Hydration Of Calcium Aluminate Cement Research Articles

Classical and new insights into the methodology for characterizing the hydration of calcium aluminate cements

Calcium aluminate cements [CACs] are typically used for cast-in-place structural work and high temperature applications due to their unique properties. The CaAl2O4 [CA] and Sm-doped CaAl2O4 [CA:Sm] cement clinkers obtained by the solid-state reaction were investigated using X-ray powder diffraction [XRD], scanning electron microscopy [SEM] and energy dispersive X-ray spectroscopy [EDS] and luminescence spectroscopy. The XRD result shows that the powders are pure CaAl2O4. As a next step, hydration behavior of as-synthesized cements was investigated by both classical methods including isothermal calorimetry and XRD, and the state-of-the-art techniques including time resolved electrochemical impedance spectroscopy [TR-EIS] and luminescence spectroscopy. The TR-EIS investigations were obtained by the Impedance Camera over the range of 100 Hz to 1 MHz frequencies. The evolution of obtained EIS and luminescence spectra easily manifested the beginning of the new solid formation and the hydration kinetics. The dynamics of hydration processes were also successfully traced by the changes of the luminescence intensity from the Sm3+ ions. This technique seems to be complementary, and consistent with other methods implemented in the cement chemistry, especially to investigate e.g. hydration of ordinary Portland cements [OPCs] and other binders.

Influence of micro‐sized CaCO3 on hydrates formation of residual calcium aluminate cement in castables during drying

AbstractThe addition of CaCO3 powder in calcium aluminate cement (CAC) is reported to accelerate the hydration of CAC and generate mono‐carbonate C4AC̅H11, which is conducive to shortening the curing time of CAC‐bonded castables at ambient temperatures. Although the hydration of CAC is relatively fast, there is still residual un‐hydrated CAC in castables after curing. The residual CAC would continue to hydrate to produce C3AH6 and AH3 during the subsequent drying procedures in the absence of CaCO3. But it is worth pondering which type of hydrate would be generated in the presence of CaCO3 during the drying at around 110°C. In this study, the effect of micro‐sized CaCO3 addition on the hydrate formation of CAC‐bonded castables during drying at 110°C was investigated. The results indicate that the C4AC̅H11 formed at the curing temperature of 30°C is not decomposed or converted during drying. The CaCO3 remained after curing does not participate in the hydration of residual CAC during drying at 110°C, and thus the residual CAC continues to hydrate to generate C3AH6 rather than C4AC̅H11.

Development and Performance Evaluation of Fibrous Pseudoplastic Quaternary Cement Systems for Aerial Additive Manufacturing

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/m3, 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.

Inhibiting effect of styrene-butadiene copolymer on the whitening of calcium aluminate cement mortar

This paper was based on the results of the research group's previous research on early whitening of mortars. The inhibition of whitening and changes of white deposit on the surface of calcium aluminate cement (CAC) mortar with styrene-butadiene copolymer (SB) were studied using a long-term accelerated whitening method under conditions of (20 ± 2) ℃ and (60 ± 5)% RH. The whitening phenomenon and white deposits of CAC mortars were studied using digital processing of images, optical microscopy (OM) and scanning electron microscopy (SEM). The results indicate that the white deposit on the surface of CAC mortar primarily consists of newly formed carbonation products, calcium carboaluminate hydrate (C4AC¯H11) and calcium carbonate (CaCO3), and with the extension of this exposure time, C4AC¯H11 will be carbonized to generate CaCO3 and Al(OH)3 (AH3). It is interesting to note that SB reduces the production of C4AC¯H11 and CaCO3 and that at SB dosing levels above 12% the C4AC¯H11 disappears and there is hardly whitening of mortar at this time. The inhibition mechanism of SB on the whitening of CAC mortar was investigated using X-ray diffraction (XRD), capillary water absorption and water contact angle tests. The analysis reveals that hydration product of CAC is mainly CAH10 after curing at 20 ℃ for 1 d, and the addition of SB did not change the type of CAC hydration products. Furthermore, incorporating SB enhances the hydrophobicity and reduces the capillary water absorption in CAC mortar, and forms a polymer film in CAC mortar. They prevent the transfer of ions from the interior of CAC mortar to the exterior and the contact of CO2 from the air with the cement hydrate on the surface of CAC mortar, thus reduces the production of white deposit on the mortar surface and inhibits the whitening of CAC mortar.

