Calcium Aluminate Cement Pastes Research Articles

Effect of sulfidic mine tailings used as mineral admixtures on the hydration of common and alternative cements

In this work, we investigate the use of pyrite-rich tailings from an operational mine as mineral admixture in different cement matrices [Portland cement, calcium aluminate cement (CAC), and calcium sulfoaluminate cement (CSA)]. Hydration and microstructure changes were studied on cement pastes produced with a 30 wt% replacement of cement with tailings, up to 200 days. Based on our results, the effect of the tailings on the hydration of Portland cement is limited to a physical effect, and no sulfide-induced degradation is observed. In the CAC and CSA pastes, minor mineral phases present in the tailings chemically react, leading to changes in the mineral phase composition of CAC and CSA hydrated pastes. In addition, in all cement pastes studied, and more effectively in the CSA pastes, most of the metal(loid)s contained in the tailings were safely immobilized. Cement chemistry notation: C: CaO; A: Al2O3; F: Fe2O3; S: SiO2; S̅: SO3; c: CO2; H: H2O.

Changes in the physicochemical properties of calcium aluminate cement paste with high alumina content under deep seas

Changes in the physicochemical properties of calcium aluminate cement paste with high alumina content under deep seas

Potential of sodium chloride solution as mixing water in calcium aluminate cement modified with siliceous minerals

Calcium aluminate cement (CAC) is advantageous in marine engineering, and using materials directly from the sea when mixing CAC can reduce both construction costs and ecological pollution to the sea. However, using seawater as the mixing water in CAC is difficult due to the strength reduction. In this work, CACs were prepared with siliceous minerals (SMs) (diatomite and silica fume (SF)) and sodium chloride solution (SCS) as the mixing water. For comparison, other samples were immersed in SCS rather than mixed with SCS. The strength, mass and length changes of CAC mortars cured at 20°C and 40°C were determined. The phase assemblages and morphology were assessed using X-ray diffraction, thermogravimetric-differential scanning calorimetry and scanning electron microscopy. Using SCS as the mixing water was beneficial to the later strength development of CAC. It promoted the formation of C2ASH8 in CAC pastes and showed no strength loss even when cured at 40°C. The strengths of the SM-modified CAC mortars mixed with SCS were higher than those immersed in SCS. The combination of diatomite and SCS significantly increased the CAC strength, especially at 20°C, while SF contributed to better stability in mass and length.

A study on the hydration of calcium aluminate cement pastes containing silica fume using non-contact electrical resistivity measurement

In this study, a non-contact electrical resistivity method is employed to investigate the hydration process of calcium aluminate cement (CAC) pastes incorporating silica fume (SF) at an early age. The CAC-SF pastes have a water-to-binder ratio (w/b) of 0.6 and a replacement of SF for CAC at 0, 5, 10, and 20% (by weight of the binder). The pastes are tested for electrical resistivity, thermal analysis (internal temperature), setting time, and compressive strength, and the hydration products of the pastes are also examined. The results indicate that the evolution of electrical resistivity is closely related to the hydration process of the CAC pastes, and the conversion stage is closely associated with the setting time of pastes. As the increase of SF replacement, the hydration process of CAC pastes was accelerated within 24 h, and the development of compressive strength was hampered at an early age but increased later. SF replacement inhibits the conversion of metastable CAC hydrates (CAH10 and C2AH8) to C3AH6 through filling, seeding, and reacting with CAC, thus reducing the heat of hydration.

Effect of the curing process on the thermomechanical properties of calcium aluminate cement paste under thermal cycling at high temperatures for thermal energy storage applications

Future perspectives to improve the energy efficiency of concentrating solar power (CSP) plants are focused on increasing temperatures above 600 °C. Among the different components of a CSP plant, the thermal energy storage (TES) medium must withstand high operating temperatures. Concrete was identified as an exciting candidate for its mechanical and thermal properties, needing further experimental research about this specific application. A fundamental concrete element is the cement binder, bringing cohesion to the composite components. As a requisite, the cement needs to be heat-resistant, and calcium aluminate cement (CAC) suits this demand. This cement is characterised by curing temperature-driven crystallisation changes, triggering an alteration of material properties. Considering that at 60 °C, the metastable hexagonal crystallisation is converted into a stable cubic crystallisation, seven curing cases were proposed in this study. After the curing process, thermo-mechanical properties of calcium aluminate cement paste were tested before and after thermal cycles from 290 °C to 650 °C. The results showed that, despite thermal cycling, the immediate hydration at 60 °C results in a higher thermal conductivity and compressive strength than standard curing at 20 °C.

