-ZrO2. Moreover, the resistivities of the samples doped with nano-ZrO2 were higher than that of the control sample (NZ-0%–CC). The efficiency of carbonation curing was mainly attributed to optimization of the substrate micromorphology by the nano-ZrO2. The addition of nano-ZrO2 decreased the orientation of the calcium hydroxide (CH) crystals in the mixture and reduced their sizes, leading to an increase in the reactivity of the CH. Moreover, with increases in the amount of nano-ZrO2 added, the calcium carbonate content of the carbonation product increased slightly. Thus, nano-ZrO2 improved the efficiency of concrete carbonation curing; high carbonation, high strength and high durability were realized by adding a small amount of nano-ZrO2 for microscopic morphology optimization.

Investigation on the reinforcing of cementitious materials by nano-hydroxyapatites

The brittleness of cement-based materials is a major obstacle to application in large-scale engineering projects. The structure of natural biological skeletons has inspired research into toughening cement. Here, we report a strategy for reinforcing cementitious materials by controlling its microstructure through two types of nano-hydroxyapatite (HAP). This strategy includes improving the dispersion of nano-HAP using polycarboxylic acid, studying the dosage of composite nano-HAP to enhance the toughness of cement-based materials, and synergistically analyzing the mechanism of action of nano-HAP using a multi-scale research framework. We found that when the content of modified nano-HAP was 0.5%, the bending strength of the specimens increased by 55% and 22% at 3d and 28d, respectively, while the compressive strength remained unchanged. Compared to ordinary cement hydration, nano-HAP drives the ordered growth of calcium hydroxide crystals inside the cement paste, leading the formation of a ‘brick-mud’ microstructure that alters the direction of crack propagation.

New insights into the nucleation of portlandite and the effects of polymeric additives

The crystallization of calcium hydroxide (Ca(OH)2, CH, portlandite) is a key process during the early stages of cement hydration. In the present work, we have revisited the formation of this mineral through nucleation and growth from supersaturated aqueous solutions, in the light of the currently emerging picture of multistage “non-classical” crystallization. To that end, we developed a titration-based assay, in which stock solutions of both relevant ions are added simultaneously into a reservoir, where supersaturation increases slowly at constant stoichiometry until nucleation occurs. This procedure allows both pre- and early post-nucleation phenomena to be analyzed quantitatively. Complementarily, the early stages of portlandite mineralization were probed by various advanced characterization techniques, including cryo-transmission electron microscopy (cryo-TEM), in-situ small-angle X-ray scattering (SAXS), pair distribution function (PDF) analysis of high-energy X-ray scattering (HEXS) data, and analytical ultracentrifugation (AUC). The experimental data show that the formation of calcium hydroxide starts with the association of ions into complexes and clusters, which subsequently coalesce to form amorphous nanoparticles – much like what has been observed in the case of calcium carbonate and other prominent minerals. Subsequently, these particles aggregate and build networks, which eventually transform into hexagonal Ca(OH)2 crystals. The presence of a soluble polycarboxylate – as a known inhibitor of portlandite crystallization – does not change the main characteristics of this multistep nucleation pathway, but it proved capable of significantly extending the lifetime of the amorphous intermediate phase and thus delaying the transition to the final crystalline phase. Our observations confirm the notion that “non-classical” crystallization is a much more common phenomenon than initially believed – and that, for minerals forming in aqueous environments, it may actually be the rule rather than the exception.

Distribution of porosity surrounding a microfiber in cement paste

This study investigates the microstructural changes of cement paste due to the inclusion of polymeric microfiber at different water-to-cement (w/c) ratios. A procedure to quantify the porosity of epoxy impregnated interfacial transition zone (ITZ) is also presented. Results show that the microstructures of the ITZ beneath and above a microfiber, with respect to the gravity direction, are largely different. Though the ITZ at both sides of the fiber are more porous than the bulk matrix, the porosity of the lower ITZ (i.e., the ITZ beneath a fiber) is significantly higher than the upper side (i.e., the ITZ above a fiber). This difference can be attributed to the combined effects of fiber on the initial packing of surrounding cement grains and on the settlement of the fresh mixture. The porosity gradients of the upper ITZs are found to be nearly identical for all the tested w/c ratios, while the porosity gradients of the lower ITZs become steeper when the w/c is higher. The lower side is also found to be the preferred location for the precipitation of calcium hydroxide crystals. Results of energy-dispersive X-ray spectroscopy (EDS) and nano-indentation analyses confirm that the chemical and mechanical properties of the ITZ are also asymmetric.