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 different foam stabilizers on the setting behavior and pore characteristics of CAC-bonded Al2O3 foamed ceramics

Calcium aluminate cement (CAC) was used in this work as a novel binder of the foam slurries to prepare Al2O3 foamed ceramics. The different foam stabilizers employed in the slurries would not only impact the foam stability, but also affect the setting behavior of CAC-containing foam slurries, consequently influencing the pore size and the heat-insulating property of the Al2O3 foamed ceramics. In this paper, modified polyethoxylated silicone (MPS), sodium alginate (SA) and carboxymethyl cellulose (CMC) were selected respectively as the foam stabilizer. The effects of the different foam stabilizers on the setting behavior of foam slurries, and the dependence of the pore size and heat-insulating property of CAC-bonded foamed ceramics on the setting behavior of foam slurries were investigated. It is found that SA promotes the hydration of CAC in the foam slurries, while MPS and CMC postpone the hydration of CAC in the foam slurries; and the foamed ceramics with SA have better structural integrity, higher porosity, smaller average pore size and lower thermal conductivity than those with MPS or CMC.

Phase and porosity changes in a calcium aluminate cement and alumina system under hydrothermal conditions

When refractory castables are dried, hydrothermal conditions may result inside the bodies if the H2O cannot escape from the material. Under such high-pressure conditions, problems such as explosive spalling can arise. As different curing temperatures during the hydration of calcium aluminate cement (CAC)-bound castables lead to the formation of different hydrate phases, different microstructures can develop in the hardened material. This study presents the changes in porosity and in the mineralogical composition of a refractory castable model system under hydrothermal conditions depending on the curing temperature (5, 23 and 40 °C).Quantitative X-ray diffraction (QXRD) measurements show that different hydrate phases are formed during curing, while C3AH6 and boehmite are formed in the same quantities after hydrothermal treatment in an autoclave at ∼11 bar/180 °C. Although the mineralogical composition after autoclaving is not different, the three samples differ in their microstructure. Mercury intrusion porosimetry measurements reveal that although the total porosity after autoclaving is the same, the 40 °C samples have a higher proportion of large pores. SEM images also show that the appearance of C3AH6 in the 40 °C autoclaved samples varies, which originates from the starting phase composition and microstructure after curing.

How C12A7 influences the early hydration of calcium aluminate cement at different temperatures

Calcium aluminate cement (CAC) can contain mayenite (C12A7) as a minor phase which can influence the early hydration. Since CAC hydration is known to be extremely temperature dependent, this study presents the valuable effect of C12A7 dissolution on the early hydration at different temperatures (10, 23 and 35 °C). A reference system without C12A7 and a mixture with 4 wt% C12A7 in the cement phase was investigated using heat flow calorimetry, QXRD and pore solution measurements. In the reference system, a clear induction phase without reactions was observed at all three temperatures, whereas with C12A7, heat flow and reactions were observed at each temperature, as C12A7 was observed to dissolve instantly after water addition accompanied by C2AH8 precipitation. Saturation indices of C2AH8 are slightly higher with the addition of C12A7, which indicates a higher supersaturation of C2AH8 and explains the measured precipitation before the actual CAC main reaction.

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.

Effects of accelerating and retarding agents on nucleation and crystal growth of calcium aluminate cement

Although calcium aluminate cement (CAC) has been widely used, one of the critical unclear issues in the field of cement chemistry focuses on certain mechanisms that manipulate the rate of cement hydration. A better understanding of hydration provides critical information such that issues, such as setting time, strength gain, and durability, can be better addressed. This study aimed to initially provide theoretical predictions using experimental information on the dissolution, nucleation, and precipitation mechanisms of ions, induction time (tind), interfacial free energy (γ), and the crystal growth mechanism of CAC containing accelerating and retarding agents in microstructural systems. We also aimed to investigate the early-age performance of CAC, including the setting time of pastes and strength development of mortars. Results found that adding accelerating and retarding agents affected the dissolution, nucleation, and precipitation mechanisms of the CAC hydration. This finally affected the phase formation of the final products.