Comparison of calcium aluminate cements on hydration and strength development at different initial curing regimes

This paper examines the influence of CA2 (monocalcium dialuminate, CaAl4O7) in calcium aluminate cement (CAC) on strength development relevant to conversion reaction in the hydration process. to ensure the different phase compositions at early ages, a curing temperature of 5 and 65 °C was applied to fabricating cement paste with two types of CAC for the first 24 h. The hydration behavior of the specimen was investigated by isothermal calorimetry, X-ray diffraction, and thermogravimetry. Simultaneously, the development of pore structure was analyzed by mercury intrusion porosimetry. The results showed that CAC containing CA2 exhibits a gradual increase in strength with no drop during test periods, regardless of initial temperature, while the typical CAC paste treated at 5 °C suffers from a strength loss due to conversion. The performance of refractory CAC may arise from CA2 hydration, producing amounts of crystalline gibbsite and thus mitigating/restricting the transformation of metastable hydrates into stable ones in order to preserve their chemical balance in the cement matrix. This phenomenon could be supported by a variation in hydrate composition and pore volume derived from the present works. However, conversion reaction should be verified in the temperature-fluctuated environment and over long-term stability in future work.

Effects of phosphogypsum on hydration properties and strength of calcium aluminate cement

• Phosphogypsum reduces setting time and flowability of calcium aluminate cement and increases compressive strength. • Phosphogypsum changes the hydration products of calcium aluminate cement. • Phosphogypsum improves pore structure of aluminate cement pastes. Phosphogypsum (PG) is a solid waste discharged from the industrial wet production of phosphoric acid. In order to improve the utilization rate of PG and understand the performance of PG mixed with calcium aluminate cement (CAC), it is necessary to determine the effect of different dosages of PG on the performance of CAC. therefore, this study investigated the hydration properties and compressive strength of PG and CAC mixtures. PG was added into CAC in an equal proportion, and the hydrated products were studied by XRD, TG-DTG, SEM and MIP methods. The research demonstrate that with the increase of PG content, the setting time and flowability of CAC pastes gradually decreased, and the compressive strength increased gradually. However, when the content of PG is greater than 15%, the growth rate of compressive strength of CAC becomes slower. The presence of PG resulted in the complete disappearance of the hydration products CAH 10 and C 2 AH 8 of CAC. With the increase of PG content, the pore size distribution of CAC paste was changed, and its main hydration products were transformed into ettringite, AFm and AH 3 , which became the main source of affecting the compressive strength of CAC.

Optimisation of early hydration, microstructure, and elevated-temperature resistance of calcium aluminate cement using steel-making slag

The strong elevated-temperature resistance of calcium aluminate cement (CAC) allows it to withstand the ultrahigh temperatures required for the in situ combustion of heavy oil; however, the high early hydration heat and strength retrogression of this cement restrict its application in the cementing engineering of heavy oil wells. Therefore, in this study, we investigated the effects of various slags on the hydration heat and temperature, compressive strength, and elevated-temperature resistance of CAC pastes. X-ray diffraction (XRD), thermogravimetry, and scanning electron microscopy (SEM) were used to evaluate the influence of slag on the hydration products and microstructure of the CAC pastes at 30, 60, and 800 °C. The results revealed that adding slag reduced the hydration heat and temperature of the CAC pastes; specifically, the maximum hydration temperature of the CAC paste containing 100% 600-mesh slag (CM6-5) decreased by 33.4%. Additionally, the slag changed the hydration products of the CAC paste from metastable products (CAH10 and C2AH8) to stable products (C2ASH8) with no conversion process, leading to the dense microstructure of CM6-5. The compressive strengths of CM6-5 cured for 14 d at 30 and 60 °C increased by 60.98% and 215.35%, respectively, compared with those of the pure CAC paste. In addition, CM6-5 exhibited excellent elevated-temperature resistance. The XRD results indicated that an elevated temperature of 800 °C changed the crystal phases of CM6-5; however, when cured for 14 d at 800 °C, CM6-5 maintained a dense microstructure, and its compressive strength upon curing at 800 °C did not result in strength retrogression.

Impact of plasticizers’ types on the performance of calcium aluminate cement

Nowadays it is almost impossible to design a cement-based mixture without the application of some plasticizing additives. There have been developed many types of plasticizers, but they are mostly directed for Portland cement applications. In the case of calcium aluminate cement (CAC), they are also widely employed, but their impact has not been properly studied yet. In this paper, four different bases of plasticizers are investigated through the view of their impact on the composition of hardened CAC paste and on the physical properties of CAC composites as well. The most convenient additive for refractories was found to be plasticizer based on modified polycarboxylates. For the application of CAC at ambient temperature the best solution was the polycarboxylates. However, the dosage of latter have to be performed carefully, because these mixtures are susceptible to bleeding. On the other hand, the melamine-formaldehyde resin was assessed as an unsuitable plasticiser for CAC.