Mechanisms of hydration heat inhibitors on the early heat release process of cement

We evaluated the effects of hydration heat inhibitors on the early hydration heat release process of cement and its main mineral components. We used a microcalorimetric method to determine the effects of various proportions and properties of hydration heat inhibitors on the hydration of portland cement, tricalcium silicate, and tricalcium aluminate: concentration (C) = 40% m/m hydroxydiphosphonic acid (HEDP) (1-hydroxyethylidene-1,1-diphosphonic acid) and C = 40% m/m diethylene triamine pentonyphosphonic acid (DTPMPA) (diethylenetriaminepentamethylene phosphonic acid). We also analyzed and tested the heat release rate and cumulative heat release during the hydration of cement and its main mineral components. The hydration heat inhibitors decreased the heat release rate of cementitious materials by means of adsorption, chelation, precipitation, complex formation, and control of calcium hydroxide crystals. Among these materials, the hydration heat inhibitor had the most substantial effect on the composition of tricalcium silicate clinker, reducing the peak temperature at the initial stage of hydration and delaying its occurrence time. These results are pertinent to controlling and selecting the early hydrothermal release process of cement systems.

Investigating the potential of trass-cement binary blend for enhancing microscopic properties, macroscopic performance, and sustainability of cement paste

• Effects of trass on micro- and macro-level properties of cement paste were evaluated. • Blending cement with trass accelerated the C 3 S and C 3 A hydration. • Higher nonevaporable water contents were detected in blended sample compared to plain one. • The values of compressive strength activity index fluctuated between 106.7 and 124.6%. • Incorporating trass into cement can lead to a cleaner, cheaper, and less energy consumer construction. Due to some physical or chemical incompatibility issues between supplementary cementitious materials (SCMs) and cement, it is necessary to investigate the microscopic properties and macroscopic performance of cement composites containing such blends before their incorporation in construction. Hence, in this study, the hydration kinetics, degree of hydration, microstructural morphology, and pore structure of cement paste blended with trass, as well as its mechanical and transport properties, were profoundly evaluated. Moreover, the potential of trass for enhancing the sustainability of construction was also assessed. The physical properties of trass accelerated the C 3 S hydration, while its chemical composition led to the reduction of induction period, in addition to the acceleration of C 3 A hydration. Significant refinement of pore structure characteristics was found in blended sample after 90 days, when the threshold and critical pore diameters were reduced by respectively 27.7 % and 10.7 %, compared to those of the plain sample. This denser microstructure was reflected in superior electrical resistivity of the blended sample, which was 2.1 times of the plain one at 90 days. Furthermore, the inclusion of trass through refining the morphology of CH crystals in the microstructure, besides lowering the calcium hydroxide (CH) content by 12.1–63.3 % and increasing the nonevaporable water content by 2.1–8.0 %, led to 6.7–24.6 % higher compressive strengths in blended sample. Regarding the sustainability, at a similar 90-day compressive strength of 40 MPa, using trass results in an approximately 21.7 % less energy consumer, 20.3 % cleaner, and 15.8 % cheaper construction. Moreover, at equal levels of energy consumption or CO 2 emission, the blended sample has greater compressive strength and electrical resistivity than the plain one, which can be indicative of an extended service life. Accordingly, the utilization of trass is recommended since it has the potential to improve the sustainability both directly and indirectly.

Evaluation of the microstructure and micromechanics properties of structural mortars with addition of iron ore tailings

The Interfacial Transition Zone (ITZ) between the cement paste and aggregates is a region of great interest for concretes because it is the composites' weakest region. ITZ presents a great amount of large calcium hydroxide and ettringite crystals, with the porosity being able to be up to 2.5 greater than that the rest of the paste. The microstructure, and consequently the ITZ, can be improved by mineral addition. These materials fill the pores in the composite, influence the hydration process and densify the matrix. Therefore, mineral additions, such as iron ore tailings (IOT), can modify microstructure of composites. There are few studies on the assessment of IOT heterogeneity on the microstructure and hardness of structural mortar. Thus, the present study analyzes the properties influences of four IOT types on the microstructure, porosity, and thickness of the structural mortars' Interfacial Transition Zone (ITZ) with IOT addition, by different evaluation methods. The composites were characterized through the Scanning Electron Microscope by backscattered electrons (SEM-BSE), line scan by Energy Dispersive Spectroscopy and nanoindentation. Results presented that IOT improved particle packing and tended to reduce the ITZ. The IOT improved nucleation since it reduces the amount of anhydrous cement particles and increases the amount of calcium hydroxide particles in the cement matrix. The nanoindentation showed that IOT-added matrix presented greater hardness and indentation module. It can be concluded that the IOT heterogeneity may affect microstructural properties and that the methods presented can be good ways to evaluate the parameters.

