Permeability and pore structure evolution of silicocalcareous and hematite high-strength concretes submitted to high temperatures

Two high-strength concretes (HSC) only differing in their aggregates—silico-calcareous and hematite—were heated at temperatures up to 450°C (1°C/min). The evolution of their microstructural parameters—porosity, pore structure, permeability—were analysed. Both concretes showed equivalent initial microstructural characteristics. From 60°C, heating generated a large capillary porosity characterized by pore accesses around 0.1 μm. The intensity and especially the width of the porosity peaks increased with temperature. For silico-calcareous HSC, macropores-50 to 0.3 μm—were detected by MIP studies at 250°C and especially at 450°C. They were correlated to microcracks visually observed at the surface of the probes. Up to 250°C, the intrinsic permeability increased similarly for both concretes. Between 250 and 450°C, permeability remained stable for hematite HSC while, for silico-calcareous HSC, a major change was noticed. A good correlation between permeability and total water porosity was observed. At 450°C, influence of the microcracks on permeability was greater than the impact of the increase of capillary pore size. As both concretes showed similar initial microstructural features, conclusion was reached that the differential behaviour can mainly be attributed to internal thermal gradients discrepancies related to the type of aggregates: hematite allows to limit thermal gradient and thus, thermo-mechanical stresses. It was globally observed that damage due to high temperature thermal treatments was lower for hematite HSC.