How C12a7 Influences the Early Hydration of Calcium Aluminate Cement at Different Temperatures

How C12a7 Influences the Early Hydration of Calcium Aluminate Cement at Different Temperatures

Effect of rice husk ash on mechanical properties, fracture energy, brittleness and aging of calcium aluminate cement concrete

The calcium aluminate cement (CAC) is considered as eco-cement due to the reduced carbon emission during its production; it has various applications due to its high early age strength and enhanced durability in harsh environments. However, the initial hydration products of CAC concrete are temperature dependent and meta stable that will gradually convert to more stable phases. Such transition from an initial product that are dense and strong to more stable phases that are weaker and more porous, causes a reduction in strength and durability over time. This article discusses the results of a comprehensive study on incorporating rice husk ash (RHA) in CAC concrete in order to limit the phase transition of CAC hydration product and stabilize its long-term properties. Various concrete mixtures with different contents of RHA (0, 2.5, 5, 7.5, and 10%) as a cement replacement material were examined. In addition to the workability properties of the fresh concrete, the microstructural and mechanical properties of hardened concrete are characterized at the ages of 7, 28 and 90 days. The findings indicate that, at 90 days, the mechanical strengths of the mixes containing RHA were higher than those of the control mix, with the maximum improvement occurring at the substitution percentage of 5%. In accordance with TGA analysis the substitution of 5% RHA in CAC concrete led to a higher hydration level, which in turn improved the mechanical properties relative to the specimen without RHA at 90 days.

Rice hush ash as supplementary cementitious material for calcium aluminate cement – Effects on strength and hydration

Replacing part of calcium aluminate cement (CAC) with reactive rice husk ash (RHA) can directly reduce the impact of rice husk disposal on environment and lower the CO2 footprint of cement. RHA is comparable in chemical composition to silica fume, and thus has the potential to similarly improve the hydration of CAC. In the present study the development of strength and hydration of CAC with various amount of RHA was investigated. By means of calorimetry, X-ray diffraction analysis, 27Al magic-angle spinning-nuclear magnetic resonance spectroscopy, mercury intrusion porosimeter (MIP) and high-resolution scanning electron microscopy (SEM), a detailed insight on the changes of CAC hydration by addition of RHA is gained. Results indicate that RHA does not only accelerate hydration but also improve the early strength of CAC. A high dosage (60%) of RHA densifies the microstructure locally, but the dilution effect induces the decrease of strength. The formed hydrates are C-A-S-H phases with smaller size than CAH and thus a refined pore size develops. Considering the effect of RHA on strength, the amount of substitution should be less than 40%.

Different Chlorides Attack on the Hydration of Calcium Aluminate Cement at 5~40°C

Calcium aluminate cement (CAC) has excellent resistance to seawater erosion, but the mechanism remains to be explored. Effects of NaCl and CaCl2 on the hydration of CAC at 5, 20 and 40°C were investigated in this paper by X-ray diffraction(XRD), thermal analyzer(TG-DSC), scanning electron microscopy(SEM), acoustic and electroacoustic spectrometer. Results show that the varieties of chlorides have great impacts on the chloride binding ability, mechanical properties and microstructure of cement pastes at different temperatures. At 5°C and 20°C, the formation of C2AH8 is suppressed by chloride attack. Though the addition of NaCl promotes the formation of CAH10, CaCl2 leads to a denser microstructure and the improvement in compressive strength. At 40°C, C2AH8 disappears by chloride attack, while C3AH6 and Friedel’s salt increase. Comparing with the attack of CaCl2, NaCl contributes to the formation of C3AH6. Therefore, it results in a the retraction in compressive strength, ascribing to a coarser structure. In addition, although NaCl is superior in chemical binding ability, CaCl2 has better physical adsorption ability which dominants the binding process, and thus leading to greater amount of bonded chloride than that with NaCl. This research provides the oretical basis for the application of CAC in marine environment.

Temperature-dependent late hydration of calcium aluminate cement in a mix with calcite – Potential of G-factor quantification combined with GEMS-predicted phase content

In continuation of earlier work on early hydration, this study evaluates the late hydration of CAC and CaCO3 using QXRD and thermodynamic modelling at different temperatures. Experiments were performed at 5, 23, 40 and 60 °C for up to one year. As stated in the preceding study, C2AHX might act as a precursor for monocarbonate in early hydration, and thus no or only little monocarbonate should form at temperatures below 20 °C. At 5 °C, monocarbonate starts precipitating after 7 d and remains alongside CAH10. At all other investigated temperatures, monocarbonate is the dominant hydrate phase. Primarily formed CAH10 at 23 °C is visible up to 14 d but then becomes unstable with respect to monocarbonate. At 23 °C and 40 °C the thermodynamically stable phase assemblage is reached within one year. However, the precipitation of C3AH6 is detected in all samples at 60 °C, which results from an insufficient w/CAC ratio for carbonate-AFm in the paste due to the inevitable evaporation of mixing water for this condition. However, C3AH6 can partly be “re-converted” at 60 °C when the sample is subsequently stored under water and monocarbonate is stable again.

