Calcium Aluminate Cement Research Articles

Chloride removal of calcium aluminate cements: Reaction and physicochemical characteristics

The chloride removal characteristics of calcium aluminate cements in chlorinated solution were investigated in the present study. Furthermore, the reaction and the physicochemical characteristics of chloride-bearing calcium aluminate cement powders in the solutions were systematically investigated via leaching of reactive components from cement powders, X-ray diffraction, and BET tests. The independent variable is the content of Al2O3 in the cement powders. The test results showed that the chloride removal rate of calcium aluminate cements increased as the content of Al2O3 increased. The reaction route for the removal of chlorides was via the transformation of C2AH8 into hydrocalumite phases, which lead to an increase in the specific surface areas and mesopores volume in calcium aluminate cements. The obtained results provide information on the reaction and physicochemical characteristics to utilize calcium aluminate cements for chloride removal in a cementitious matrix.

Preparation and properties of calcium aluminate cement with Bayer red mud

Aiming at the problems of large discharge, the low utilization rate, and severe environmental pollution of Bayer red mud (BRM), this paper studied the feasibility of preparing calcium aluminate cement (CAC) with BRM as a raw material. The calcination regime of preparing CAC with BRM was investigated in the system of the CaO-Al2O3-SiO2. The experiment was designed by the simplex centroid design method. The optimal proportion range of CAC without BRM was determined with the compressive strength as the index. In addition, the evolution of the microstructure was studied by scanning electron microscope (SEM). Results demonstrate that the phases CA, CA2, and C2AS were formed at a temperature of 1300 °C. It was determined that the best performance was obtained with a Bayer red mud content of 8% by X-ray diffraction (XRD) and hydration heat. Moreover, the strength of CAC designed with Bayer red mud could reach 45.8 MPa after curing for 14 days, which was 18.7 MPa higher than that of CAC prepared without BRM at the same curing age.

Interfacial bonding properties of styrene-butadiene rubber and ethylene vinyl acetate emulsion-modified OPC–CAC–G repair mortar

Inorganic repair materials such as ordinary Portland cement–calcium aluminate cement–gypsum (OPC–CAC–G), which consist of ordinary Portland cement, aluminate cement, and gypsum, have advantages of fast hardening, high early strengths, and minimal expansion; however, they also have the disadvantages of low bond strengths and weak interface areas, which often lead to repair failure. The interfacial bonding properties of repair materials can be improved by adding highly flexible polymers. In this study, the effects of styrene-butadiene rubber (SBR) and ethylene vinyl acetate (EVA) on the interfacial bonding properties of OPC–CAC–G bonding and the mechanism of the effect of the microstructure are examined using X-ray diffraction, scanning electron microscopy, and Fourier-transform infrared spectroscopy. The results show that and EVA are both effective in improving the interfacial bond strength of OPC-CAC-G repair mortars. SBR is suitable for the modification of repair mortars with an optimum dose of 20%; EVA is suitable for the use as an interface agent on the surface of damaged concrete substrates with an optimum concentration of 40%.

A critical review of the role of ettringite in binders composed of CAC–PC–C$ and CSA–PC–C$

AbstractEttringite‐accelerated binders composed of CAC–PC–C$ (calcium aluminate cement–Portland cement–calcium sulfate) and CSA–PC–C$ (calcium sulfoaluminate cement–Portland cement–calcium sulfate) have been used widely for indoor applications, such as self‐leveling floor screeds, underlayment, and tile adhesives owing to their rapid setting, early strength gain, and shrinkage compensation properties. These properties also make these binders appealing candidates for outdoor rapid repair (e.g., highways, bridge decks, and airfield pavements). However, a central question remains: Does ettringite remain stable in outdoor exposure conditions? If so, which factors will contribute positively/negatively to the long‐term stability of ettringite in these systems? To address these questions, this critical review presents the current state of knowledge regarding the hydration of ternary binders composed of CAC–PC–C$ and CSA–PC–C$ with respect to ettringite formation and the factors affecting the stability of ettringite thereafter. The purpose of this review paper is to synthesize and analyze current research regarding conditions that promote or deter ettringite stability, establish what information is missing, unclear, or contradictory, and identify remaining research needs to address the identified knowledge gaps.