Thermo-mechanical stability of supplementary cementitious materials in cement paste to be incorporated in concrete as thermal energy storage material at high temperatures

The incorporation of recycled materials in concrete as a partial replacement of cement is becoming an alternative strategy for decreasing energy-intensive and CO2 emissions imputable to the cement manufacture, while investigating new potential uses of such multifunctional materials for environmental sustainability opportunities. Therefore, low-cost and worldwide availability of by-products materials is being assessed for sensible heat thermal energy storage applications based on cementitious materials. A greater concern is especially required focusing on the thermal stability of cement paste under high temperature cycled conditions. Moreover, compatibility between cement type and supplementary cementitious materials is determinant for the proper performance reliability. In this study, benchmark cement types were selected, i.e., ordinary Portland and calcium aluminate. Six supplementary cementitious materials were added to both types of cement in a content of 10 % and 25 %. Thermo-mechanical properties were studied before and after 10 thermal cycles from 290 to 650 °C. Results after thermal cycling showed that calcium aluminate cement paste mixtures maintained their integrity. However, most ordinary Portland cement paste mixtures were deteriorated: only mixtures with 25 % cement replacement with chamotte, flay ash, and ground granulated blast furnace slag remained without cracks. Calcium aluminate cement paste mixtures obtained the highest compressive strength, for partial replacement of cement with 10 % of chamotte, ground granulated blast furnace slag, and iron silicate. The incorporation of supplementary cementitious materials did not increase the thermal conductivity.

Investigations of the Influence of Nano-Admixtures on Early Hydration and Selected Properties of Calcium Aluminate Cement Paste.

In this work, the hydration of calcium aluminate cement (CAC, Al2O3 ≥ 70%) paste with nano admixtures (0, 0.05%, 0.1% and 0.2%) of nano-silica (NS) and carbon nano-cones (NC) when W/CAC = 0.35 was investigated. The methods of calorimetry, thermal analysis, X-ray diffraction (XRD), IR spectroscopy, and scanning electron microscopy (SEM) were used. In addition, the physical and mechanical properties of hardened cement pastes were determined after 3 days of hardening. NS was found to shorten the induction period of CAC hydration and accelerate the time of the secondary heat release effect, especially in the specimens with the highest NS content. The incorporation of NC (up to 0.2%) slows down the hydration process. After 3 days of hydration, the formation of hydration products, such as C2AH8, CAH10, C3AH6 and AH3 hydrates, was observed in CAC pastes, however, the quantitative compositions were different depending on the kind of nano admixture and its amount. SEM results obtained show differences in the effect of NS and NC on the formation of the structure of cement paste during its hardening. Significant changes in CAC paste microstructure were caused by the addition of NS and NC admixtures. Compressive strength was found to increase with the increase of NS and the optimal NS content was found to be 0.10 wt.%. The modification of the cement paste with an NS admixture results in a higher amount of hydrates, lower total porosity, and a higher amount of the smallest pores in the microstructure of the sample. NS and NC influence the hydration behaviour of CAC in different ways, which causes characteristic changes in the microstructure and properties of hardened samples.

Thermal behaviour and inertia effect in calcium aluminate cement pastes with microsilica

The main contribution of this research is the study of the effect of thermal inertia in the activation energy in calcium aluminate cement pastes. For this, several formulations of calcium aluminate cement pastes (CACPs) with 51 wt% and 71 wt% of alumina (Al2O3) were made at 0.4 water-to-cement ratio, and with additions of 0.0 wt% and 20 wt% of silica content. The characterisation was done using X-ray fluorescence and X-ray diffraction, and the behaviour of the phases with temperature was studied using thermal gravimetric analysis, including the corresponding derivative thermogravimetry (DTG). The influence of the thermal inertia on the activation energies and its dependence on the heating rate is established. The effect of thermal inertia on peaks of DTG curves and activation energies is analysed. The activation energies for dehydroxylation of the gibbsite were calculated based on the Kissenger–Akahira–Sunose method. The activation energy values obtained, in kJ/mol, for CACP71 – 0, 20 wt% and CACP51 – 0, 20 wt% silica content were 19.9, 6.37 and 19.8, 17.2, respectively. The influence of silica on phase formation, activation energies and the effect of thermal inertia is also analysed.

