Calcium Hydroxide Crystals Research Articles

Study on the mechanical behavior and micro-mechanism of concrete with coal gangue fine and coarse aggregate

Coal gangue is waste rock generated from coal mining, which results in vast land occupation in mines and severe environmental pollution. Owing to the similarity between coal gangue and natural aggregates, concrete preparation from coal gangue has been considered effective for solving gangue problems. In this study, to further improve the on-site utilisation rate of coal gangue, coal gangue was used to replace fine and coarse aggregates for preparing mortar and concrete. The macro and microscopic characteristics of coal gangue mortar (CGM) and coal gangue concrete (CGC) were investigated while considering the effects of coal gangue fine aggregate (CGFA) replacement ratio, curing time, and water-to-cement ratio. To investigate the macro properties, flexural, compressive, and splitting tensile tests were performed. The results show that CGM and CGC with an appropriate CGFA replacement ratio of 25% exhibited optimal performance. Additionally, by replacing all aggregates with CGFAs and coal gangue coarse aggregates, the 28 d compressive strengths of samples W0.5R100 and W0.4R100 were 44.8 and 52.24 MPa, respectively, which satisfied the C30–C50 concrete strength requirements. Moreover, a model is proposed for predicting the splitting tensile strength of the CGC to address the overestimation of existing national standards. For microscopic analysis, X-ray diffraction, thermogravimetric analysis, and scanning electron microscopy were conducted to illustrate the hydrated products and micromorphology of CGC under different conditions, as well as to discuss the effect of the mechanism on strength. Based on the micro analysis, the mechanism of CGFAs is summarised as a coal gangue–Ca(OH)2–H2O reaction system: volcanic-activated SiO2 and Al2O3 in CGFAs react with calcium hydroxide (CH) crystals, and more CH is consumed; subsequently, more calcium silicate hydrate gels and ettringite crystals are produced in the secondary hydration reaction, contributing to a denser microstructure and improvement in concrete strength at the late stage. In particular, gismondine is observed as a new hydration product in the reaction of CGC, which differs from that of normal concrete. This study provides a basis for the large-scale utilisation of coal gangue in situ.

Effects of bacteria-embedded polylactic acid (PLA) capsules on fracture properties of strain hardening cementitious composite (SHCC)

Strain hardening cementitious composite (SHCC) is a special class of ultra-ductile material which has autogenous self-healing capability due to its intrinsic tight crack widths. To further improve its healing ability, healing agent (HA) can be incorporated in SHCC, enabling it also the autonomous self-healing mechanism. In this study, the effects of adding bacteria-embedded polylactic acid (PLA) capsules on the mechanical properties of SHCC with different amounts of HA (i.e., 1.25%, 2.5%, 5% by weight to binder) were investigated. Experiments were conducted to examine the composite performance, matrix properties and single fiber pullout behavior of the SHCCs, followed by microscopy characterization of the fiber/matrix interface microstructure. Results show that the inclusion of the PLA-HA up to 5% by weight to binder influenced the tensile performance (i.e., tensile strength and ductility) of SHCC only to a very small extent but significantly reduced the average residual crack widths. The inclusion of HA at a high dosage (5%) increased the crack tip toughness (Jtip) of the matrix by lowering elastic modulus and increasing fracture toughness. Single fiber pullout results show that the fiber/matrix bond properties were enhanced by the addition of the HA, which can be attributed to the formation of a denser interfacial transition zone (ITZ) with less calcium hydroxide crystals as revealed by the scanning electron microscope (SEM) micrographs. The improved bond properties led to higher fiber bridging complementary energy and thus partially sustained the tensile strain capacity as verified by the micromechanical model.