Microwave thermogravimetry as a tool to evaluate the drying of refractory castables – The effects of volumetric heating

Refractory castables present special features that render them unique and the most appropriate solution for lining specific high-temperature industrial equipment. Hydraulic bonded monolithics develop mechanical strength at room temperatures due to the hydration of some of its constituents, such as calcium aluminate cement. Their most significant drawback is the drying step, which takes place before firing and requires several hours of slow heating rates and plateaus to reduce the likelihood of facing explosive spalling. This phenomenon is a result of water vapor pressurization, as well as thermomechanical stresses. As an alternative to reduce the time and CO2 generation, microwave heating can be a promising solution as the volumetric nature of the temperature increase can make this process safer. Besides this benefit, the likelihood of using green energy and the higher efficiency of this method add to the potential benefits. The present work analyses the mass loss of refractory castables by proposing a microwave thermogravimetry equipment. The results suggest that water removal occurs at earlier temperatures and, in total, nine-fold less energy consumption estimation Numerical model results showed the distinct thermal gradients and the resulting drying profiles, highlighting the importance of properly controlling the microwave power during drying. Thus, these findings indicate that the microwave process might be a valid strategy for a safer, greener and faster drying of castable refractories.

Investigation of water stability and microcharacteristics of magnesium potassium phosphate cement modified by calcium aluminate cement and wollastonite

The early water stability of magnesium potassium phosphate cement (MKPC) blended with calcium aluminate cement (CAC) and wollastonite (WS) was investigated. Macroscopically, the presence of CAC prolongs the setting time of MKPC becauseof the stimulating effect of the admixture on the composite retarder. The synergistic effect between CAC and WS enhances the early water stability and compressive strength of MKPC subjected to water curing. From the microscopic aspect, the X-ray diffraction and Fourier transform infrared spectroscopy results indicate that the CAC phase participates in the reaction of the MKPC system to produce crystalline (metavariscite) and amorphous (hydroxyapatite and gibbsite) products. In contrast, although WS does not participate in the reaction, it inhibits the development of cracks and strengthens the brittle matrix. Furthermore, the results of the scanning electronic microscopy–energy dispersive spectrometry, mercury intrusion porosimetry, and Brunauer-Emmett-Teller nitrogen adsorption–desorption analysis confirm that the amorphous products coating the surface of struvite-K can change its solubility. Meanwhile, the crystalline product is embedded in the gap of struvite-K crystals, refining the pore structure and preventing water from entering the interior of the hardened paste.

Hydration and phase conversion of MgO-modified calcium aluminate cement

This study evaluated the hydration and phase conversion of MgO-modified calcium aluminate cement (CAC). MgO was used to replace CAC at various dosages (0, 5, 10, 15, and 20 wt%) and the samples were exposed to a 60 °C atmosphere to expedite the conversion of the hydration products in CAC after curing initially at 20 °C. Microstructural analysis was carried out before and after expedited curing using various characterization techniques. During the initial curing, MgO can suppress the phase conversion, hindering the formation of stable phases (i.e., C3AH6 and AH3). After 28 days of expedited curing, a phase conversion can occur, with metastable phases converted to stable phases, due to which a strength reduction can be observed in all samples. Nevertheless, the development of hydrotalcite in samples modified with MgO could lessen the degree of the compressive strength loss. The addition of MgO can maintain the structural integrity before exposure by suppressing the conversion of metastable hydrates and after exposure through the formation of an Mg-Al LDH tetrahedral Al network as a secondary hydration product, maintaining the strength after exposure.

Effects of carbonation on mechanical properties of CAC-GGBFS blended strain hardening cementitious composites

Calcium aluminate cement (CAC)—based strain hardening cementitious composites (SHCC) has been developed and used for the rehabilitation of sewerage pipelines. In addition to well-known microbiologically induced corrosion, CO2 concentration in the sewerage environment is high, which may cause significant carbonation of pipelines. Thus, this paper aims to investigate the effects of carbonation on the mechanical performance of CAC-based SHCC. Two types of CAC-based SHCC with different strength grades and a referenced OPC-based SHCC were prepared. The accelerated carbonation test was conducted in a carbonation chamber with a 5% CO2 concentration. The compressive and tensile behaviour of SHCC was tested first, and microstructure analysis, e.g., X-ray diffraction and scanning electron microscopy, was then performed. The results showed that CAC-based SHCC specimens exhibited robust strain-hardening performance as well as large deformation capacity in tension due to the fiber-bridging effect. Also, the compressive and tensile strength was significantly improved as well as achieving a higher tensile strain capacity after carbonation when compared with OPC-based SHCC. Microstructure analysis revealed that the metastable phases in carbonated CAC-based SHCC were converted into stable phases and calcium carbonate polymorphs, densifying the binder matrix. The obtained results of this paper may provide new insight into utilizing carbonation to avoid the unstable conversion of hydrates in calcium aluminate cement.

