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Calcium aluminate cement (CAC) is a cement type that has superior properties such as rapid strength gain, high resistance to high temperatures and harmful chemicals. However, the result of the using of CACs in the production of structural elements occur strength decreases at later ages as a result of a series of chemical reactions called 'conversion reactions' seen in these cements. In this study, the hydration kinetics and the crystalline and amorphous structures formed as a result of hydration were investigated in CAC containing different amounts and types of main oxides-based binary and ternary systems. Considering the results obtained, the main hydration product seen in these specimens was ettringite. Unlike many studies in the literature, metastable structures which cause conversion reactions, such as CAH10, C2AH8, have not been observed. Instead of conversion of the phases, ettringite needles were became thin and elongate which causes the paste structure porous and thus causing expansion and strength reduction at the later stages of hydration in some mixtures produce high amounts of ettringite. On the other hand, it is understood that the formation of strätlingite was limited strength decreases in systems where high silica fume is used. [ABSTRACT FROM AUTHOR]

As known, calcium oxide (CaO) is an alkaline material, which can be used widely to increase the clay-containing soils load carrying capacity, to produce aerated concrete and calcium aluminate cement. In the last few years, introducing CaO into alkali-activated materials (AAMs) became a hot topic and attained more attention than other times. Generally, CaO can be incorporated into AAMs as an additive/a part of the main precursor and a sole activator without/with an auxiliary activator. Incorporating CaO into the matrices may improve some properties and worsen others. This mainly depends on the ratio of CaO, curing conditions, activator type and activator concentration, precursor type and testing age. This review collected, summarized and analyzed the available studies focused on the effect of CaO on the fresh (reaction kinetic, workability, setting time) and hardened (mechanical strength, durability and length change) properties of AAMs. In addition, some recommendations for future works were included. The results showed that the inclusion of CaO in AAMs decreased workability and setting time. In spite of there are contradictory results about the effect of CaO on the compressive strength of AAMs, most of them reported higher compressive strength, especially at the early ages. The incorporation of CaO up to 5% in the matrix is more effective than the incorporation of higher ratios. The inclusion of CaO in the matrix decreased water absorption, decreased total porosity, increased wetting/drying as well as acid attack resistivity. The CaO (5–10%) can be used as a sole activator for precursors. Auxiliary activators such as Al2(SO4)3, Na2CO3, Na2SiO3, Na2SO4, CaSO4, NaOH, Ca(NO3)2, NaNO3, Mg(NO3)2, Mg(HCOO)2, Ca(HCOO)2, SO3, gypsum and MgO can be used to enhance the compressive strength of CaO-activated materials, especially at the early ages. [ABSTRACT FROM AUTHOR]

The high level of gas permeability can effectively reduce the explosive spalling risk of refractory castables. The hydratable magnesium carboxylate (HMC) is expected to improve the permeability of castables owing to the thermal decomposition of the HMC hydrates. This study compared the gas permeability and explosive spalling resistance of HMC bonded refractory castables (HMCC) with calcium aluminate cement bonded refractory castables (CACC). Thermal decomposition of (Mg3(C6H5O7)2∙11H2O) (hydrates of HMC), drying behavior, and the pores size distribution of castables were investigated. The level of gas permeability of HMCC is higher than that of CACC, which was confirmed by the higher values of Darcian k1 and non‐Darcian k2. The degas temperatures of HMC hydrates (156°C) and HMCC (432°C) are lower than those of CAC hydrates (289°C) and CACC (536°C) at a heating rate of 20°C/min, respectively. The large‐size and more permeable pores in HMCC were obtained according to the mercury intrusion porosimeter (MIP) results, which formed the connected paths for gases (H2O, CO2, C2H4, CO, CH4) released from the castables. [ABSTRACT FROM AUTHOR]

Calcium sulfate (CaSO4), an essential retarder in cement, retards the hydration of tricalcium aluminate (C3A) and tetracalcium aluminoferrite (C4AF) phases. However, its retarding mechanism remains unclear. This paper focused on the adsorption of CaSO4f on C4AF and C3A surfaces based on isothermal calorimetry, the measurement of the ionic concentrations in a diluted system, and density functional theory to enhance the understanding of the retardation mechanism. The results showed that the retarding effect of CaSO4 on C4AF was stronger than that on C3A due to the slower CaSO4 consumption rate, lower driving force for CaSO4 adsorption, and surface coverage of Fe(OH)3 gel. The adsorption of CaSO4 hindered Ca dissolution more markedly on C4AF than C3A, which was pronounced on Fe‐free C4AF surfaces. The adsorption of CaSO4 weakened the affinity of water on C4AF and C3A surfaces, lowering the driving force for H2O adsorption. The adsorption of H2O and CaSO4 promoted the dissolution of Al on the [AlO6] octahedral surface of C4AF, which may be responsible for the maintenance of a higher Al concentration in the solution. Based on the above results, the adsorption of CaSO4 on initial C4AF and C3A hydration was explained. [ABSTRACT FROM AUTHOR]

Refractory castables, i.e., refractory aggregates and ultra-fine particle mixtures with calcium aluminate cement (CAC) and deflocculants, were created 40 years ago for the metallurgy and petrochemical industries. These materials demonstrate outstanding performance even over 1000 °C. Typically, they have no structural reinforcement, resisting compression stresses because of the combination of temperature and mechanical loads. This study is a part of the research project that develops high-temperature resistance composite material suitable for fire and explosion protection of building structures. However, this application is impossible without structural reinforcement, and the bond performance problem becomes essential under high temperatures. This experimental work conducts pull-out tests of austenitic stainless 304 steel bars and typical structural S500 steel bars embedded in refractory castables after high-temperature treatments. This study includes plain and ribbed bars and considers two castable materials designed with 25 wt% CAC content for 50 MPa compressive strength after drying (typical design) and 100 MPa strength (modified with 2.5 wt% microsilica). This test program includes 115 samples for pull-out tests and 88 specimens for compression. As expected, the tests demonstrated the plain bars' inability to resist the bond stresses already at 400 °C; on the contrary, ribbed bars, even made of structural steel, could ensure a mechanical bond with cement matrix up to 1000 °C. However, only stainless steel bars formed a reliable bond with the high-performance castable, determining a promising object for high-temperature applications. Still, the scatter of the test results did not ensure a reliable bonding model. In addition, the castable strength might not be optimal to ensure maximum bond performance. Thus, the test results clarified the research objectives for further developing the reinforced composite. [ABSTRACT FROM AUTHOR]