Influences of PCE superplasticizers with varied architectures on the formation and morphology of calcium hydroxide crystals

The impact of polycarboxylate (PCE) superplasticizers on calcium hydroxide (CH) precipitation and morphology were studied aiming at further understanding retardation effect of PCEs on cement hydration. Three comb-like PCEs with varied architecture and two linear copolymers (PAS) charged by carboxylic and sulfonic groups were synthesized. CH precipitate was prepared by simultaneous feeding NaOH and CaCl2 solutions in bulk solution containing various polymers. Analytical techniques of solution conductivity, ICP−OES, TG, XRD and SEM were used. Results show that the required [Ca] for massive CH precipitation is greatly increased by the presence of the polymers, suggesting inhibited nucleation and growth of CH crystals. The large incorporation of the polymers remarkably reduces the crystal size and consequently increases the specific surface area of the obtained CH. The polymers with higher density of carboxylic groups present stronger impacts on CH precipitation while such effect of the solely sulfonic group charged PAS is negligible.

Use of reclaimed water for unreinforced concrete block production for the self-construction of houses

AbstractExperiments were conducted to evaluate the possibilities of using treated wastewater for the production of unreinforced concrete blocks. Compressive strength, water absorption and morphology tests of concrete blocks, produced from different makeups of mixing water, drinking water, drinking water spiked with ammonium and phosphate, and the effluent of the city's wastewater treatment plant, were evaluated. Results showed that the compressive strength of blocks manufactured using treated wastewater was as high as of the blocks produced using drinking water. Ammonium, phosphate and chlorine were found not to have a negative effect on the strength of the blocks. Water absorption tests confirmed the results of the compressive strength, as lower humidity was found in cases of higher strength. In the process of cement hydration, crystals of calcium silicate and calcium hydroxide were observed by morphology tests. From the variability in the results, it could be concluded that the quality of the mixing water was not the only factor that influenced the strength of the unreinforced concrete blocks. The observed differences in strength could, for example, also be attributed to the manufacturing process.

Deformation of Steel Slag Asphalt Mixtures Under Normal Temperature Water Immersion

To study the deformation resistance of steel slag asphalt mixtures (SSAMs) under rainy conditions, limestone–asphalt mixtures (LAMs) and SSAM were soaked in water at room temperature for 120 days and rutting tests and triaxial compression tests were carried out. The results show that the deformation resistance of SSAM was improved after 120 days of immersion, the cohesion did not decrease significantly, and the internal friction angle increased by 25.1%; the deformation resistance of LAM decreased significantly, the cohesion decreased by 27.1%, and the internal friction angle decreased by 21.1%. To better understand the reason for the increased anti-deformation ability, adhesion tests of asphalt and microscopic studies of the steel slag surface were performed. The experimental results showed that the cohesion of steel slag did not decrease significantly after immersion because of the excellent adhesion between steel slag and asphalt. The increased internal friction angle was caused by calcium hydroxide and other crystals formed on the surface of the steel slag mixture after immersion, which increased the surface roughness of the steel slag aggregates and the internal friction angle of the SSAM. The mechanical properties of semirigid asphalt pavement were analyzed by ANSYS. It was determined that the shear stress of this type of pavement is large, and it can easily produce permanent deformations. Under the influence of moisture, the anti-deformation ability of SSAMs can grow, which promotes the high-temperature deformation resistance of asphalt pavement. Based on a test road used for 2 years, the SSAM pavement exhibited no pavement problems, such as cracking, loosening, or rutting, which indicates good practical road performance.

Performance of prestressed concrete beams using magnetic water for concrete mixing

Civil engineering network is virtually orient to develop high-performance materials with eco-friendly construction to welfare of the society. Undoubtedly, water plays an important role in the performance of concrete to provide workability, strength, and other durability properties. Magnetic water being salt free and its properties are turning out to be as good as potable water. When water is passing through a magnetic field, water molecules get rearranged inside the structure, which allows the water to seep through more effectively than normal water in cementitious particles, thereby enabling the hydration process to operationalize faster and effective. Based on the preliminary studies, it has observed that workability of M40 grade concrete mixed with magnetic water is improved significantly without compromising its strength and also it has been observed that the strength achievement is faster than the conventional concrete with the considerable reduction of cement content. This research paper intends to investigate the efficient utilization of magnetic water in M40 grade concrete and to study the enhancement of its properties on hardened concrete and structural behavior of prestressed concrete (PSC) beams such as load carrying capacity, ductility factor, and energy absorption capacity. These properties are comparatively studied for pre-tensioned reinforced concrete (RC) beam subjected to central concentrated load for concrete mixing to use the normal water and magnetic water. Utilization of magnetic water on pre-tensioned concrete beams is improving the flexural strength by 25%. The characterization study on concrete with magnetic water such as X-ray diffraction (XRD) and Fourier-transform infrared (FT-IR) spectroscopy substantiates hydrated products of cements at early ages. Further the scanning electron microscopic (SEM) studies demonstrate the high degree of hydration of cement with separation of calcium hydroxide (CH) crystal using magnetic water.

