Influence of carbonated pozzolana on sulfate attack of cement stone at low temperatures

This study investigates the influence of natural pozzolana (opoka) additive on the sulfate attack process of Portland cement stone at low temperatures. In the samples, 5%, 15% and 25% (by weight) of the Portland cement is replaced with additive. The specimens are then hardened for 28 days in water and then soaked in a 5% sodium sulfate (Na₂SO₄) solution for 6 months at 4°C. It is estimated that under normal conditions, opoka decreases the compressive strength of cement stone. A decrease in strength is the result of the slow pozzolanic reaction; moreover, part of the pozzolana carbonate component acts as an inert filler. In this study, however, pozzolana additive increased the compressive strength of samples after treatment for 3 months in a 5% sodium sulfate solution at 4°C. This effect is related to the ongoing pozzolanic reaction and the formation of a greater amount of calcium silicate hydrates. With increasing maintenance of samples for up to 6 months in a 5% sodium sulfate solution, it is found that pozzolana promotes the formation of thaumasite. This process is initiated by calcite, which is an integral part of pozzolana, and there is a greater formation of calcium silicate hydrates in samples with opoka.