Aerial additive manufacturing (AAM) represents a paradigm shift in using unmanned aerial vehicles (UAVs, often called 'drones') in the construction industry, using self-powered and untethered UAVs to extrude structural cementitious material. This requires miniaturisation of the deposition system. Rheological properties and known hydration times are important material parameters. Calcium aluminate cement (CAC) systems can be advantageous over purely ordinary Portland cement (OPC) binders as they promote hydration and increase early strength. A quaternary OPC/pulverised fuel ash (PFA)/CAC/calcium sulphate (CS) system was combined with polyvinyl alcohol (PVA) fibres and pseudoplastic hydrocolloids to develop a novel AAM material for miniaturised deposition. CAC hydration is affected by environmental temperature. Intending material to be extruded in situ, mixes were tested at multiple temperatures. OPC/PFA/CAC/CS mixes with PVA fibres were successfully extruded with densities of ≈1700 kg/m 3 , yield stresses of 1.1–1.3 kPa and a compressive strength of 25 MPa. Pseudoplastic OPC/PFA/CAC/CS quaternary cementitious systems are demonstrated to be viable for AAM, provided mixes are modified with retarders as temperature increases. This study can significantly impact industry by demonstrating structural material which can be extruded using UAVs in challenging or elevated in situ construction, reducing safety risks. [ABSTRACT FROM AUTHOR]

High Performance Aerogel Concretes (HPAC) are produced by embedding silica aerogels in high‐performance matrices, which were previously based on finely ground Portland cement CEM I 52.5 R and microsilica. On the one hand, this ensured a comparatively high compressive strength, but on the other hand it was associated with large shrinkage strains and a high global warming potential (GWP). In the following, approaches are presented on how these disadvantages can be counteracted by replacing Portland cement with alternative (ACM) or supplementary cementitious materials (SCM). In the experimental investigations, calcium aluminate cement, calcium sulfoaluminate cement, calcium sulfoaluminate belite cement and a CSH phase binder were used as ACMs; limestone powder, calcined clay, fly ash, glass powder, rice husk ash, brick dust and ground granulated blast‐furnace slag were used as SCMs. Based on extensive fresh and hardened concrete tests, it could be shown that with the (partial) replacement of Portland cement by ACMs or SCMs, High Performance Aerogel Concretes can be produced that have significantly reduced shrinkage deformations and a lower GWP with an unchanged good relationship between compressive strength and thermal conductivity. [ABSTRACT FROM AUTHOR]

The use of cementitious materials in deep sea environments, focusing on the durability challenges of low‐temperature seawater and hydraulic pressure, are presented in this paper. The authors conducted laboratory and deep‐sea field experiments, thermodynamic calculation, and evaluated the performance of different types of binders in deep sea conditions. Durability of rebar in cementitious materials was investigated. In situ construction methods using cementitious materials was demonstrated on the deep‐sea field. Testing results revealed that deep sea conditions, especially low‐temperature seawater, can accelerate the disintegration of cement hydrates, and the use of pozzolanic admixture and calcium aluminate cement can improve its resistance. Our study provides valuable insights and applicability of cementitious materials to deep sea infrastructure construction. [ABSTRACT FROM AUTHOR]

A titration method is used to analyse the acid neutralization/consumption capacity (ANC) of cement binders, which involves plotting the steady‐state pH against the amount of acid added to a diluted suspension of hardened cement samples. Despite ANC being employed for several decades, there is no reported application of ANC for metakaolin‐based geopolymers. This study compares the resistance of metakaolin geopolymers to acetic acid and hydrochloric acid with that of Calcium Aluminate Cement (CAC) and Portland Cement (PC). The proposed ANC test setup involves small acid dosage steps using an auto‐titrator, resulting in a high‐resolution titration curve. The effect of binder chemical composition on acid resistance is evaluated through cumulative and differential ANC analysis, providing a new rapid approach to evaluate the durability of geopolymers in low pH environments. [ABSTRACT FROM AUTHOR]

It was analysed how a low Na5P3O10 (STPP) concentration influences the hydration of a calcium aluminate cement. It is known that the hydration of a CAC‐Calcite system precipitates C2AHX as a precursor phase at 23°C during hydration before the formation of hemi‐ or monocarbonate and the additional formation of CAH10 and AH3. In our study the addition of 0.2 and 1.0 wt.% STPP (bwoc) to a CAC‐Calcite system at 23 °C and with a w/b of 0.33 was investigated and is compared with mixes without STPP addition. Heat flow calorimetry data showed that STPP accelerates the main reaction. Moreover, an additional heat flow event within the first 30 minutes was detected by STPP addition. However, from the results of QXRD analysis, crystalline hydrate phases are formed only during the main reaction. These are CAH10, C2AHX, AH3, hemicarbonate and monocarbonate. STPP shows an influence on the amount of crystalline C2AHX, otherwise, no difference in the qualitative phase content is observed. Gillmore‐Needle‐Testing (IMETER device) could observe an accelerated hardness development during the early heat flow event despite initially reduced viscosity. Pore water studies and thermodynamic modelling (PhreeqC) indicate the formation of an amorphous C‐P‐H phase during this early heat flow event. [ABSTRACT FROM AUTHOR]

