Unraveling the expansion mechanism in lightweight aggregates: Demonstrating that bloating barely requires gas

Abstract

Lightweight aggregate bloating process has been studied by a simple experiment using an archetypal clay to know the actual amount of gas involved in expansion. Considering the relationship between gas loss (LOI) and volumetric changes over time, three main stages are identified: 1) a preheating stage of massive gas loss (close to 80% of the total) with hardly any volumetric change; 2) a very brief transition stage, in which sintering (shrinkage) and closed (micro)porosity formation begin, accompanied by a sudden gas loss (close to 100% of the total); 3) the bloating stage itself, in which an appropriate viscosity is reached, allowing the available residual gas (<0.1 wt%) to increase the aggregate volume due to the growth in size of the micropores formed in the transition phase and probably also to the development of new porosity. Therefore, the proportion of gas-generating components estimated to obtain a highly expanded lightweight aggregate would be much lower than previously thought: e.g., only 0.06 to 0.2% of carbonates (calcite or dolomite) or 0.2 to 1% of Fe2O3 would actually be involved in bloating. These results suggest that obtaining an adequate viscosity appears to be much more decisive for bloating than gas release capacity.