Assessment of the Performance of Calcined Clay as a Partial Replacement for Cement in Rammed Earth Floor

Abstract

Excessive dependence on cement for the stabilization of rammed earth in construction degrades the environment. Therefore, sustainable and easily available alternatives are needed. The aim of this study was to assess the performance of calcined clay as a partial replacement for cement in rammed earth floor construction for low-income housing. The pozzolanic activity of clay collected from selected wetlands in three sub-counties of Murang’a County, Kenya, (Kahuro, Kiharu and Maragua Sub-Counties) was analysed after thermal activation. Thermal treatment (calcination) was carried out in an electronic muffle furnace at 600, 700, and 800°C for a duration of 2 hours. Three methods (electrical conductivity (EC), Frattini test, and strength activity index (SAI)) were used to assess the pozzolanic activity of clay. Two soils were stabilized: murrum soil and black cotton soil. Each soil was stabilized by mixing it with a binder in accordance with BS 1377. The binder consisted of Portland cement and calcined clay mixed at various proportions. 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- concentrations to 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. The temperature range of 600-800°C was considered effective for clay calcination and provided sufficient pozzolanic activity (EC reduction > 0.12 S/m in the first 2 minutes and SAI > 0.75). For general construction, the KS02-1070:1993 standard requires a minimum compressive strength of 2.5 MPa after 28 days. This performance was achieved with murrum soil stabilized with a binder content in the range of 10-15% and 25% cement replacement with calcined clay. For stabilized rammed murrum, cement replacement with 25% calcined clay did not lead to a substantial increase in capillary water absorption after 24 hours of contact with water (0.15 and 0.11 kg water/kg soil block for a binder content of 10% and 15%, respectively). On the other hand, a replacement beyond 25% presented undesirable water absorption. Moreover, stabilized black cotton soil presented excessive water absorption of 0.28 kg water/kg soil block (10% cement content). It was not possible to stabilize black cotton soil sufficiently to achieve the required compressive strength. With a 25% cement replacement with calcined clay, Murrum soil was sufficiently stabilized as long as the binder content was in the range of 10-15%. Thus, a blend of calcined clay and Portland cement can provide an adequate mix for use in rammed earth floor construction.