The Design of Experiment (DOE) is a widely recognized approach to designing and analyzing experiments. It is a statistical method utilized to simultaneously examine the impact of multiple factors on the results. In this study, two different DOE methods, including mixture and factorial design were employed to evaluate the influence of various factors on the performance of ternary binders. To assess the effect of each component such as Portland cement (PC), calcium aluminate cement (CAC) and sulphate (CŜHx) on the workability and strength of ternary binders, mixture design was utilized. A two‐stage factorial design was implemented to investigate the impact of various factors on the fresh and hard properties of ternary binders. In the first stage the influence of composition, supplementary cementitious material (SCM) and cement replacement ratio were examined. In the second stage the impact of composition, CAC and CŜHx type were analyzed. Principal Component Analysis was also employed to analyse the continuous data obtained from the XRD patterns. The study results indicated that increased CAC and CŜHx content in the mixture enhances the workability of the pastes. High compressive strengths were found to be a result of increased cement content in the mixture and low SCM substitution. [ABSTRACT FROM AUTHOR]

Cementitious materials employed in deep‐sea marine engineering must have excellent durability. In this study, the durability of a ternary binder‐based paste composed of Portland cement, calcium aluminate cement, and anhydrite was investigated after deep‐sea exposure at a depth of 3515 m for 438 days. The recovered paste specimen showed significant expansion. However, the hydrate phases in the ternary binder exhibited no changes with the exception of the formation of Friedel's and Kuzel's salts. One reason for this was the transformation from unstable monosulfate to stable ettringite under low‐temperature seawater conditions. However, the additional ettringite formation could lead to significant expansion of the paste specimen. [ABSTRACT FROM AUTHOR]

The subject of the research is features of synthesis of alumina cements on the basis of compositions of system CaO – CoO – Al2O3 – MoO3 with use of cobalt‐molybdenum‐containing waste for reception of refractory materials with a complex of the set properties. The processes of phase formation in raw mixtures have been investigated. X‐ray phase studies of clinkers fired at different temperatures and holding times proved that as a result of the interaction of the raw materials of the mixture in the material synthesizes a mixture of hydraulically active calcium mono‐ and di‐aluminate and refractory cobalt spinel, which provides the resulting complex high strength, accelerated curing time, fire resistance. The absence of phases corresponding to molybdenum compounds is explained by the fact that they are part of hydraulically active aluminates as limited solid solutions, deforming the crystal lattice and increasing hydraulic activity. Thus, it was found that the high strength of cement is due to the presence of calcium hydroaluminates such as C2AH8, aluminum hydroxide, as well as unhydrated grains of calcium aluminates, which will contribute to further long‐term strength. It is this coexistence of phases in both the crystalline and colloidal states provide high strength cement stone. [ABSTRACT FROM AUTHOR]

The amount of early ettringite crystallization is known to influence the rheology of cementitious materials. It is dependent on various factors, e.g., the reaction temperature, the chemical composition of the cement and the amount of PCE. In this study, we investigate the rheology and reactivity of synthetic model cements with different contents of C3S, C3A and β‐hemihydrate (β‐HH). Their reactivity is studied using in‐situ calorimetry and their rheology is studied using a rotational rheometer. Our results demonstrate that higher C3A contents in the clinker increase the early heat release of the cements which is associated with an increased formation of ettringite. Increased C3A contents from 2.5 wt.% to 12.5 wt.% enhance the early heat by four orders of magnitude. The static yield stress of the different cement pastes was determined with an oscillation based rheometry method 17 minutes after contact with water. Interestingly, the static yield stress increases exponentially with the heat of hydration. These findings have implications for the understanding of OPC hydration, as well as for chemical optimization and the design of OPC‐based materials. [ABSTRACT FROM AUTHOR]

To address the environmental problems caused by the storage of solid waste, such as red mud (RM), silica fume (SF), carbide slag (CS), and flue gas desulphurisation gypsum (FDG), and the poor corrosion resistance of Portland cement (PC), anti-erosion grouting materials (AGM) are synthesised from RM, SF, CS, and FDG. The synthesis, structural characterisation, and properties of AGM are reported and compared with those of typical PC. The results show that the AGM matrix is composed mainly of hydration phases: 68.3 wt.% β-dicalcium silicate (β-C2S), 21 wt.% tetracalcium ferroaluminate (C4AF), and 8.4 wt.% tricalcium aluminate (C3A). AGM prepared with 8 wt.% FDG exhibits the best compressive strength and rheological characteristics. Hydration of the AGM leads to the formation of ettringite, calcium alumino-ferrite hydrate (C3(A, F)H6), and aluminium gel, resulting in a compressive strength of 29.2 MPa at 210 days. The anti-erosion coefficients K of AGM after exposure to different erosion solutions for 28 days are > 0.99, indicating better corrosion resistance than PC. This anti-erosion property is due to a mineral composition characterised by low tricalcium silicate(C3S) content and high C4AF content. This study provides a feasible and promising method for the preparation of for coastal engineering materials. [ABSTRACT FROM AUTHOR]

In this paper, a new approach in the development of calcium aluminate (CA) coatings for ceramic foam filters for steel melt filtration is presented. This approach is based on applying an environmentally friendly binder system based on lactose and tannin. The use of such a binder was previously demonstrated for the matrix of carbon-bonded alumina filters. In this research, the influence of the total solid content, the binder content, and the centrifugation speed on the coated ceramic foam filters' final properties was studied. All of the prepared coating slips were analyzed in terms of their rheology. The most promising slips were applied on carbon-bonded alumina filters, which were prepared using a lactose- and tannin-based binder system. For the selected coating slips, three different centrifugation speeds (600, 1000, and 1400 rpm) were used to optimize the coating layer thickness and the overall amount of the coating material applied to the filter. The coated and thermally treated filters were tested in terms of their cold crushing strength. Furthermore, the microstructure of the filters and mainly the coating layer thickness were studied. It was observed that both the centrifugation speed and the total solid content in the coating slip have a major impact on the filter properties. The most promising results were obtained for a coating containing 72.5 wt% of total solids applied at 1000 rpm centrifugation speed. This batch of coated samples showed a cold crushing strength of more than double the value obtained for the uncoated reference, while having a uniform coating layer with the thickness of ∼177 μm. [ABSTRACT FROM AUTHOR]

