About the mechanism of increasing the sulfate resistance of concrete with the addition of finely dispersed calcium carbonate

One of the trends in modern construction is the use of highly dispersed additives to produce Ultra-High Performance Concrete in terms of both strength and durability, especially for high-rise construction projects. The durability of concrete is determined by two main parameters – the reactivity of the cement stone components in relation to an aggressive environment and the permeability to an aggressive environment. The article discusses the use of finely dispersed calcium carbonate as an additive that increases the durability of concrete in general, and sulfate resistance in particular. The mechanism of action of calcium carbonate is based on a combination of the effect of a micro filler and chemical interaction with Portland cement minerals. Calcium carbonate interacts with the hydration products of tricalcium aluminate, reducing the amount of C₃A available for interaction with gypsum, followed by the formation of the main destructor when exposed to sulfates, ettringite. Calcium carbonate at high dosages reduces the strength of concrete. To correct this negative effect, it is proposed to use nanoscale calcium carbonate along with the already widely used micro-sized CaCO₃. The use of nanoscale calcium carbonate increases the total reaction capacity of the introduced additive, while reducing its total amount and, thus, preventing a decrease in concrete strength with a significant increase in its sulfate resistance. Nanoscale calcium carbonate densify the zone of interfacial interaction at the “cement matrix – filler” boundary, forming complex compounds with cement monominerals, which is confirmed by the data of physico-chemical studies. To further enhance the durability of concrete, the combined use of finely dispersed calcium carbonate with silica is proposed in order to bind free lime into low-base calcium hydrosilicates.