Analytical Method for Evaluating Calcium Diffusion Coefficient of Partially Leached Cement Paste

This paper aims at presenting an analytical method for evaluating the calcium diffusion coefficient of partially leached cement paste. The analytical method is developed based on a two-scale model of partially leached cement paste. On the first scale, the solid phase in partially leached cement paste is modeled as a two-phase composite material, composed of a matrix of calcium silicate hydrate (C─S─H) gel and inclusions of unhydrated cement and undissolved calcium hydroxide (CH) crystal. The relative calcium diffusion coefficient is derived analytically by the differential effective medium scheme. On the second scale, partially leached cement paste is treated as a system consisting of the solid phase and spheroidal capillary pores; the percolation behavior of capillary pores near the critical volume fraction is considered. In combination with percolation theory, the effective medium approach is modified to evaluate the calcium diffusion coefficient of partially leached cement paste. The depolarization factor is expressed in terms of the critical volume fraction of capillary pores, and the percolation exponents are calibrated from computer simulation data. Finally, the validity of the analytical method is verified with computer simulation data and experimental results collected from the literature. It is concluded that the proposed analytical method can predict the calcium diffusion coefficient of partially leached cement paste with reasonable accuracy.

Deterioration mechanism of concrete under long-term elevated temperature in a metallurgic environment: A case study of the Baosteel company

A long-term elevated temperature environment is typically found in the metallurgy industry, which has adverse effects on the properties and microstructure of concrete structures. A few studies investigated the effect of short-time high temperature on the properties and microstructure of concrete, but the effect of long-term elevated temperature is seldom analysed. In this study, the deterioration mechanism of concrete under long-term elevated temperature was investigated by determining the thermal stresses on basis of finite element model (FEM) and analyzing the changes in its chemical composition and microstructure through X-ray diffraction (XRD), pore structure analysis (PSA) and scanning electron microscope (SEM). The results indicated that the maximum temperature stress of surface concrete could exceed 4 MPa, which was close to the ultimate tensile strength of normal concrete. The results also showed that calcium hydroxide (CH) could decompose at less than 200 ℃ because of the crystallization of tiny CH crystals, doping with other ions, extended heating time, and xonotlite (α-C2SH), which was formed on the internal surface of the concrete and decomposed on its external surface. Owing to these reasons, cracks and holes were generated, resulting in severe deterioration of the concrete structure.

A new type of inorganic binder curing agent for soft soil

Aiming at the defects of traditional curing agent in strengthening soft clay, a new type of inorganic binder type soil curing agent has been developed. Compared with P.O 42.5 cement, the new soil stabilizer has smaller fineness and larger specific surface area. The strength of the soil solidified by the new curing agent is lower at the initial stage, but it increases rapidly after 24 hours. When the dosage is 10%, the 28-day strength of the soil solidified by the new curing agent is 2.1 times that of the soil solidified by cement. The soil solidified by the new curing agent can form more calcium hydroxide crystals and hydrated calcium silicate gel with more compact structure. After solidification, there are fewer fine particles in the soil.