Premature decommissioning of a steel-teeming ladle lining due to critical localized damage, which significantly reduces lining functional properties, is one of the most acute problems in modern metallurgy. The main type of products used for steel-teeming ladle lining is periclase-carbon. In order to improve corrosion resistance and resistance to thermomechanical loads, additional additives, such as spinel, and high alumina CMA cement, are introduced into periclase-carbon products. In this article the effect of calcium aluminate-based additive on the properties of periclase carbon refractories is investigated. It is assumed that there is a potential for formation of magnesium aluminate spinel (MA) in periclase-carbon products with calcium-aluminate additives during heat treatment. Fused periclase is used as the raw material, the carbon component is represented by natural flaky graphite in an amount of 10 wt.%, and a combined binder. An additive based upon calcium aluminates is introduced, with a fractional size from 0 to 3 mm, while the amount of main fractions of fused periclase is reduced by the corresponding amount of the additive introduced. The compressive strength, apparent porosity, and bending strength values before and after coking firing are analyzed. The thermal coefficient of linear expansion, phase composition and microstructure of the products are also investigated. [ABSTRACT FROM AUTHOR]

The influence of Fe 2 O 3 on the hydration kinetics of tricalcium aluminate (C 3 A) was studied in order to clarify the mechanism of improving hydration resistance of CaO by in-situ synthesized tricalcium aluminate. The Krstulovic-Dabic model was used to investigate the hydration processes of Fe 2 O 3 -C 3 A solid solution and calculate the corresponding kinetic parameters. The hydration products were analyzed by the X-ray diffraction and scanning electron microscope. The results indicated that the Krstulovic-Dabic model simulated the hydration processes of Fe 2 O 3 -C 3 A solid solution at different stages effectively. The hydraulic activity of Fe 2 O 3 -C 3 A solid solution decreased with the addition of Fe 2 O 3. Reasonable amount of Fe 2 O 3 addition reduced the hydration rate in the initial stage of Fe 2 O 3 -C 3 A solid solution hydration, while the hydration rate of Fe 2 O 3 -C 3 A solid solution was increased with excessive amount Fe 2 O 3. The hydration process was controlled by multiple mechanisms due to an incomplete layer of hydration products was formed on the surface. [ABSTRACT FROM AUTHOR]

In this contribution, the widely used non-ecological production process of ceramic foam filters was modified into an environmentally friendly procedure, in which no hazardous materials are involved. Firstly, a large number of carbon-bonded ceramic foam filters were prepared by this approach. In the next step, a thin coating consisting of graphene-doped, carbon-bonded calcium aluminate was applied over these filters. Reference samples as well as coated samples were analyzed in detail with a focus on the phase composition, microstructural analysis, and mechanical as well as thermal properties. The results confirmed that the application of the developed coating improved the cold crushing strength of the ceramic foam filters. In addition, by application of graphene nanoadditives, it is also possible to vary the surface microroughness which will subsequently enable improved efficiency of inclusions elimination from metal melts during the filtration process. [ABSTRACT FROM AUTHOR]

To improve the dispersion of carbon nanotubes in refractory castables and exert the strengthening effect of carbon nanotubes on castables, the core-shell CNTs/CAC (carbon nanotubes/calcium aluminate cement) was synthesized by the catalytic carbon-bed sintering method. The effects of catalyst content on the microstructure of CNTs/CAC powders were evaluated by Raman spectroscopy, SEM, and HRTEM. The results suggested that the optimal addition amount of catalyst was 0.75 wt%. The diameter of the bamboo-like carbon nanotubes was ∼50 nm and the length of the carbon nanotubes was ∼120 μm. Carbon nanotubes and small calcium aluminate particles were interspersed with each other and wrapped in a compact calcium aluminate shell with a thickness of 3∼5 μm. Moreover, the DFT calculation was employed to investigate the growth mechanism of carbon nanotubes. Electrons were transferred from Ni to H, O, and C atoms, increasing bond length and weakening the strength of C–H and C O bonds which was conducive to the growth of carbon nanotubes. The formation of the core-shell structure was in direct connection with the liquid-phase eutectic phase. Calcium carbonate and alumina reacted preferentially to form C 12 A 7 , and then C 12 A 7 continued to react with alumina to form CA and CA 2. With the increase of sintering temperature, a large amount of liquid phase was formed. Carbon nanotubes were wrapped by the liquid phase of calcium aluminate to form a core-shell structure. The CNTs/CAC cement bonded Al 2 O 3 –SiC castables had great mechanical properties and thermal shock resistance. Compared with Al 2 O 3 –SiC castables bonded with Secar71, the HMOR and thermal shock residual strength ratio of Al 2 O 3 –SiC castables bonded with CNTs/CAC cement were increased by 24.7% and 22.1%, respectively. [ABSTRACT FROM AUTHOR]

Optical thermometers are advantageous for temperature measurement in electromagnetic fields and aggressive environments; however, their composition mostly relies on materials doped with expensive and resource-limited rare earth ions. In this article, we describe the application of calcium aluminate doped with transition metal ions (Mn2+ and Cr3+) in optical thermometry, employing optical fibres for signal transmission. Upon excitation with 450 nm laser diode radiation, changes in the luminescence of Mn2+ ions in the 500–550 nm band are followed along with changes in the Cr3+ band at 750–800 nm. The application has been tested in the temperature range from 20 °C to 800 °C. The temperature dependence of Cr3+ luminescence encompasses the high-temperature range, whereas the luminescence band of Mn2+ ions gives an increase in the total intensity and provides a more consistent change in the range from 400 °C to 550 °C. [ABSTRACT FROM AUTHOR]

