Parameter influence on the rankinite binder paste and mortar accelerated carbonation curing

Abstract

This paper presents a study based on accelerated carbonation curing of low-lime non-hydraulic calcium silicate—rankinite (3CaO·2SiO2 or C3S2)—binder paste and mortar. Rankinite hardening process involves a reaction with gaseous CO2 in the presence of moisture, resulting in the formation of calcium carbonate and Ca-modified silica gel. The carbonation process of this type of binder depends on many features, among which CO2 pressure, duration, and temperature influence were investigated. Thermogravimetric analysis was performed to quantify the amount of calcium carbonates formed after the carbonation reaction, which were later used to calculate the degree of reaction, thus indicating the process efficiency. XRD and FTIR analysis were conducted to determine the prevailing mineral composition after the curing. Based on experimental results, the mineral composition of carbonated rankinite binder consisted of three polymorphs of calcium carbonate, such as calcite, vaterite, and aragonite, together with Ca-modified silica gel. However, the only polymorph of calcium carbonate observed in the mortar samples was calcite. According to the TGA data, the highest carbonation degree reached by paste samples was less than 40%. Nonetheless, the significant differences observed in the XRD curves, led to a conclusion, that pastes and mortars do not undergo the same path of carbonation reaction, indicating that aggregates, i.e., sand (quartz), has a major impact on the formation and polymorphism of calcium carbonate,in mortars potentially supporting calcite, rather than other polymorph formation.