Characterisation of calcium aluminate cement hydration: comparison of low-field NMR and conventional methods

The early hydration process of calcium aluminate cement (CAC) was characterised by low-field nuclear magnetic resonance (LF-NMR), isothermal calorimetry (IC), X-ray diffraction (XRD) and scanning electron microscopy (SEM). It was revealed that LF-NMR characterisation of the hydration process gives results very similar to those of IC; both show that the hydration reaction is faster in about 10–17 h. In addition, the hydration process of CAC could be quantitatively divided into four stages according to the second derivative of LF-NMR signal amplitude. LF-NMR is also used in the study of CAC microstructure. The results of the T2 distribution curve prove that there are mainly two kinds of pores of different sizes in the slurry, and they gradually become smaller, especially in more than 10 h. The weighted average T2 and specific surface area decreased and increased, respectively, due to the gradual densification of the slurry. Owing to the conversion of CAH10 and C2AH8 to C3AH6, the solid specific surface area decreased, resulting in the LF-NMR specific surface area curve slowing down or even decreasing in about 20–50 h of hydration. XRD and SEM results further validate the characterisation of hydrated CAC microstructure by LF-NMR.

Calcium aluminate cement in castable alumina: From hydrate bonding to the in situ formation of calcium hexaluminate

Calcium hexaluminate (CA6) is an intrinsically densification-resistant material, therefore, its porous structures are key materials for applications as high-temperature thermal insulators. This article reports on the combination of calcined alumina and calcium aluminate cement (CAC) in castable aqueous suspensions for the in situ production of porous CA6. The CAC content (10–34 vol%) and the curing conditions ensure structural integrity prior to sintering and maximize the development of hydrated phases. Changes in physical properties, crystalline phases, and microstructure were investigated after isothermal treatments (120–1500 °C), and three sequential porogenic events were observed. The hydration of CAC preserved the water-derived pores (up to 120 °C), and the dehydroxylation of CAC hydrates (250–700 °C) generated inter-particles pores. Moreover, the in situ expansive formation of CA2 and CA6 (900–1500 °C) hindered densification and generated intra-particle pores. Such events differed from those observed with other CaO sources, and resulted in significantly higher pores content and lower thermal conductivity.

Effects of Sodium Hexametaphosphate Addition on the Dispersion and Hydration of Pure Calcium Aluminate Cement.

The effects of sodium hexametaphosphate (SHMP) addition on the dispersion and hydration of calcium aluminate cement were investigated, and the relevant mechanisms discussed. The content of SHMP and the adsorption capacity of SHMP on the surface of cement particles were estimated using plasma adsorption spectroscopy and the residual concentration method. The rheological behavior of hydrate, ζ-potential value of cement particles, phase transformation and the microstructure of the samples were determined by coaxial cylinder rheometer, zeta probe, X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results indicate that SHMP readily reacted with Ca2+, forming complexes [Ca2(PO3)6]2− ions which were subsequently adsorbed onto the surfaces of cement particles. When the content of SHMP was 0.05%, the adsorption ratio reached 99%. However, it decreased to 89% upon further increasing the addition of SHMP to 0.4%. The complexes [Ca2(PO3)6]2− adsorbed onto the surfaces of cement particles inhibited the concentration of Ca2+ and changed ζ-potential, resulting in enhanced electrostatic repulsive force between the cement particles and reduced viscosity of cement-water slurry. The experimental results indicate that the complexes [Ca2(PO3)6]2− covering the surfaces of cement particles led to a delayed hydration reaction, i.e., they extended the hydration time of the cement particles, and that the optimal addition of SHMP was found to be about 0.2%.

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.

Effect of Different Kinds of Zinc (II) on Early Hydration of Calcium Aluminate Cement

The early hydration of calcium aluminate cement (CAC) with different kinds of zinc (II), such as ZnSO4·7H2O, ZnO, Zn(NO3)2·6H2O and ZnCl2, was analyzed. Changes in consistency, setting time, hydration heat flow, hydration heat amount, ion concentration in solution, and hydration products were found upon the addition of different Zn2+. The water consumption of standard consistency of CAC is decreased with different Zn2+. Zn2+ can delay the initial hydration of CAC. The induction period of cement with Zn2+ is longer than that of CAC, especially the reaction time of the acceleration period is extended. Zn2+ can promote hydration hydrate of CAC at 24 h. The characteristic diffraction peaks of CA and CA2 in CAC with different Zn2+ are significantly reduced. It can inhibit the formation of CAH10 and promote the formation C3AH6 and AH3 in hydration products at 24 h.