Influence of different CA2/CA-ratios on hydration degree, AH3 content and flexural strength investigated for a binder formulation of calcium aluminate cement with calcite

The study was aimed to investigate the influence of the CA2/CA-ratio in an application-orientated binder formulation of calcium aluminate cement (CAC) with calcite. Six mixtures containing varying fractions of two calcium aluminate cements with different CA2- and CA contents, calcite and additives were investigated after 1, 7 and 28 d at 23 °C. By means of QXRD the hydration degrees of CA2 and CA, as well as hydrate phase contents, especially AH3, were determined and correlated with the mechanical response under biaxial flexural stress and pore size distribution using micro-computed tomography (μCT) of three contrastive mixtures. The strength values increase slightly up to a CA2/CA ratio of 70:30, above this ratio the mechanical properties deteriorate significantly. The macro-porosity acting as strength-limiting defects correlate well with this threshold, though not the content of AH3 or the degree of hydration of CA.

Chloride Corrosion of Reinforced Calcium Aluminate Cement Mortar

Chloride Corrosion of Reinforced Calcium Aluminate Cement Mortar

Predicting Compressive Strength and Hydration Products of Calcium Aluminate Cement Using Data-Driven Approach

Calcium aluminate cement (CAC) has been explored as a sustainable alternative to Portland cement, the most widely used type of cement. However, the hydration reaction and mechanical properties of CAC can be influenced by various factors such as water content, Li2CO3 content, and age. Due to the complex interactions between the precursors in CAC, traditional analytical models have struggled to predict CAC binders' compressive strength and porosity accurately. To overcome this limitation, this study utilizes machine learning (ML) to predict the properties of CAC. The study begins by using thermodynamic simulations to determine the phase assemblages of CAC at different ages. The XGBoost model is then used to predict the compressive strength, porosity, and hydration products of CAC based on the mixture design and age. The XGBoost model is also used to evaluate the influence of input parameters on the compressive strength and porosity of CAC. Based on the results of this analysis, a closed-form analytical model is developed to predict the compressive strength and porosity of CAC accurately. Overall, the study demonstrates that ML can be effectively used to predict the properties of CAC binders, providing a valuable tool for researchers and practitioners in the field of cement science.

Hydration of blended ladle slag and calcium aluminate cement

Partial replacement and co-hydration of calcium aluminate cement (CAC) with ladle slag was investigated in this study as a pathway in valorizing the slag and reducing the economic cost of CAC. The impact of this replacement on the physical and microstructural properties were analysed using different techniques such as mechanical strength test, freeze-thaw, sulfate attack, XRD, SEM etc. Thermodynamic modelling was used to predict the phase assemblages of the blended cement using the hydration kinetics of the system. Experimental results showed the reference CAC mortar and the substituted mortar exhibited comparable strength gain at 7 and 28 days, and durability as measured by sulfate attack, abrasion, and freeze-thaw resistance. A low water-to-binder ratio (0.35) lessened the effect of conversion on the hydrates, as XRD showed metastable CAH10 and C2AH7.5 as the hydrates at 7, 28 and 60 days. This however can convert later to the thermodynamically stable phase C3AH6. Thermodynamic modelling suggests these two metastable phases as major binding phases, while Si-hydrogarnet and FeOOH appeared a minor trace in the binder.*Cement chemistry notation used, where C = CaO, A = Al2O3 and H = H2O

Microstructure Evolution Mechanism of Quaternary Phase Paste Containing Metakaolin and Silica Fume

To promote the sustainable development of high-performance calcium aluminate cement (CAC)-based material and the efficient utilization of supplementary cementitious materials (SCMs) in CAC, the evolution mechanism of the microstructure of quaternary phase (Q phase, Ca20Al26Mg3Si3O68) paste containing metakaolin (MK) and silica fume (SF) was investigated. The strength of SF-blended paste decreases with time when cured at 40 °C, while that of MK-blended paste shows sustained strength gain. The microstructure of outer products (OPs) for SF-blended paste suffers from significant damage when the age increases from 3 to 28 d, whereas that for MK-blended paste exhibits no significant difference, showing a dense morphology. The microstructure evolution of OP is mainly controlled by the reactions between SCMs (i.e., MK and SF) and early metastable hydrates (i.e., CAH10 and C2AH8). The dissolved silica from SF reacts with CAH10 to form C2ASH8 but cannot completely hinder its conversion to C3AH6, while the preferentially dissolved alumina from MK can stabilize CAH10 due to the common-ion effect. As the rate of conversion of C2AH8 to C3AH6 is much faster than that of CAH10, the rapid conversion of C2AH8 cannot be effectively suppressed by MK or SF.