Alloys and oxidation-resistant coatings utilized in high-temperature applications can be degraded by aerosols that deposit onto surfaces during operation. Understanding how the deposit composition influences the hot corrosion mechanisms is essential to develop more durable materials. This work advances the understanding of the effect of complex oxide and sulfate deposits on the degradation of an alumina-forming FeCrAlY alloy in comparison to reactions with single-crystal sapphire. The deposit compositions were developed to systematically understand the effect of anion makeup (mixed oxides, oxide–sulfate, and sulfates) and the effect of adding Na and K salts. CaSO4 was used as a control. The mixed oxide and oxide–sulfate deposits increased the frequency of thermally grown oxide (TGO) intrusions in FeCrAlY but did not produce a noticeable change in the sapphire. Pure CaSO4 and mixed sulfate reacted with the TGO and sapphire to form calcium aluminates and led to roughening of the specimen–reaction product interface. The primary difference between the CaSO4 and mixed sulfate deposits was the increased uniformity of the attack by the latter due to its tendency to melt and spread. Comparison between the change in the degradation features in the presence of CaSO4 and mixed sulfate deposits on both types of specimens expands the current understanding of sulfate-based hot corrosion. [ABSTRACT FROM AUTHOR]

Possible applications of a new matrix were studied for incorporating radioactive aqueous NaNO3 solutions arising at the nuclear fuel cycle enterprises. The matrix is formed by solidifying the solutions under study with cement consisting of a mixture of industrial calcium aluminate with gypsum. The compressive strength of the matrix was investigated as a function of the cement composition. The positive effect of sodium nitrate on the matrix strength was discussed and explained. As a result of the study, a matrix containing up to 24 wt.% NaNO3 was obtained, meeting all the requirements imposed on the cemented radioactive waste. [ABSTRACT FROM AUTHOR]

The deterioration of the microscopic pore structure of concrete under external sulfate attack (ESA) is a primary cause of degradation. Nevertheless, little effort has been invested in exploring the temporal and spatial development of the porosity of concrete under ESA. This study proposes a mechanical-chemical model to simulate the spatiotemporal distribution of the porosity. A relationship between the corrosion damage and amount of ettringite is proposed based on the theory of volume expansion. In addition, the expansion strain at the macro-scale is obtained using a stress analysis model of composite concentric sphere elements and the micromechanical mean-field approach. Finally, considering the influence of corrosion damage and cement hydration on the diffusion of sulfate ions, the expansion deformation and porosity space-time distribution are obtained using the finite difference method. The results demonstrate that the expansion strains calculated using the suggested model agree well with previously reported experimental results. Moreover, the tricalcium aluminate concentration, initial elastic modulus of cement paste, corrosion damage, and continuous hydration of cement significantly affect concrete under ESA. The proposed model can forecast and assess the porosity of concrete covers and provide a credible approach for determining the residual life of concrete structures under ESA. [ABSTRACT FROM AUTHOR]

This article analyses the combined effect of a mixture of natural zeolite and ground soda lime silicate glass on the durability properties of concrete. The specimens were prepared by replacing 25% of the cement with a mixture of zeolite and glass at different proportions with zeolite particles of two different sizes and cured for 28 and 90 days. Chloride migration, the depth of water penetration under pressure, and freeze–thaw cycle tests were performed, and compressive strength was determined. The combined effect of the mixture on the composition and microstructure of the cement matrix was studied by analysing the results of XRD, TGA, SEM-BSE, and porosity tests. The test results showed that irrespective of the cement "dilution" effect, the mixture of additives had a beneficial combined effect: increased durability of concrete, a denser structure of the cement matrix, the formation of calcium aluminate hydrates, faster rate, and higher degree of hydration. [ABSTRACT FROM AUTHOR]

This study investigates the influence of various factors on the performance of ternary binders, utilizing statistical approaches. The research focuses on the influence of varying compositions of Portland Cement-Calcium Aluminate Cement-Calcium Sulphate (PC-CAC-CŜ), types and amounts of mineral powders, and chemical admixtures in ternary binders. Using the Taguchi design, the study required a limited number of experimental trials, utilizing a standard orthogonal array of seven factors across three levels. These factors encompassed binder composition (C1-C2-C3), mineral powder types (limestone, quartz, slag), replacement ratio (0%, 25%, 50%), retarder (0%, 0.1%, 0.2%), superplasticizer, viscosity modifying agent (stabilizer) and accelerator (0%, 0.05%, 0.1%). Measurements on hydration kinetics, dimensional stability, compressive strength, and microstructural analyses like X-ray diffraction were conducted. Principal Component Analysis (PCA) was employed to interpret the continuous data derived from heat of hydration curves, length change curves and X-ray diffraction (XRD) patterns. Results indicated that retarder quantity and binder type significantly impacted paste workability. Higher powder content led to reduced strength, whereas increased accelerator improved strength. A strong correlation was observed between accelerator content and the dimensional stability. The primary hydration product's formation was predominantly influenced by the PC-CAC-CŜ ratio, accelerator, and cement substitutions. [ABSTRACT FROM AUTHOR]

Calcium aluminate cement (CAC) is one of the alternative cements that is widely used for special applications. However, during the hydration process degradation of CAC microstructure, the so-called hydrate conversion process, hexagonal calcium aluminate hydrate (CAH10) transforms into a cubic (C3AH6) phase, resulting in increased porosity and reduced strengths. It is known that alternative means for stabilizing the CAC conversion are conducted by introducing fly ash (FA) in CAC, where its microstructure is attributed to aluminosilicates. However, no study has yet been conducted on different FA compositions influencing CAC performance. This study aims to evaluate the effects of different compositions of FA on CACs' fresh and hardened characteristics. Results revealed that the microstructure was denser when CAC was mixed with FA. Regarding reactivity, CAC with calcium-rich FA systems is 13% faster than the silica-rich one. The higher the density and the lower the porosity of calcium-rich FA mixtures were found compared with silica-rich FA in both micro- and macro-structures. As seen in the microscopic structure, this is due to the calcium-rich phase formation. [ABSTRACT FROM AUTHOR]

