Effect of Low Calcite Addition on Sulfate Resisting (SR) Portland Cements: Hydration Kinetics at Early Age and Durability Performance After 2 Years

The limestone addition to sulfate resisting Portland (SR) cement is often used to adjust artificially the amount of aluminate (C3A) in order to keep it less than 3%. However, CaCO3 addition is well known to induce thaumasite formation (CaCO3·CaSO4·CaSiO3·15H2O) in Portland cement during external sulfate attack. This compound decreases the concrete durability because of its high volume expansion. In this work we studied the addition of limestone within low amounts (≤ 6.21%) into SR Portland cement. Thus, three commercial samples (A, B and C) containing limestone addition were analyzed by XRF technique and the results were used to estimate the mineralogical composition by Bogue method. XR diffraction coupled to Rietveld algorithm was then performed to determine the real amounts of mineralogical phases. The results were as following: C3S contents were 49.70, 45.30 and 50.60%; C3A: 1.40, 1.10 and 2.5% and calcium carbonate addition 6.21, 2.10 and 2.10%, respectively. The effect of calcium carbonate on hydration kinetics at early age has been studied and discussed. Durability of studied cement samples was followed during 2 years into Na2SO4 and MgSO4 solutions (10 g/L). Thaumasite formation was observed only in cement mortars corresponding to the sample 6.21% CaCO3, C3S (49.70%) and C3A (1.4%) at 22 °C, this result is different from earlier works in which thaumasite precipitates at low temperature (5 °C) and high CaCO3 contents (> 15%). This result proved the SR Portland cement must contain very low limestone addition, less than 6% and much lower if used at 5 °C or below.