Impact of sulphate source on the hydration of ternary pastes of Portland cement, calcium aluminate cement and calcium sulphate

The present work investigates the hydration and evolution of solid phase assemblage as a function of sulphate source in ternary Portland cement (PC), calcium aluminate cement (CAC) and calcium sulphate (CS¯HX) cement pastes. Two binders are compared, a PC‒rich and a CAC ‒ CS¯Hx ‒rich paste, containing gypsum and anhydrite as sulphate carrier, using a multi-method approach including calorimetry, XRD, TGA, MAS NMR spectroscopy, microscopy and thermodynamic modelling. The overall phase assemblage is very similar for the two-sulphate source in both PC-rich and CAC ‒ CS¯Hx−rich systems. However, the quantitative X-ray analysis, TGA and the 27Al, 29Si NMR show that the amounts of the crystalline hydrates and X-ray amorphous phases are influenced by the type of sulphate. In the long-term hydration, the anhydrite-bearing formulations exhibit the highest amount of ettringite, whereas the gypsum-containing samples develop a higher fraction of AFm phases and X-ray amorphous hydrates. This difference may relate to faster dissolution kinetics of gypsum compared to anhydrite in the studied ternary blends. For the CAC ‒ CS¯Hx −rich pastes, gehlenite from CAC (C2AS) shows hydraulic activity, which primarily results in the precipitation of strätlingite. Higher amounts of strätlingite are identified in the gypsum bearing samples, suggesting that the type of sulphate source impacts the hydration of silicate-bearing phases. Finally, the phase assemblages from thermodynamic modelling (using the GEMS software) are found to be in good agreement with those observed experimentally, although some differences occur as a result of kinetic effects.

Effect of drying methods on the microstructures and properties of cured calcium aluminate cement pastes

Effect of drying methods on the microstructures and properties of cured calcium aluminate cement pastes

Calcium aluminate cements under high‐temperature compression tests

AbstractThis investigation is about the damage generated with high‐temperature compression tests in pure calcium aluminate cement pastes (CACP) with quartz. Two types of formulations of CACP were manufactured and later exposed to a compression of .3 MPa at 800°C for 16 h. Samples had a water to cement ratio (W/C) of .40. The formulations were characterized using laser granulometry, X‐ray fluorescence, X‐ray diffraction, thermogravimetric analysis, and scanning electron microscopy. The theta projection model parameters for creep were estimated using nonlinear regression fit methods. The activation energy, minimum time of creep, minimum creep rate, and other parameters were calculated. The creep micro‐mechanisms were established as well. The activation energy was obtained exclusively by creep for the formulations CACP 71–0 wt% and CACP 71–20 wt% of 2.98 kJ/mol and 7.04 kJ/mol, respectively.

Stabilisation/solidification of municipal solid waste incineration fly ash by phosphate-enhanced calcium aluminate cement

Landfill disposal of municipal solid waste incineration fly ash (MIFA) presents significant environmental and economic burden. This study proposed a novel and high-efficiency approach for stabilisation/solidification (S/S) of MIFA by phosphate-modified calcium aluminate cement (CAC). Experimental results showed that the presence of Pb (the most leachable metal contaminant in the MIFA) retarded the early-stage reaction of CAC, resulting in an extension of setting time and a significant decline of compressive strength of CAC pastes. The incorporation of phosphate additives (10 wt% of binder), especially for trisodium phosphate, in CAC system effectively mitigated the negative impact of Pb on the CAC reaction and reduced the Pb leachability. Elemental mapping results illustrated that Pb2+ coordinated with phosphate to generate insoluble precipitates (e.g., Pb3(PO4)2). The S/S treated MIFA samples fulfilled the compressive strength and leachability requirements for on-site reuse. Overall, this study demonstrated that phosphate-modified CAC is a promising binder for S/S of hazardous MIFA.

Impact of sand and filler materials on the hydration behavior of calcium aluminate cement

AbstractIn earlier work, we have observed discrepancies relating to the early hydration of calcium aluminate cement (CAC) when comparing data from heat flow calorimetry of CAC paste with results from mortar strength tests using the crushing method. Here, we investigated on this phenomenon and found that the sand which is used as a filler exerts a major influence on CAC hydration resulting in acceleration. Furthermore, in particular fine filler materials such as, for example, microsilica, fine limestone powder, and especially α‐ and γ‐Al2O3 also produced a strong hydration accelerating effect which is dependent on their specific surface area. The mechanism underlying the acceleration is that under alkaline conditions their negative surface charge attracts calcium ions as was confirmed via inductively coupled plasma atomic emission measurements. Such a layer generates favourable conditions for the nucleation of CAC hydration products (C‐A‐H phases). The resulting crystalline hydrates which form on the surface of the filler particles submerged in CAC cement pore solution were visualized via SEM imaging. This way, specifically selected fillers can significantly accelerate CAC hydration and save precious lithium salts which are commonly used to boost the early strength of CAC.