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We investigated the reaction kinetics and initial chemical conditions in the production of silicon alloys, employing aluminum as the reductant for calcium silicate slag, to enhance process economics and scalability to industrial levels. The apparent kinetics and transient chemical conditions were studied by immersing solid aluminum into molten slag, allowing the reaction to proceed for varying durations without external agitation, before quenching the reaction for chemical and microscopic analyses of the resulting silicon alloy and slag. The majority of the conversion was observed within the first 15 s at 1650 °C, driven by significant chemical interactions and interfacial turbulence introduced upon aluminum immersion. For Al-SiO2 stoichiometries ranging from 0.5 to 1.2, the slag phase reaction conformed to first-order kinetics during the initial two minutes, when it approached equilibrium. The mass transfer coefficients for Al2O3 were estimated at 1–2 × 10−4 m/s, comparable to those for SiO2 and CaO. A constant mass transfer coefficient could not be established for stoichiometries of 1.6 and 2, as these deviated from the standard slag mass transfer relationship and did not adhere to established relationships. Despite near-complete reactions, alloy–slag mixing was extensive, decreasing with lower stoichiometry values.

In heavy oil recovery, calcium aluminate cement (CAC) is in the working environment of "low-temperature hardening and ultrahigh temperature service." However, the formation of C 3 AH 6 under low temperatures results in a decrease in strength and reduce the cementing quality. In this study, titanium extraction slag (TES) was used to inhibit CAC strength deterioration. TES, characterized by a high Ti content, presents challenges in terms of utilization and poses significant ecological risks owing to its large accumulation. Cementite hydration with 0%, 20%, 30%, and 40% TES relative to CAC was examined at 30 °C for 28 d. The high C 3 AH 6 content of the pure CAC increased the strength deterioration, pore size, and cementite carbonation. With 20% TES, a dilution effect was observed without strength improvement. Furthermore, 30% TES generated layered double hydroxides and converted C–S–H into C–A–S–H, thereby increasing compressive strength. By-products were generated with 40% TES, which inhibited the strength development while generating C–A–S–H to maintain the compressive strength. Therefore, TES can inhibit the strength decline of CAC, and the byproducts of the LDH structure can improve corrosion resistance. [ABSTRACT FROM AUTHOR]

The article studies the possibility of using a matrix of gibbsite and kuzelite for the preservation of liquid radioactive wastes consisting of aqueous NaNO3 concentrates. The matrix is formed during the solidification of these wastes with cement, consisting of calcium aluminate and gypsum. The requirements for cemented solid waste are met by a matrix obtained from a mixture of the following composition: 45.2%-NaNO3 solution 53.04%; gypsum—5.4%; calcium aluminate—36.16%; diatomite powder treated with polydiallyldimethylammonium chloride (polyDADMAC)—5.4%. The matrix has the following characteristics: ratio of matrix and cemented solution volumes—1.45; average 137Cs leaching rate for 90 days—1.4∙10−4 g/(cm2∙day); strength—15.2 MPa, including after the tests for water resistance, 30 freezing-thawing cycles, and irradiation to a dose of 1 MGy—6.8, 15.4, and 9.6 MPa, respectively. [ABSTRACT FROM AUTHOR]

In the presented study, ye'elimite-aluminate-calcium (YAC) cement was synthesized. Complete synthesis of crystalline phases was achieved at a temperature of 1300 °C, which is 150 °C lower than the temperature standardly used in the processes of obtaining calcium aluminate cements (CAC). The greatest amount of ye'elimite phase (Klein complex), roughly 87% by mass, was acquired utilizing a sulphur ion transporter derived from artificial dihydrate gypsum obtained in the flue gas desulphurization process (variation I). In the case of anhydrite, the amount of synthesized crystalline ye'elimite in the clinker was 67% by weight (variant II). Depending on the synthesis conditions in the clinkers, the quantity of obtained calcium aluminates (C12A7, CA, and CA2) ranged from 20 to 40% by weight. Studies on the hydration process of YAC cement samples showed that the main products are hydrated calcium aluminates and dodecahydrate calcium alumino-sulphate. In sinters of YAC and OPC, no crystalline ettringite was observed. Hydration analysis of Chinese cement revealed the presence of crystalline ettringite and dodecahydrate calcium alumino-sulphate, as well as hydrated calcium silicates of the CSH type accompanied with pseudo-crystalline Al(OH)3. The obtained clinkers from variants I and II constitute a special binder, which, due to its phase composition after hydration, can be used in the construction of reactors for biogas production in eco-energy applications. [ABSTRACT FROM AUTHOR]

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

In order to promote the resource utilization of steel slag, based on the stability evaluation of steel slag, using lime as alkali activator, the unconfined compressive strength test, X-ray diffraction (XRD) and scanning electron microscope (SEM) were used to study the effects of different alkali-activated slag mix ratios and alkali-activated slag content on the compressive strength of slag-stabilized steel slag mixture. The hydration products and strength enhancement mechanism of slag-stabilized steel slag mixture was discussed. The research showed that the strength of the mixture increased first and then decreased with the increase of lime content, and the optimal mix ratio of lime and mineral powder was 1 ∶4. 2. Under this mix ratio, the 7 d strength of the mixture with 4. 5% alkali activated mineral powder content reached 6. 27 MPa, which met the technical requirements of high-grade pavement. With the increase of alkali-activated slag content, the compressive strength of the mixture increased, the elastic section of the load-deformation curve increased gradually, the plastic section and the descending section decreased, and the plastic deformation capacity decreased. Lime can better stimulate the potential activity of the slag-steel slag system. The hydration products included hydrated calcium silicate, hydrated calcium aluminate and Jennite. The internal honeycomb and fibrous cementitious structure made the internal structure of the mixture denser and improved the macroscopic strength and volume stability of the mixture. [ABSTRACT FROM AUTHOR]

