Thermal analysis of hydrated calcium aluminates
01 Sep 1996-Journal of Thermal Analysis and Calorimetry (Kluwer Academic Publishers)-Vol. 47, Iss: 3, pp 765-774
TL;DR: Differential scanning calorimeter has been used to study the dehydration characteristics of hydrated calcium aluminates such as CA, CA2 and C12A7 where C and A stand for CaO and Al2O3 respectively.
Abstract: Differential scanning calorimeter (DSC) has been used to study the dehydration characteristics of hydrated calcium aluminates such as CA, CA2 and C12A7 where C and A stand for CaO and Al2O3 respectively. Dehydration of CAH10 and C2AH8 (whereH=H2O) occur ∼ at 160–180°C and 200–280°C respectively. These two phases are unstable and ultimately get transformed to AH3 and C3AH6. Dehydration of AH3 and C3AH6 occur between 290 and 350°C and overlap at lower scanning rate. The activation energy for dehydration of the stable AH3 and C2AH6 phases has been found to be 107.16 and 35.58 kJ mol−1 respectively. The compressive strength of the hydrated calcium aluminates has been determined. The result shows that in the case of CA, almost 90% of ultimate strength has been attained in 1 day whereas in CA2, ultimate strength has been attained in 14 days and in C12A7 in 1 day. DSC results have been correlated with the rate of strength developments.
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TL;DR: The hydraulic properties of the Ca 7 ZrAl 6 O 18 (C 7 A 3 Z) phase as well as the hydration products and thermal decomposition mechanism of this hydrated phase were studied in this article.
Abstract: The hydraulic properties of the Ca 7 ZrAl 6 O 18 (C 7 A 3 Z) phase as well as the hydration products and thermal decomposition mechanism of this hydrated phase were studied. Microcalorimetric analysis has shown that the C 7 A 3 Z phase reacts with water very quickly, especially in the first 2 h after the start of the experiment. Hydration of calcium zirconium aluminate proceeds with the formation of high refractory calcium zirconate (with melting point 2345 °C), apart from the hydrated, nearly amorphous material. According to the DTA–TG–EGA, FT-IR and SEM/EDS examinations it has been found that not only the hydrates CAH 10 , C 2 AH 8 and C 4 AH 19 are present, but also C 3 AH 6 (C = CaO, A = Al 2 O 3 , H = H 2 O), the only hydrated calcium aluminate which is a thermodynamically stable phase above 40 °C. Unhydrated Ca 7 ZrAl 6 O 18 and CaZrO 3 phases have been found by XRD, but crystalline hydrates have not been detected.
21 citations
TL;DR: In this paper, the internal moisture of twelve hardened cement pastes fired at high temperature, made with four aluminous hydraulic binders and three different Water/Binder ratios, was registered under diverse thermo-hydrous conditions, including at very high temperature.
Abstract: Refractory castables based on aluminous hydraulic binders are commonly used in aluminium casthouses (furnaces, ducts, etc.). Their selection is based on their good mechanical strength, thermal behavior and compatibility with molten aluminium. However, few studies focus on their hydrous evolution in operation, whereas this property can also have an influence on the produced metal quality. In this article, the internal moisture of twelve hardened cement pastes fired at high temperature, made with four aluminous hydraulic binders and three different Water/Binder ratios was registered under diverse thermo-hydrous conditions, including at high temperature. The water trapped by physisorption and chemisorption can be significant for some products, and it strongly depends on the mineralogy and porosity of the hardened cement paste. The more the binders contain alumina phase, the more the hardened cement pastes mobilize and render moisture.
21 citations
TL;DR: In this article, the results of experimental study concerning the hydration behavior of Ca 7 ZrAl 6 O 18 with water to solid (w/s) ratios of 0.5 and 1.0 at 60°C are presented.
Abstract: In this paper, the results of experimental study concerning the hydration behavior of Ca 7 ZrAl 6 O 18 with water to solid (w/s) ratios of 0.5 and 1.0 at 60 °C are presented. Hydration products were studied by XRD, DTA-TG and SEM/EDS after 24 h; 3, 7, 14 and 21 days of curing and hydration. Ca 7 ZrAl 6 O 18 in paste form, at w/s ratio of 0.5, showed one clearly visible endothermic effect at about 310 °C due to the dehydration of crystalline C 3 AH 6 . In the paste at w/s ratio of 1.0, the thermal decomposition of C 3 AH 6 occurred at about 317 °C, and was preceded by an endothermic effect with a minimum at 277 °C corresponding to the crystalline Al(OH) 3 decomposition. The results showed that the hydration of Ca 7 ZrAl 6 O 18 phase was especially a time dependent effect in regard to the separation of CaZrO 3 . SEM observations have shown typical morphological changes for each hydration period and water/solid ratios.