Mechanical Properties of Calcium Aluminate Cement Reinforced with Plasma-Treated Basalt Fibers for In Situ Combustion.

Brittleness and poor tensile/flexural properties restrict the application of calcium aluminate cement (CAC) in oil and gas wells. Reinforcing CAC with fibers is an effective method for improving its strength and toughness and overcoming the shortcomings of its mechanical properties. In this article, as an auxiliary cementing material, slag does not affect the thickening time of CAC. After adding slag, the cement slurry meets the thickening time during cementing construction, and basalt fiber is selected as the toughening material. The enhancement effect of basalt fiber on the mechanical properties of CAC slag composites is studied, including the evaluation of the macroscopic mechanical properties and microstructure at a high temperature (500 °C). The optimum composition of basalt and fiber-reinforced CAC was determined. Basalt fibers were added to CAC at different contents of 0, 0.1, 0.2, 0.3, 0.4, and 0.5% based on the weight of the cement. All the results showed that the introduction of basalt fibers could significantly enhance the strength of the cement at high temperatures. Compared with the control samples, an additional increase in the compressive and tensile strengths of the samples of 35.1 and 85.2%, respectively, was achieved at high temperature with approximately 0.4% fiber content. Plasma treatment further improved the reinforcing effect of the basalt fibers, where the high-temperature compressive and tensile strengths of the samples increased from 28.88 and 1.52 to 35.23 and 1.87 MPa, respectively, an increase of 21.98 and 20.6%, respectively, compared with the untreated basalt fibers. When the cement paste is cured by simulated curing for 28 d, the high-temperature compressive strength and tensile strength with plasma modification increased from 28.26 and 1.5 to 29.1 and 2.15 MPa, respectively, an increase of 3.0 and 43.3%, respectively. The structure of the formed hydrates was studied using scanning electron microscopy. Furthermore, toughening of the basalt fiber-reinforced CAC-based composites resulted mainly from crack bridging and fiber pull-out.

Physicochemical properties of a calcium aluminate cement containing nanoparticles of zinc oxide.

This study evaluated the effect of different nanoparticulated zinc oxide (nano-ZnO) and conventional-ZnO ratios on the physicochemical properties of calcium aluminate cement (CAC). The conventional-ZnO and nano-ZnO were added to the cement powder in the following proportions: G1 (20% conventional-ZnO), G2 (15% conventional-ZnO + 5% nano-ZnO), G3 (12% conventional-ZnO + 3% nano-ZnO) and G4 (10% conventional-ZnO + 5% nano-ZnO). The radiopacity (Rad), setting time (Set), dimensional change (Dc), solubility (Sol), compressive strength (Cst), and pH were evaluated. The nano-ZnO and CAC containing conventional-ZnO were also assessed using scanning electron microscopy, transmission electron microscopy, and energy-dispersive X-ray spectroscopy. Radiopacity data were analyzed by the 1-way analysis of variance (ANOVA) and Bonferroni tests (p < 0.05). The data of the other properties were analyzed by the ANOVA, Tukey, and Fisher tests (p < 0.05). The nano-ZnO and CAC containing conventional-ZnO powders presented particles with few impurities and nanometric and micrometric sizes, respectively. G1 had the highest Rad mean value (p < 0.05). When compared to G1, groups containing nano-ZnO had a significant reduction in the Set (p < 0.05) and lower values of Dc at 24 hours (p < 0.05). The Cst was higher for G4, with a significant difference for the other groups (p < 0.05). The Sol did not present significant differences among groups (p > 0.05). The addition of nano-ZnO to CAC improved its dimensional change, setting time, and compressive strength, which may be promising for the clinical performance of this cement.

Phase Transformations and Properties Evolution of Alumina-Based Refractory Castables Containing ZnO and SiO2

Although most of the studies presented in the literature are focused on MgAl2O4 formation and its role on alumina-based refractories performance, ZnO has been reported as a promising spinel inducer. Aiming to investigate the influence of ZnAl2O4 (ZA) and MgAl2O4 (MA) generation on the properties of alumina-based castables, three vibratable compositions containing calcium aluminate cement or hydratable alumina as binders and 1 wt% of silica fume, were evaluated in this work. Flexural strength, apparent porosity, hot elastic modulus, corrosion cup-tests, thermodynamic simulations, were carried out to analyze the performance of such ceramics. The results indicated that ZnAl2O4 was mainly formed above 800 °C, favoring an earlier sintering of the samples. Besides that, the softening of the castables was observed above 1200 °C, which resulted in the elastic modulus decay of the samples during their first heating cycle due to the formation of SiO2-rich liquid phase in the resulting microstructure. Cement-free samples obtained after calcination (600 °C for 5h) presented enhanced corrosion resistance when placed in contact with molten slag at 1500°C. Although, silica fume addition to the castables negatively affected their corrosion performance, it helped to counterbalance the expansion associated with the spinel and calcium aluminates formation.