Durability and Mechanical Properties of Self-compacting Concretes with Combined Use of Aluminium Oxide Nanoparticles and Glass Fiber

The presence of fibers in concrete specimens has an effective role on how the specimens were failed. In this study, the effects of aluminium oxide nanoparticles on the workability, mechanical and, durability properties of SCCs containing glass fibers were investigated. Glass fibers contents of 0, 0.5, 1, and 1.5 % by volume of concrete and aluminium oxide nanoparticles contents of 0, 0.5, 1, 1.5, 2, and 3 % by weight of cement were used. The properties of fresh concrete were evaluated according to EFNARC consideartions. The mechanical properties were evaluated by compressive strength, splitting tensile strength, and ultrasonic pulse velocity tests. The durability of the specimens was also measured using water absorption tests, water penetration depth and, electrical resistivity. Combined use of 2% aluminium oxide nanoparticles and 1% glass fiber has increased the compressive and tensile strengths of SCCs by 59% and 119.2%, respectively. Aluminium nanoparticles have a very high specific surface area and their reactivity causes them to react rapidly with calcium hydroxide to produce silicate-hydrate gels. Therefore, calcium hydroxide crystals are reduced and the cavities in the cement gel are filled and the compressive strength is increased. The use of aluminium oxide nanoparticles along with glass fibers reduces the water absorption rate compared to the sample without these materials. This is one of the effective properties of aluminium oxide nanoparticles, which increases the resistance to adverse environmental factors by reducing water absorption.

Effects of polycarboxylate superplasticizer-modified graphene oxide on hydration characteristics and mechanical behavior of cement

Graphene oxide (GO) has received great attention as a nanosized reinforcing agent in cement-based materials. However, GO easily aggregates in an alkaline cement matrix and suffers from poor dispersibility, which reduces the beneficial effect on the final properties of cement composites. In this study, GO-polycarboxylic acid superplasticizer (GPC) was synthesized by free-radical copolymerization of GO with methyl allyl polyoxyethylene ether (TPEG), sodium styrene sulfonate (SSS) and acrylic acid (AA). The chemical structure of the GPC was characterized by X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR). To understand the influence of GO on the cement hydration process, we analyzed hydration heat, phase composition, and TG/DSC behavior. The pore structure and micromorphology of the cement composites were investigated by mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM). The results showed that the GO layer spacing increased after copolymerization, and the GO surface was covalently grafted with PC macromolecules. The addition of GPC to cement yields uniform dispersion of GO and reduces the adverse effects of GO on the fluidity of the cement slurry. GPC can accelerate cement hydration, promote crystallization of calcium hydroxide (CH) in the cement slurry, refine grain size, and enhance mechanical properties of the cement-based materials.

Strength and hydration of metakaolin-containing ultra-high-performance paste

The influence of metakaolin (MK) addition on the mechanical and hydration performance of ultra-high-performance (UHP) paste was investigated. UHP mortars were prepared to evaluate the compressive strength when MK was used as a cement replacement at levels of 0, 5, 10, 15 and 20%. The hydration heat evolutions of pastes blended with MK were examined. The chemically bound water and MK reaction degree were determined to assess the hydration reaction degrees of the pastes. Differential thermal analysis, X-ray diffraction, mercury intrusion porosimetry and scanning electron microscopy were carried out to investigate the phase composition and microstructure of pastes. The results showed that the mechanical performance of UHP mortar at an early age is improved by the addition of MK due to its filling effect and pozzolanic reaction. The reaction rate of MK in the pastes significantly decreased at a later age and at a low water-to-binder ratio, but the incorporation of MK still had an improving effect on the mechanical strength of UHP mortars at a later age. The incorporation of MK reduced the content of calcium hydroxide crystals and increased the content of calcium silicate hydrate and calcium aluminium silicate hydrate phases in the pastes. The hardened MK-dosed paste samples showed improved microstructure with lower porosity and a refined pore structure.