We have fabricated the most versatile MAl2O4 (M=Zn, Mg, Ca) spinel nanopowder by citrate sol–gel method using ethylene glycol as a chelating compound. X-ray diffraction method has been employed to analyze the crystalline behavior of prepared nanopowders. The sample annealed at 800 °C indicates the cubic phase for zinc and magnesium aluminates while the monoclinic phase has been noticed for calcium aluminates. Structural parameters like d-spacing, lattice constant 'a', microstrain, stacking fault were evaluated for prepared nanopowders. The Debye–Scherrer formula is being used to calculate the prepared powder's crystallite size, which is further compared with the results of W–H analysis. Surface morphology and elemental composition were examined by SEM–EDS. Methyl blue dye has been selected for evaluating the degradation efficiency of aluminates under sunlight. The study reveals that ZnAl2O4 has the highest degradation efficiency (~ 98%) owing to its large surface area, optimum bandgap, and suitable nanocrystalline size. [ABSTRACT FROM AUTHOR]

The paper considers different aspects of natural and forced carbonization of binding agents and cement systems. It is shown that depending on the composition of binding agents, curing conditions, and transient factors, carbonization of hydrated cement can not only lead to corrosion and irreversible degradation, but also serve as a powerful productive endeavor for strengthening of concrete structures, self-healing of cracks in concrete, shrinkage reduction, and carbonization corrosion of constructional materials. Based on theoretical analysis of experimental data and the literature, a working hypothesis is developed concerning the observed processes of the structure formation based on the interaction of carbon dioxide and its derivatives with calcium aluminates and aluminate ferrites with the formation of hydrated calcium carboaluminates of various composition. Portlandite fixation is crucial for concrete structures capable of resisting to both carbon dioxide and sulfate corrosion and, sometimes, in introducing chemical additives, can lead to the structure strengthening. [ABSTRACT FROM AUTHOR]

The by-products of the circulating fluidized-bed boiler combustion (CFBC) of coal exhibit self-hardening properties due to the calcium silicates generated by the reaction between SiO2 and CaO, and the ettringite generated by the reaction of gypsum and quicklime with activated alumina. These reactions exhibit tendencies similar to that of the hydration of ordinary Portland cement (OPC). In this study, the self-hydration and carbonation reaction mechanisms of CFBC by-products were analyzed. These CFBC by-products comprise a number of compounds, including Fe2O3, free CaO, and CaSO4, in large quantities. The hydration product calcium aluminate (and/or ferrite) of calcium aluminate ferrite and sulfate was confirmed through instrumental analysis. The CFBC by-products attain hardening properties because of the carbonation reaction between calcium aluminate ferrite and CO2. This can be identified as a self-hardening process because it does not require a supply of special ions from the outside. Through this study, it was confirmed that CFBC by-products generate CaCO3 through carbonation, thereby densifying the pores of the hardened body and contributing to the development of compressive strength. [ABSTRACT FROM AUTHOR]

Calcium aluminate cement (CAC) is broadly used as a binder for ceramic materials as it can provide suitable setting times, green mechanical strength and refractoriness. When applied to directly-foamed macroporous refractories, this binder can set the fresh foam before bubble coalescence, favouring higher porosity. Nevertheless, its large particles tend to thermodynamically stabilize bigger bubbles, resulting in samples with larger pores and broader cell size distribution, which is undesirable for high-temperature thermal insulation applications. An alternative to overcome this issue is CAC milling, which can be carried out in water by using sodium gluconate (SG) to produce a stable suspension of this binder. By using 1 wt% of SG, it was possible to grind the CAC in water for 6 h without hydration. When applied to macroporous samples, milled CAC provided faster setting kinetics and in situ formation of hibonite after firing. Additionally, smaller average pore size and higher cold crushing strength were attained when used for the production of ultrastable foam-derived refractories, whereas less significant changes were observed for samples based on surfactant-stabilized foams. The study also showed that the CaO/Al 2 O 3 particle size ratio was an important feature to control the shrinkage/expansion effects after sintering, based on the morphological aspect of the resulting hibonite (CA 6) phase. [ABSTRACT FROM AUTHOR]

To overcome the bottleneck in improving the high-temperature mechanical properties of SiC castables bonded with vulnerable oxides, a lab-synthesized special carbon nanofibers/calcium aluminate cement (CNFs/CAC) was used as a binder for SiC castables. The microstructural evolution, thermal shock resistance and high temperature modulus of rupture properties were investigated. The results showed that uniform and continuous SiC nanowire networks were generated in the matrix of SiC castables bonded with CNFs/CAC. The continuous network-like SiC nanowires were grown in the liquid phase using carbon nanofibers as templates. The high temperature modulus of rupture (HMOR) of the SiC castables bonded with CNFs/CAC was 52.2% higher than that of SiC castables bonded with Secar71. The residual strength ratio of the SiC castables bonded with CNFs/CAC after thermal shock increased by 28.3% (from 70.5% to 90.5%) compared to that of the SiC castables bond with Secar71. This is because the oxide binder was significantly reinforced by the SiC nanowires. [ABSTRACT FROM AUTHOR]

This study aims to approximate the optimum sulfate content of cement, applying maximization of compressive strength as a criterion for cement produced in industrial mills. The design includes tests on four types of cement containing up to three main components and belonging to three strength classes. We developed relationships correlating to 7- and 28-day strength with the sulfate and clinker content of the cement (CL), as well as the clinker mineral composition (tricalcium silicate, C3S, tricalcium aluminate, C3A). We correlated strength with the ratio %SO3/CL and the molecular ratios MSO3/C3S and MSO3/C3A. The data processing stage proved that artificial neural networks (ANNs) fit the results' distribution better than a parabolic function, providing reliable models. The optimal %SO3/CL value for 7- and 28-day strength was 2.85 and 3.00, respectively. Concerning the ratios of SO3 at the mineral phases for 28-day strength, the best values were MSO3/C3S = 0.132–0.135 and MSO3/C3A = 1.55. We implemented some of the ANNs to gain a wide interval of input variables' values. Thus, the approximations of SO3 optimum using ANNs had a relatively broad application in daily plant quality control, at least as a guide for experimental design. Finally, we investigated the impact of SO3 uncertainty on the 28-day strength variance using the error propagation method. [ABSTRACT FROM AUTHOR]

