Effect of thermal treatment on the pozzolanic activity of natural clay from selected Kenyan wetlands

Abstract

The use of cement in construction is costly and degrades the environment. Therefore, affordable and easily available alternatives are needed. This study evaluated the effect of thermal treatment on the pozzolanic activity of clay soils. Three wetland clays collected from three sub-counties of Murang’a County, Kenya (Kahuro, Kiharu and Maragua Sub-Counties) were studied after thermal activation. Thermal treatment (calcination) was carried out in an electronic muffle furnace (Advantec KL-420) at 600, 700, and 800°C for a duration of 2 hours. The chemical oxide composition was evaluated using X-ray fluorescence (XRF). Three methods (electrical conductivity, Frattini test, and compressive strength test method) were used to assess the pozzolanic activity of clay. A decrease in the electrical conductivity of the lime-clay solution was observed over a 24 hour period. Results of the Frattini test showed that clay calcined at 600°C and 700°C reacted with Ordinary Portland Cement (OPC) - CEM-I and reduced CaO and OH concentrationsto levels below the solubility curve of Ca(OH)2. Calcination of clay from Kahuro and Maragua sub counties, at 600°C and 800°C, respectively, increased the strength activity index of cement blocks from less than 0.75 to about 1 after 28 days of curing. Therefore, a temperature range of 600-800°C was considered effective for calcination. We conclude that thermal treatment increases the pozzolanic activity of natural clay for use as a replacement for cement. Therefore use of blended cement may be a sustainable alternative to the use of cement alone. Keywords: calcined clay, pozzolanic activity, thermal treatment, pozzolana, cement.