A study on the characteristics of a novel inorganic solidified foam for the prevention and control of the spontaneous combustion of coal

• Response surface methodology was used to optimize formulation of solidified foam. • Mechanism of foaming and curing reaction of NICF was clarified. • Rice husk ash with high pozzolanic activity replaced fly ash. • Core parameters of the mixing device were determined by numerical simulations. The spontaneous combustion of coal seriously threatens the safe operation of coal mines. In order to solve the problems of the traditional inorganic solidified foam (TICF), i.e., a long gelation time, difficulty in uniform mixing, and low efficiency for blocking air leakage, a novel inorganic solidified foam (NICF) was proposed in this study to prevent the spontaneous combustion of coal; it was prepared by using rice husk ash (RHA), xanthan gum (GX), and sodium silicate (WG) as modifiers. Plackett-Burman experiments and response surface methodology were used to optimize the expansion ratio, stability coefficient, and gelation time of NICF and to determine the formulation of NICF in order to achieve rapid sealing of air leakage channels. A spiral hollow mixing device (SHMD) was developed based on the characteristics of NICF. The mixing effect of an aqueous foam (AF) and compound seriflux (CS) was numerically simulated by the ANSYS FLUENT software, and the SHMD parameters were determined with a high expansion ratio and uniform mixing of grout. The mechanism of the foaming and curing reaction of NICF and its mechanism for preventing the spontaneous combustion of coal were obtained by means of scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and X-ray diffraction (XRD). The experimental results indicated that WG hydrolyzed to produce an orthosilicic acid sol and sodium hydroxide in the process of particle hydration, and the sol could bind the particles to the hydration products. In addition, WG dissociated to generate a large amount of orthosilicate ions ([SiO 4 ] 4- ), which combined with the calcium ions present in a high concentration on the surface of the hydrated particles; this considerably reduced the gelation time of NICF. Ca 2+ on the surface of the hydrated particles chelated with the active functional groups such as the hydroxyl and carboxyl groups in GX, improving the compressive strength of NICF in the initial hydration stage. Compared with fly ash, rice husk ash has higher pozzolanic activity, and it undergoes a secondary hydration reaction with numerous calcium hydroxide crystals, increasing the hydration degree. This study can provide a theoretical basis for reducing the gelation time of NICF and explaining its foaming and curing reaction mechanism.

Effects of admixtures on the mechanical characteristics and microstructure of coral aggregate mortar

Coral mortar is often used to reinforce building foundations in island-reef engineering. To enhance the mechanical characteristics of coral aggregate mortar, and hence facilitate the development of ocean engineering, this paper studies the influences of silica fume (SF), metakaolin (MK), blast furnace slag (BFS) and fly ash (FA) on the workability, uniaxial compression process and microstructure of coral aggregate mortar (CAM). Ultrasonic pulse velocimetry is used to investigate the effects of various additives on ultrasonic propagation speed and cumulative voltage energy (CVE). The microstructure and hydration products of different CAMs are analyzed using scanning electron microscopy. Experimental results show that silica fume is more effective than blast furnace slag and fly ash in improving density and water retention of CAM. Silica fume can promote the consumption of calcium hydroxide crystals and the formation of calcium silicate hydrate, thus improving the compressive strength of CAM. There is a good correlation between S-wave velocity and elastic modulus in CAM. The CVE of the P-wave in CAM with external admixture is significantly higher than that in ordinary CAM. Addition of blast furnace slag and fly ash makes the CVE of the ultrasonic wave in CAM decrease earlier and faster. The results provide a guide for the improvement of the strength of CAM.

Comparison of lightweight and normal-weight aggregate concrete

In this experimental study, lightweight aggregate concrete (LWAC) and normal-weight aggregate concrete (NWAC) were compared within the contexts of the size of calcium hydroxide (CH) crystals in interfacial transition zone (ITZ), compressive strength, and oven-dry density. Six LWAC and NWAC mixtures were prepared for this study. Thirty-six images obtained from Scanning Electron Microscope (SEM) were used to determine the size of CH crystals in ITZ of LWAC and NWAC. Eighteen test specimens (three for each of the six LWAC and NWAC mixtures) were prepared in 150x300 mm sizes and in the form of cylinders for the compressive strength tests and also eighteen test specimens in 100x100x100 mm sizes and in the form of cubes for the oven-dry density tests. It was determined that the size of CH crystals in ITZ of LWAC is 8.43% less than (on average), compressive strength of LWAC is 39.09% more than (on average), and oven-dry density of LWAC is 10.97% less than (on average) the NWAC’s that has the same volumetric proportions of ingredients. The findings of this study show that lightweight aggregate that has high particle density, angular shape, rough surface texture, and a structure that enables chemical reaction with CH crystals will be beneficial for the ITZ microstructure and properties of concrete. It is considered that these properties should be taken into consideration in the selection of lightweight aggregate for structural concrete production.