Hydration of limestone and dolomite cement

In light of the need to introduce new composite cements and the reduction of CO2 emissions, the article presents the results of the study of Portland cement hydration with dolomite added for 180 days. They are compared with the development of cement hydration with the addition of ordinary Portland cement and limestone. The effect of additions of limestone and dolomite powder on the properties of composite cements cured at 20 °C, 40 °C and 60 °C was investigated using a multi-method approach: compressive strength measurement, thermogravimetric analysis (TGA), quantitative X-ray diffraction (XRD), scanning electron microscopy and thermodynamic modelling. Higher curing temperature accelerates the early cement hydration and increases early compressive strength. However, after hydration times longer than 7 days, the higher curing temperature turns out to be detrimental for the compressive strength. This behaviour is typical for all investigated cements. The compressive strength of dolomite cement samples is similar or even higher than the strength of limestone cement samples at investigated temperatures. No evidence of expansive reactions was found during our investigations. The experimental results confirm that dolomite is not thermodynamically stable. At 20°C dolomite reacts very slowly. Increasing the temperature accelerates the dissolution of dolomite considerably. The reaction degree of dolomite after 180 days is 5-10%, 45% and 90% at 20, 40 and 60°C respectively. At 20°C and 40°C, in the presence of limestone, the formation of hemicarbonate that transforms to monocarbonate is observed instead of monosulphate presented in Ordinary Portland Cement (OPC), in agreement with data published earlier. Dolomite dissolution influenced the hydrate assemblage by the formation of additional hydrotalcite. At 60 °C, ettringite is not stable in ordinary Portland cement and limestone cement. However experimental data suggest that dissolving dolomite stabilized ettringite at that temperature. Thermodynamic modelling predicts well the phase assemblage of the investigated samples. Modelling enables the calculation and prediction of the limestone reaction in cement. Additionally, comparison of calculated porosity with the measured strength data shows a good agreement and enables the prediction of the effect of temperature on the strength development of Portland clinker based cements. The results obtained show that the reaction of dolomite in the cement matrix is initially similar to the behavior of limestone, and the same applies to the properties of dolomite cement. The further reaction of dolomite leads to a modification of hydrated compounds, but without prejudice to the performance of the cement with dolomite added. The results show that thermodynamic modeling can be successfully used for developing new cements in industrial laboratories.
Author: M. Zajac, M. Ben Haha

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