Impacts of two alkanolamines on crystallization and morphology of calcium hydroxide

As one of the main hydration products, Ca(OH)2 has important effects on mechanical properties and durability of hardened cementitious materials. In this study, Ca(OH)2 precipitates were synthesized by continuous feeding of CaCl2 and NaOH solutions into a bulk solution containing triethanolamine (TEA) or diethanolamine (DEA). Analytical techniques including electric conductivity measurement, TOC, ICP − OES, XRD, FTIR and SEM were employed to investigate the impacts of the alkanolamines on the precipitation process and morphology of the synthetic Ca(OH)2. Results show that TEA strongly hinders the nucleation and growth of Ca(OH)2 precipitate due to the complexation effect with Ca2+ ions in solution. The chemical incorporation of TEA into the lattice structure greatly disrupts lattice structure of the obtained Ca(OH)2 precipitates and alters the morphology from typical hexagonal prism of pure portlandite to vertically stacked or randomly agglomerated Ca(OH)2 lamellae crystals with much smaller size. In contrast, DEA exhibits little effect on the precipitation process and morphology of portlandite.

Effects of elevated temperatures on strength and microstructure of mortar containing nano-calcined montmorillonite clay

This study focuses on the residual mechanical strengths and microstructure of nano-calcined montmorillonite clay (NCMC) cement mortars at high temperatures. NCMC was prepared via thermal activation of nano-clay, and the produced mortars with 0%, 1%, 3%, and 5 wt% NCMC replacements were subjected to temperatures of up to 900 °C. The fire-resistance properties of the produced mortars and their role were analysed through thermogravimetric analysis, X-ray diffraction, and scanning electron microscopy.The results revealed that the use of thermally treated NCMC reduced the density and widths of microcracks, improved the mass-loss behaviour, reduced the amount of calcium hydroxide crystals, and strengthened the matrix, which explains their higher residual mechanical strengths.

Effect of different particle sizes of nano-SiO2 on the properties and microstructure of cement paste

Abstract Nano-materials modified cement-based materials have attracted wide attention due to their advantages of improving strength and durability. In this paper, the effect of nano-SiO2 (NS) with particle sizes of 15 and 50 nm on the mechanical properties and microstructure of cement paste was studied. The results showed that 50 nm NS provided a greater compressive strength than that of 15 nm NS, while 15 nm NS afforded a denser microstructure than that of 50 nm NS. A fluctuation in the compressive strength was revealed using a physicochemical reaction equation, and the microstructure was interpreted by a pore structure analysis. In addition, the orientation index of calcium hydroxide (CH) with 15 nm NS could be reduced significantly in the early stages (the early stages refer to the first three days from the maintaining of specimens) compared with when the 50 nm NS was used. The experimental results also showed that NS helped increase the mechanical strength of cement paste, advance the endothermic peak of CH, and refine the size of the CH crystals. The microstructural changes at different stages of cement paste with different particle sizes of NS were investigated by X-ray diffraction, scanning electron microscopy, mercury intrusion porosimetry and differential thermal analysis. This study is expected to promote the research and application of nano-materials in the cement industry by clarifying the performance of NS with different particle sizes.

Nonclassical Crystallization of Calcium Hydroxide via Amorphous Precursors and the Role of Additives

In many systems prenucleation clusters (PNC), dense liquids and solid amorphous phases precede the formation of crystalline phases, which can grow via a nanoparticle aggregation mechanism. Despite ...

Effect of Lithium Compound Addition on the Dehydration and Hydration of Calcium Hydroxide as a Chemical Heat Storage Material.

Many studies on calcium hydroxide [Ca(OH)2] as a chemical heat storage material have been conducted. Generally, calcium hydroxide undergoes a dehydration reaction (heat storage operation) efficiently at about 400 °C or higher. In this study, we aimed to lower the dehydration reaction temperature and increase the dehydration reaction rate to expand the applicability of calcium hydroxide as a chemical heat storage material. For the purpose of improving the dehydration reactivity, calcium hydroxide with added lithium compounds was prepared, and the dehydration/hydration reactivities were evaluated. From the results, it was confirmed that the addition of the lithium compounds lowered the dehydration reaction temperature of calcium hydroxide and enhanced the reaction rate. The dehydration reaction of Ca(OH)2 with Li compounds proceeded efficiently even at 350 °C, and the reversibility of the dehydration/hydration reaction was confirmed. The reason for the improvement of the calcium hydroxide dehydration reactivity upon the addition of a lithium compound was examined from the viewpoint of its crystal structure. It was presumed that when lithium ions enter the calcium hydroxide crystals, the crystals became fragile and the dehydration reaction was accelerated.