Insight into the influences of β-hemihydrate and dihydrate gypsum on the properties and phase conversion of white calcium aluminate cement

It is still uncertain whether β-hemihydrate and dihydrate gypsum influence the properties, especially the phase conversion, of white calcium aluminate cement (CAC). Therefore, in this study, we prepared CAC pastes/mortars containing β-hemihydrate and dihydrate and comparatively assessed their setting times and compressive strengths. Simultaneously, the influence of β-hemihydrate and dihydrate on their chemical shrinkage, hydration-heat, pore size distributions, crystalline, and thermogravimetric characteristics was investigated. It is interesting that 0.5% β-hemihydrate and dihydrate significantly postponed the setting, whereas more than 0.5% β-hemihydrate and dihydrate slightly prolonged the initial setting and significantly shortened the final setting. Both β-hemihydrate and dihydrate decreased the compressive strength of CAC pastes hardened for 1 and 3 days to different extents; however, CAC pastes with a β-hemihydrate or dihydrate content less than 1% exhibited a high compressive strength at 28 days. Beyond this dosage, the compressive strength of the specimens was found to reduce. In addition, CAC pastes containing more than 0.5% β-hemihydrate or dihydrate exhibited high chemical shrinkage over 3–12 h. Furthermore, β-hemihydrate and dihydrate enhanced the gel micro and meso pores. They accelerated or decreased CAC hydration in the fast reaction and first deceleration periods, prolonged or shortened the induction period, enhanced or reduced the hydration heat before or after 12 h. These cases are closely associated to their contents. Their inclusion restricted CAH10 formation while promoting the generation of C2AH8, AFm, and AH3 and conversion of CAH10 to C3AH6 after 3 days. Besides, there are some significant differences in their influence on the properties and phase conversion of CAC pastes. For these differences, CAC pastes/mortars with β-hemihydrate and dihydrate presented different setting times and compressive strengths.

Understanding the binding and leaching of Cr(VI) in calcium aluminate cement based solidified/stabilized pastes

This study investigated the potential of calcium aluminate cement (CAC) to solidify/stabilize Cr(VI) (Na2CrO4). The hydration and microstructure development of CAC pastes in the presence of Na2CrO4 were examined by means of isothermal calorimetry, XRD, TG/DTG and SEM-EDS. The leaching properties of Cr(VI) were evaluated by TCLP and GANC methods. The results demonstrated that addition Na2CrO4 significantly retarded the early-age hydration of CAC depending on the dosage, while caused an acceleration of long-term hydration. The formations of CAH10 and C2AH8 were inhibited, instead AH3, CrO4-C3AH6 and CrO4-U phase were found as the main hydrates. These changes of hydrate assemblages led to a strong decrease in compressive strength. Complying with the TCLP regulatory limit of 5 mg/L, up to 1.0% of Cr(VI) could be effectively immobilized in the CAC paste. Two binding phases, CrO4-C3AH6 and CrO4-U phase, were responsible for controlling Cr(VI) leaching in the CAC pastes.

Effect of high temperature and additions of silica on the microstructure and properties of calcium aluminate cement pastes

In this study different formulations of calcium aluminate cement pastes (CACP) with 51 wt% (wt% -percentage by weight) of Al2O3 (CACP 51) were made at 0.4 water to cement ratio (W/C) with additions of 0 wt%, 10 wt% and 20 wt% silica (CACP 51-0 wt%, CACP 51-10 wt% and CACP 51-20 wt%, respectively). All samples were exposed to oxidative environment for 4 h at T = 1000˚C, in a furnace under an air atmosphere. The behavior of the constituent phases according to the temperature was also studied through TGA and DTG. The characterization was done using SEM, XRF, XRD, the Williamson-Hall Model (WHM)) and FTIR. Compression, absorption, density and porosity tests were done. It is observed that: C12A7 decreases between T = 800 °C and T = 1000 °C while the quantity CA and CA2 increases, silica improves compression strength at T = 20 °C and T = 1000 °C, Silica favors the formation of and avoids the conversion process of the CAC pastes. Knowledge of the behavior of the CACP added with silica between the temperatures of 20 °C and 1000 °C, is essential to understand its behavior at higher temperatures. A review of the state of the art shows an incipient literature in this aspect, for this reason this research is relevant for the refractory industry based on CACP with silica.