Thermal stability and pozzolanic activity of raw and calcined mixed-layer mica/smectite

Abstract

Samples of synthetic mixed-layer mica/smectite were calcined at 560°C, 760°C and 960°C. Chemical, physical and mineralogical properties of both untreated and calcined samples, before and after being mixed with Ca(OH) 2 in the presence of simulated cement pore solution, were studied in detail. Technological properties such as rheology and compressive strength of the clay mortars were also studied. The investigation indicates that the mixed-layer mica/smectite is a fairly good pozzolanic material and calcination is an effective approach to improve its pozzolanic activity. Dehydration and dehydroxylation of the sample at 560°C increase its pozzolanic activity considerably. Calcination from 560°C to 760°C does not cause further significant improvement, whereas calcination at 960°C again increases the compressive strength of the mortars to 113% (compared to the reference ordinary Portland cement (OPC) mortar). Most of six studied standard clay minerals are compositionally suitable to be used as pozzolanic materials. The common reaction products of clays and cement are CSH and C 4AH x . The untreated clay minerals with stable layer structures have low pozzolanic activity and have negative effects on the compressive strength. Particle sizes of pozzolanic materials of calcined clay minerals and some other artificial pozzolans correlate well with the compressive strength of their cement mortars.