Performance of low-grade kaolinitic clay as a cement substitute in mortar: A comparative study with fly ash

Different pozzolanic materials perform differently in blended hydrated cementitious systems depending on their unique chemical and mineralogical make-up. In this study, the pozzolanic properties of a naturally occurring calcined low-grade kaolinitic clay (a siliceous-aluminous material) was investigated and compared with pulverised coal fly ash, complying with BS EN 450, which is a silica-calcium-based pozzolanic material. Blended cements were prepared by partially substituting Portland cement with 10-30% (by weight) calcined clay and fly ash. The performance of these mineral admixtures was evaluated based on their influence on mortar properties such as workability, hydration, pozzolanic reactivity and compressive strength. From the results, pastes containing calcined clay required more water to form a workable paste and consequently increased setting times, as compared to fly ash. Both blended cement samples were found to exhibit similar water absorption and porosity characteristics. The 28 days compressive strength results of fly ash/cement blended mortar showed a slight (about 4.6%) compressive strength advantage over the calcined clay/cement blended cement. This could be due to the slower pozzolanic reactivity observed in pastes containing calcined clay, as measured by the Frattini test. Overall, the calcined clay, even though slightly deficient in metakaolinite content, obtained results comparable to fly ash and could be considered as an alternative in cementitious systems.