Thermal treatment and pozzolanic activity of sepiolite

Samples of standard sepiolite were calcined at 370, 570 and 830 degrees C corresponding to the various stages of thermal reaction detected by DTA and TG. Both the raw and calcined samples, before and after being mixed with Ca(OH)(2) in the presence of simulated cement pore solution, were studied with XRD, SEM, EPMA and chemical solubility. The reaction rate of the mixtures was monitored by a chemical shrinkage test. Technological properties of the raw and calcined sepiolite-cement mortars were studied with rheology and compressive strength tests after reaction for 2, 7, 28 and 91 days at 40 degrees C. The results indicated that sepiolite is rather inactive as a pozzolanic material and has high water demand. The calcination to 370 and 570 degrees C does not upgrade it significantly. The most effective calcination temperature is 830 degrees C, which increases the compressive strength of its mortar to 84% of the compressive strength of the reference ordinary Portland cement (OPC) mortar.