Recent Advances in Alternative Cementitious Materials for Nuclear Waste Immobilization: A Review

Since the emergence of the problem of nuclear waste conditioning, cementation has become an important and developing part of the waste management system, owing to its simplicity and versatility. The continued development of the cementation technique is driven by the improvement and expansion of cementitious materials that are suitable and efficient for nuclear waste solidification. Advances in cement theory and technology have significantly impacted improvements in nuclear waste cementation technology, the quality of fresh and hardened waste forms, waste loading rates, and the reliability and sustainability of the nuclear industry. Modern mineral matrices for nuclear waste immobilization are a broad class of materials with diverse chemical–mineralogical compositions, high encapsulation capacities, and technological and engineering performance. These matrices include not only traditional Portland cement, but also non-Portland clinker inorganic binders. This review focuses on recent trends and achievements in the development of calcium aluminate, calcium sulfoaluminate, phosphate, magnesium silicate, and alkali-activated cements as cementitious matrices for nuclear waste stabilization/solidification.

Effects of temperature on chloride removal from solution with calcium aluminate cement

The effects of reaction temperature on the chloride removal characteristics of the calcium aluminate cement in chlorinated solution were investigated in the present study. The reaction temperature varied from 30 °C to 70 °C. The sodium chloride pellets were used to create chlorinated conditions. The effects of the chloride removal on the physicochemical characteristics of calcium aluminate cement were investigated via leaching of reactive components, X-ray diffraction, and thermogravimetric tests. The test results indicated that an increase in the reaction temperature in the present study was favorable for the formation of hydrocalumite phase, leading to an increase in the chloride removal rate.

Recent progress in microsilica-gel bonded no-cement castables

This paper focuses on the recent advance in no-cement technology, mainly focusing on their properties, benefits, and prospects when calcium aluminate cement is replaced by a “dry-version” silica binder, using microsilica powder to replace silica-sol. A microsilica-gel bond system has been designed using microsilica powder and a tailor-made dispersant package to give excellent flow and to control the setting behaviour of the NCCs. The flow, set-behaviour and mechanical properties have been studied in comparison with silica-sol bonded NCCs and cement-bonded low cement castables (LCC). The microsilica-gel bonded NCCs not only exhibit higher flow, better workability and adequate green strength compared to the silica-sol bond system, but also entail easier handling, storage, and transportation thanks to the all-in-the-bag solution. The microsilica-gel bond system contains only a minor amount of cement as gelling agent; hence, the hot properties are much better than that of LCCs. Phase transformations and bond mechanism were investigated by SEM and XRD techniques after pre-firing at 1400 and 1500°C. Furthermore, the effect of drying agents on both lab- and industrial-scale explosion resistance was studied. The drying behaviour and explosion resistance of microsilica-gel bonded NCCs was further significantly improved by using a speciality drying agent (EMSIL-DRY™); a perfect 400 kgs block was produced with no problems using a fast-firing program (20°C to 850°C at a heating rate of 50°C).

Numerical Simulation of Thermal Conductivity and Thermal Stress in Lightweight Refractory Concrete with Cenospheres

The main objective of this paper was to investigate the heat transfer of modified lightweight refractory concrete at the microscopic scale. In this work, such material was treated as a porous composite based on the compound of calcium aluminate cement and aluminosilicate cenospheres. The presence of air inclusions within the cenospheres was an essential factor in the reduction in thermal performance. Due to the intricacy of the subject investigated, our research employed numerical, theoretical, and experimental approaches. Scanning electron microscopy (SEM) imaging was performed to study the composite microstructure with a special focus on geometry, dimensions, and the distribution of cenospheres. Based on the experimental analysis, simplified geometrical models were generated to reproduce the main features of the composite matrix and cenospheres. A finite element framework was used to determine the effective thermal conductivity of such domains as well as the thermal stresses generated in the sample during the heat flow. A considerable difference in thermal properties was revealed by comparing the simulation results of the pure composite matrix and the samples, indicating a varying arrangement of cenosphere particles. The numerical results were complemented by a theoretical study that applied analytical models derived from the two-phase mixture theory-parallel and Landauer. A satisfactory agreement between numerical and theoretical results was achieved; however, the extension of both presented approaches is required.