Search Results
Abstract
A new-design conduction microcalorimeter is described, which has been used to measure the heat of cement hydration evolved in the initial period of hydration. The calorimeter is 30 cm3 in volume; the heat loss coefficient is 27.2700.015 W V–1, the time constant is 300 s.
1962 Geometry of structures consisting of water molecules in hydrated crystals J. Struct. Chem. 3 206 226 . J
Calorimetry in the studies of cement hydration
Setting and hardening of Portland cement–calcium aluminate cement mixtures
Abstract
Calorimetry was applied to an investigation of the early hydration of Portland cement (PC)–calcium aluminate cement (CAC) pastes. The heat evolution measurements were related to the strength tests on small cylindrical samples and standard mortar bars. Different heat-evolution profiles were observed, depending on the calcium aluminate cement/Portland cement ratio. The significant modification of Portland cement heat evolution profile within a few hours after mixing with water was observed generally in pastes containing up to 25% CAC. On the other hand the CAC hydration acceleration effect was also obtained with the 10% and 20% addition of Portland cement. As one could expect the compressive and flexural strength development was more or less changed—reduced in the presence of larger amount of the second component in the mixture, presumably because of the internal cracks generated by expansive calcium sulfoaluminate formation.
The hydration reactions of calcium aluminate in water and in Na2CO3 solution have been investigated using calorimetric, DTA, DTG and XRD methods.
The hydrated rare earth orthophosphates LnPO4 ·xH2O (Ln=La-Dy) contain zeolitic water in the structural channels, which is released reversibly up to 300°. The thermal stabilities of the hydrates depend upon the nature of the Ln atom. The dehydration temperature decreases with decreasing ionic radiusr of Ln3+, according to the general equationθ=(r−a)/b (whereθ is the DSC and/or DTG dehydration peak temperature, anda andb are empirical constants depending on the experimental conditions).
Abstract
Nafagrel hydrochloride has two kinds of stable pseudopolymorphs such as hemihydrate and monohydrate. The dehydration of crystal water of these hydrates took place in one step under nitrogen gas atmosphere, whereas the two dehydration steps could be detected for the monohydrate under self-generated atmosphere such as the quasi-sealed and/or the completely sealed systems. These observations indicated that the crystal water of the monohydrate consisted of two different crystal waters. Prediction of the stability for the hydrates using the kinetic parameters indicated that the dehydration of the monohydrate occurred faster than that of the hemihydrate.
Abstract
In this study the process of hydration of fly ash, produced by a municipal solid waste (MSW) incinerator, is examined in water and in a solution enriched with Ca(OH)2. The examined fly ash samples are characterized by a high content of alkaline chlorides and anhydrite and by remarkable amounts of heavy metals. Investigations using differential thermal analysis/thermogravimetry (DTA/TG) and X-ray diffractometry (XRD)show particular kinetics of the formation of the ettringite phase. The development of such a hydrated phase is much more intense in the presence of an excess of Ca(OH)2so as to consume the ‘free’ sulphate in the case of the more reactive fly ash. Experimental results from thermal analysis and X-ray diffractometry show the presence of different hydrated phases during the interaction between fly ash and aqueous solution. The analytical determinations, related to the aqueous solution, point out an interesting decrease in concentration of metals Pb, Zn and Cr(VI), relating to the middle period of the interaction.
Abstract
An increase of the specific surface area of solid phases is often desirable e.g. for the bioavailability of pharmaceuticals or in chemical processes. Such an increase can a.o. be achieved by suspending crystalline substances in a solvent that induces phase transformations. Hence, the original substance has to be in a metastable state in the solvent. If the stable phase after transformation has in addition a very low solubility in the solvent, a dendritic growth is forced to occur because of the high local supersaturations during the phase change. This dendritic growth of the stable phase in term leads to needle- or whisker-like crystals, which have the desired larger specific surface area in comparison to the initial crystalline substance.In order to investigate this phenomenon several hydrates of salts were chosen, which undergo phase transformations to their anhydrates accompanied by a corresponding loss of crystal water when suspended in excess in lower alcohols. Consequently, anhydrous forms were created by dehydrating these hydrates. The transformation rate or in this case the dehydration level can thus be indirectly measured by Karl-Fischer titration. The thermodynamic background of the dehydration phenomena can be clarified by solubility studies of the hydrates and anhydrates in water/alcohol-mixtures.
Summary
The hydration products in two aged cement pastes (DTA/DTG/TG) were compared with those in fresh ones (static heating, SH) and were also studied by mass spectrometry (MS), IR and thermo XRD-analysis. The products considered here were: the sorbed water, the CSH gel including hydrates, portlandite, calcite, aragonite and vaterite. Except carbonates their content was higher in the stronger paste C-43, than in C-33, but lowered with ageing (only the CSH gel water remained approximately unchanged). The sorbed water content became with time lower and similar in both pastes (it evaporated up to 155-185C in TG); the escape of the rest moved to higher temperatures (500-700C). The three DTG peaks at 200-400C indicated jennite-like phase in the CSH gel; the mass loss (155-460C) was higher on ageing due to development of organic matter, especially in C-43 (DTA, TG, IR). Portlandite content changed little and carbonate content increased considerably. They decomposed in air at 470 and 720-740C, in argon at 450 and 680-710C and in vacuum at 400 and 630C, respectively (DTG peak, XRD). Between 500 and 700C the simultaneous evolution of H2O and CO2was observed by MS, which is attributed to dehydroxylation of jennite-like phase and/or to decomposition of some carbonate hydrate and/or hydrocarbonate (three peaks on CO2evolution curve, MS). The d(001) peak of portlandite exceeded the nominal value and will be analyzed separately.
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.
