An investigation on very high volume slag pastes subjected to elevated temperatures

• HVS pastes exhibited lower compressive strength than neat PC paste at all ages.
• HVS pastes showed higher residual strength at 400 and 600 °C than their references.
• At 800 °C, HVS pastes kept 50–90% of their original strengths, whilst PC kept 27.5%.
• pH value of pastes decreased with increasing slag content and treatment temperature.

In this investigation, high volume slag (HVS) pastes were studied under heat treatment. Portland cement (PC) was replaced with ground granulated blast-furnace slag (shortened as slag) at levels of 0%, 85%, 90%, 95% and 100%, by weight. The pastes were exposed to elevated temperatures ranging from 200 °C to 800 °C with an increment of 200 °C for 2 h. Compressive strengths and pH values before and after firing were measured. The various decomposition phases formed were identified using X-ray diffraction (XRD) and thermogravimetric (TGA) analyses. The morphology of the formed hydrates was studied using scanning electron microscopy (SEM). Visual inspections were also conducted. The results showed an increase in the reference compressive strength of HVS pastes with heat treatment up to 600 °C, then a reduction at 800 °C was observed. The relative compressive strength of HVS pastes was higher than that of 100% PC paste. The optimum slag content was 85% which exhibited residual compressive strength comparable to that of 100% PC at 600 °C and 800 °C. The pH values of the pastes decreased with increasing heat treatment and slag content.