Effect of Carbonate Minerals and Calcination of Carbonatites and Kamafugites on Their Pozzolanic Performance and Early Age Concrete Properties

Carbonates are primary minerals of the carbonatites and kamafugites sourced from Toro-Ankole geological region of the East African Rift system. Consequently, these materials are silica undersaturated. They are currently utilized as mineral additions in production of Portland pozzolana cements in Uganda. No published work exists to show how their unique composition might affect their pozzolanic performance and other concrete properties. This study investigated the effect of the carbonate minerals in the natural pozzolans and calcination on setting time, standard consistency, workability, soundness, early heat of hydration and strength development of blended cements. Two (2) samples, a carbonatite and a kamafugite sourced from deposits located in the Toro-Ankole geological region of the East African rift system were calcined in a furnace at 825 °C for one hour. The samples were then subjected to XRD analysis for mineralogical composition characterisation and to establish the effect of calcination at 825 °C on mineralogy. Setting time, standard consistency, workability, soundness and strength development were studied following standard procedures for testing blended Portland cements. Calcination led to a gain in compressive strength for both test pozzolans, the kamafugites showing a higher gain in strength than the carbonatite. The higher gain in pozzolanic performance for the kamafugites is likely due to kaolinite, a secondary mineral in the test pozzolan whose pozzolanic reactivity is activated by thermal destabilization. Calcination also led to pacification of the early age properties of cements blended with test carbonatites and kamafugites. The study reveals the carbonate minerals in the test pozzolans as a considered factor in accelerating early hydration of Portland cement. Hydration progression of Portland cement controls the important properties of fresh concrete (workability, setting and paste microstructure), which in turn directly impacts on the strength and durability properties of hardened concrete. Cements blended with carbonate bearing natural pozzolans therefore present interesting perspectives on how paste microstructure composition and durability performance properties might be impacted.