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Bauxite residue (BR) is a by-product from the aluminum industry, generated during the Bayer process to obtain alumina. It has valuable iron and alumina contents, with soluble ions that can interact with the cement compounds, which could make BR a new source of supplementary cementitious material. This work aims to determine the influence of different contents of BR in substitution to two types of Portland cement regarding the hydration reactions and chemical shrinkage during the first 48 h of hydration, as well as to monitor the crystalline phases with XRD and the amount of bound water and the hydrates formed with thermogravimetric analysis. The results show the interaction of BR and cement, with the formation of new products, such as N-A-S-H, C-A-S-H, aluminum silicate hydrate (AS), and calcium aluminate hydrate (CAH), that culminate in changes in the shrinkage as BR is added. It was also observed that BR accelerates and enhances the slag reactions due to its high alkalinity and soluble ions content and the fineness of the material added to the cement. [ABSTRACT FROM AUTHOR]

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). [ABSTRACT FROM AUTHOR]

The present study aims to clarify the influence of ammonium metatungstate (AMT) on composition, microstructural evolution and properties of Al 2 O 3 –SiC–C (ASC) refractory castables. Castables with different AMT contents were prepared using brown corundum, white corundum, silicon carbide, calcium aluminate cement, and AMT as major raw materials. The results show that hot modulus of rupture, thermal shock resistance and slag resistance were significantly improved for castables with addition of AMT (lower than 1.0 wt%), due to the optimized microstructure from the formation and development of mullite and SiC whiskers in castables. Along with the in-situ formed whiskers in pores, formation of Fe 7 W 6 from the reaction between W ions in castables and Fe ions in the penetrated molten slag led to the enhanced slag viscosity and inhibited continuous penetration (penetration index decreased from 30.5% to 20.2%) for castables with AMT ranging from 0 to 0.5 wt%. The research of this work provides a new insight into microstructure and high-temperature properties enhancement of ASC refractory castables. [ABSTRACT FROM AUTHOR]

The present work focused on the evolution characteristics in the microstructure and properties of calcium aluminate composite ceramics after heat treatment for raw materials at different temperatures. The results showed that the apparent porosity of all the sintered samples firstly decreased to the minimum at a heat treatment temperature of 1200 °C and then increased with increasing heat treatment temperature. The compressive strength was greatly enhanced after the heat treatment for raw materials due to the significant decrease in porosity, presenting the opposite relationship with the change of porosity. The formation of the interlaced structure in the sintered samples was conducive to improving the densification of the calcium aluminate ceramics. This work provided new avenues of thought regarding the manufacturing of calcium aluminate ceramics and promoted its better application in metallurgy. [ABSTRACT FROM AUTHOR]

Based on the principle of minimum Gibbs free energy, a thermodynamic model is established to predict the composition of inclusions in the Al‐killed steel during the calcium treatment process. The Wagner model combined with coexistence theory is applied to calculate the Gibbs free energy of the steel‐inclusion system. The composition evolution of inclusions is accurately predicted through the current thermodynamic model which is widely applied to predict the modification of inclusions by the calcium treatment process with different contents of total oxygen (T.O), total sulfur (T.S), and total calcium (T.Ca) in the molten steel. Alumina inclusions can be modified to liquid calcium aluminate inclusions with appropriate T.Ca contents. Solid CaS or CaO inclusions can be formed as a result of the excessive calcium addition to molten steel, while the insufficient calcium addition can hardly modify alumina inclusions to liquid ones. The effect of the T.O and T.S contents on the liquid window is also discussed. The relationship between the value of T.S/T.O in the molten steel and the composition of inclusions is also studied. Based on the established thermodynamic model, a software for the precise control of inclusions is also developed to guide the calcium treatment operation timely in the accrual refining process. [ABSTRACT FROM AUTHOR]

We herein report the synthesis of monoclinic calcium aluminate (CaAl2O4) nanoparticles via a facile auto-combustion method followed by calcination. We performed the auto-combustion method using aluminium nitrate and calcium nitrate as oxidants and different fuels as reductants such as urea, glycine, and a mixture of urea and glycine, with various fuel-to-oxidant equivalence ratios (Φ c). Then, the combusted samples were calcined at different temperatures; 600 and 800 °C. The products were characterized by means of X-ray diffraction, Fourier transform infrared spectroscopy, thermo-gravimetric analysis, field-emission scanning electron microscope, and high-resolution transmission electron microscope. CaAl2O4 nanoparticles with an average crystallite size of 40.4, 38.8, and 33.7 nm were obtained after calcination at 800 °C using the aforementioned fuels, respectively. TEM images revealed that CaAl2O4 nanoparticles tend to form partially sintered aggregates owing to the high thermal treatment temperature, so they have non-uniform shapes. The produced CaAl2O4 nanoparticles exhibited good absorptivity toward Ni(II) and As(III) ions form aqueous media. The maximum sorption capacities (qm) of CaAl2O4 for the removal of Ni(II) and As(III) were found to be 58.73 and 43.9 mg.g−1, at pH 7 and 5, respectively. The equilibrium isotherms and adsorption kinetics studies revealed that the adsorption data fitted well Freundlich isotherm and pseudo-second-order models, respectively. Besides, the adsorption of Ni(II) and As(III) ions on CaAl2O4 nanoparticles is physisorption. Overall, the obtained results indicated that calcium aluminate nano-adsorbent is a good candidate for the removal of Ni(II) and As(III) ions from wastewater, due to its high efficiency, stability, and re-usability. [ABSTRACT FROM AUTHOR]