The reinforcing effects and mechanisms of multi-layer graphenes on mechanical properties of reactive powder concrete

The utilization of multi-layer graphenes (MLGs) as reinforcement for reactive powder concrete (RPC) is motivated by their flat morphology, unique stacking structure, high stiffness/strength, and high electron mobility. This paper studied compressive strength, flexural strength and flexural stress-strain relationship of MLGs reinforced RPC (MRRPC). Electrical properties including electrical resistivity and electrochemical impedance spectroscopy were tested to characterize RPC’s pore structure firstly. Besides, the microstructure of MRRPC was observed using scanning electron microscope, and CaO/SiO2 ratio of C-S-H gel was obtained by energy dispersive spectrometer. The results show that the MRRPC filled with 0.5 wt% MLGs exhibits maximum compressive strength of 123.2 MPa. MRRPC filled with 0.75 wt% MLGs presents maximum flexural strength of 11.08 MPa, along with 18.7% enhancement to ultimate flexural strain. Electrical properties’ results suggest pore structure of RPC can be modified by MLGs. Negative correlation between compressive strength and two electrical parameters including electrical resistivity increase and pore solution resistance is observed. Moreover, CaO/SiO2 ratio of C-S-H gel is decreased and size of calcium hydroxide crystal is reduced along with the addition of MLGs, implying MLGs can fill pores and reduce weakened areas in RPC matrix effectively.

Effect of ZnO Nanoparticles on Cement Mortar for Enhancing the Physico-Chemical, Mechanical and Related Properties

Zincoxide nanoparticles have been incorporated into the cementitious materials for enhancing their physico-chemical and mechanical properties. The decrease in particle size will leads the increase in surface to volume ratio as compared to conventional micro-particles. In this investigation to evaluate physical, chemical and mechanical properties of Ordinary Portland Cement (OPC) containing zinc oxide nanoparticles. Zinc oxide nanoparticles used with replacement of various properties such as 0%, 1%, 2%, 3%, 4% and 5% by weight with ordinary Portland cement. The ordinary Portland cement mortar, nano zincoxide cement mortars prepared with the water-binder ratio (w/b ratio) as 0.4 and cement-sand ratio as 1:3 by weight. The prepared mortars are analyzed through compressive strength, setting time, flexural strength, porosity, water absorption, sulphate attack, electrical resistivity and with SEM analysis. The setting time of zinc oxide nano particle significantly decreased and enhanced the compressive strength of Portland cement mortar. The flexural strength outcomes showed that the cement mortars with zinc oxide nanoparticle were higher rather than Portland cement mortar. Microstructure of mortar contained zinc oxide nano particles illustrate that the nano zinc oxide particles filled the pores significantly and so large size of calcium hydroxide crystals reduced afterwards more hydration products formed.

Strengthening Behavior of Cemented Paste Backfill Using Alkali-Activated Slag Binders and Bottom Ash Based on the Response Surface Method.

A new type of cemented paste backfill (CPB) was prepared by using the bottom ash (BA) from a thermal power plant as an aggregate, alkali-activated slag as a binder, and an air-entraining agent as an admixture. Based on the central composite design (CCD) response surface method, the mix ratio was optimized, and scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) was performed on the optimal mix ratio. ImageJ software was utilized to determine the porosity of the experimental samples at various curing ages. The results indicate that the optimal mix ratio of the aggregate-binder ratio is 3.28, the alkali dosage is 3%, the solid content is 67.44%, and the air-entraining agent dosage is 0.1%. As the curing age increases, the porosity of CPB gradually decreases. A calcium aluminosilicate hydrate (C-A-S-H) gel is the main hydration product of alkali-activated slag. At the beginning of the hydration reaction, the slag gradually dissolves, and the C-A-S-H product binds the BA together. At 14 d, complete calcium hydroxide (CH) crystals appeared in the hydration product. Finally, the degree of C-A-S-H crystallization increased further to form a dense structure.