20 citations
TL;DR: In this paper, the ion concentration, PH value of pore solution and the hydration of two low iron calcium aluminate cements based on bauxite (BCACs) with different sulfur content were investigated by XRD, ICP-OES, PH meter and SEM.
Abstract: The ion concentration, PH value of pore solution and the hydration of two low iron calcium aluminate cements based on bauxite (BCACs) with different sulfur content were investigated by XRD, ICP-OES, PH meter and SEM. The results indicate that the sulfur in the bauxite and coal results in the formation of ye’elimite (C 4 A 3 S) in BCAC. The more the sulfur content in BCAC the more the C 4 A 3 S formed. The hydration of C 4 A 3 S decreases the ion concentrations of Ca, Al and the PH values but increases the concentration of S. Hydration and hydrates of CA are influenced by the hydration of C 4 A 3 S in BCAC. CAH 10 is found in the hydration of both BCAC samples, but C 2 AH 8 is only found in the hydrates of the BCAC with lower SO 3 content.
17 citations
TL;DR: In this article, the TG and DTG curves of different Portland cement pastes (types I, II, III and G), with a water-to-cement ratio (W/C) equal to 0.5, were analyzed at different ages, at same operating conditions.
Abstract: The capture of CO2 and SO2 from industrial gas effluents has been done usually by lime-containing products. For this purpose, cement pastes also can be used, due mainly to their calcium hydroxide content formed during hydration. To select the best cement for this purpose, TG and DTG curves of different Portland cement pastes (types I, II, III and G), prepared with a water-to-cement ratio (W/C) equal to 0.5, were analyzed at different ages, at same operating conditions. The curves were transformed into respective cement calcined and initial mass basis, to have a common and same composition reference basis, for a correct quantitative hydration data comparison. This procedure also shows that there is an unavoidable partial drying effect of the pastes before starting their analysis, which randomly decreases the W/C ratio at which were prepared, which indicates that, when results are compared on respective paste initial mass basis, assuming that the ratio W/C has not changed, possible calculation errors may be done. Type I, II and G analyzed cements have shown similar hydration characteristics as a function of time, while the analyzed type III cement has shown a different hydration behavior, mainly due to its highest Al2O3 and lowest SO3 contents, promoting the formation of hydrated calcium aluminates, by the pozzolanic action of the excess of alumina, consuming Ca(OH)2, which final content at 28 days was the lowest one, among the hydrated cements.
15 citations
Cites background from "Thermal analysis of hydrated calciu..."
...It is important to note that calcium aluminate hydrates may decompose from 180 to 390 C [22], confirming the previous occurrence, as well as indicating that in the case of cement III hydration, the increase in combined water loss during the first 14 days attributed so far to tobermorite and ettringite dehydration between 35 and 200 C can also be due to the dehydration of a kind of calcium aluminate hydrate, which begins to decompose at 180 C....
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10,842 citations
TL;DR: The chemistry of hydration of monocalcium aluminate, CA, has been studied at several temperatures using conduction calorimetry, X-ray diffraction and other techniques as discussed by the authors.
Abstract: The chemistry of hydration of monocalcium aluminate, CA, has been studied at several temperatures using conduction calorimetry, X-ray diffraction and other techniques. At 4 °C, hydration to the decahydrate CAH 10 occurs about 15 hours after mixing; this hydration time increases with increasing temperature up to 30 °C, and C 2 AH 8 appears as a hydration product. At 40 °C rapid hydration to C 2 AH 8 is followed over a period of weeks by the ‘conversion’ reaction producing C 3 AH 6 . The reaction of CA to form crystalline hydrates was monitored by X-ray diffraction analysis; the results indicate that hydration also produces significant amounts of noncrystalline material. The enthalpies of the reactions involved in hydration and ‘conversion’ were measured by conduction calorimetry.
63 citations
TL;DR: In this article, the authors studied the hydration behavior of Secar 71 refractory aluminous cement using conduction calorimetry, X-ray diffraction and thermal analysis techniques.
Abstract: The hydration behaviour of Secar 71 refractory aluminous cement has been studied over a range of temperature using conduction calorimetry, X-ray diffraction and thermal analysis techniques. The first reaction is the hydration of monocalcium aluminate, CA, producing a sharp calorimeter peak within 12 hours of mixing. Following this reaction the other major component, calcium dialuminate (CA 2 ), undergoes slow hydration over a period of weeks. The enthalpies of these two reactions have been measured. The initial hydration products of both components are CAH 10 , or C 2 AH 8 with alumina gel. These can subsequently convert to C 3 AH 6 ; after one month significant conversion is observed even at room temperature. A method of estimating the degree of conversion based on X-ray measurement of C 3 AH 6 is proposed.
46 citations
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31 citations
TL;DR: In this paper, the hydration process of a high alumina cement was investigated through ultrasonic wave propagation techniques, and a correlation between wave amplitude and velocity and breaking strength of the cement was demonstrated.
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6 citations