Calorimetric studies of special cements

The kinetics and even the mechanism of cement reaction with water can be successfully investigated by use of microcalorimetry. In this study this method was applied to follow the hydration of the new family of portland cements containing C₁₂A₇ ^* and C₁₁A₇·CaF₂ addition as well as special cement with C₃A replacement by calcium sulphoaluminate. It has been found that C₁₁A₇·CaF₂ acted as hydration retarder. The heat evolution curves for C₁₂A₇ containing samples without CaF₂ are very similar to those for the reference portland cement samples. XRD and SEM studies confirm the results described above, relating to the retardation of alite hydration. The process is positively modified by the addition of anhydrite. In the presence of calcium sulphoaluminate (4CaO·3Al₂O₃·SO₃) the hydration at early stage occurs with the rapid formation of large amount of the ettringite phase. The calcium fluoride acts as a set retarder. The full compatibility of calorimetry with SEM and XRD results should be underlined. In cement chemistry the following notation is used:C=CaO,A=Al₂O₃,S=SiO₂,H=H₂O etc. for the main oxide constituents of portland cement clinker and hydrates.     

Mineral Composition of Build-up in Cement Kiln Preheater

Use of a short rotary kiln with a cyclone preheater allowed the internal recirculation of volatile constituents, essentially consisting of compounds of potassium, chlorine and sulphur. These compounds underwent partial condensation on the raw material grains, composed mainly of calcite. The increasing concentration of volatile constituents created convenient conditions for the crystallization of new phases, particularly sylvite. Beautiful crystals of this phase were formed, probably by the VLS mechanism. Thermal analysis revealed that a liquid phase was formed in the system at the relatively low temperature of about 630°C, which enhanced the reaction of silica with calcium carbonate, and spurrite was formed. Thus, the build-ups were composed mainly of calcite, sylvite and spurrite, and in some cases also of calcium oxide and anhydrite. Sulphospurrite, gehlenite, calcium langbeinite, dicalcium silicate and calcium aluminate, 12CaO·7Al₂O₃ were found as minor components. This revised version was published online in July 2006 with corrections to the Cover Date.     

Studies of Capillary Porosity of Clinker Phases During Hydration

The capillary porosity of clinker phases, i.e. 3CaOSiO₂, β2CaOSiO₂, 3CaOAl₂O₃ and 4CaOAl₂O₃Fe₂O₃, at the early stages of hydration has been studied by the methods of Differential Scanning Calorimetry and nitrogen adsorption (BET). It was established that pores of 3–70 nm were formed during the hydration of 3CaOSiO₂ and the maximum of their distribution was found at about 10 nm. The hydration of 2CaOSiO₂ is much slower and the porosity is one order of magnitude lower. During the hydration of 3CaOAl₂O₃ the content of crystalline hexagonal hydrates prevailed and the porosity was in the range 5–90 nm with the average pore diameter of about 16 nm. This average pore diameter was much smaller for thermoporosimetry and lay at about 7 nm. The hydrated 4CaOAl₂O₃Fe₂O₃ sample had the porosity in the range 3–90 nm with the maximum of the pores distribution at about 4 nm. There are some differences between the porosities measured by BET and thermoporosimetry. Principally thermoporosimetry gives no information about larger capillary pores in the range 30–50 nm. This revised version was published online in July 2006 with corrections to the Cover Date.