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Alkalis, carbonatites, compressive strength, filler effect, heat of hydration, Kamafugites, natural pozzolan
Kamafugites and carbonatitic tuffs, though not common natural pozzolans, are the main option for supplementary cementitious materials in regions surrounding the Western part of the East African Rift valley system. The tuffs are silica undersaturated, carbonatitic and largely ultrapotassic. The current study sought to characterise their physical and chemical properties and compare them with those of published works on natural pozzolans. Two samples, a carbonatite and a kamafugite, from the Toro-Ankole geological region of East Africa are investigated for their effect on specific properties of pastes and mortars when used as natural pozzolans in Portland cement. The volcanic tuffs milled to three fineness levels and blended with Portland cement at 0%, 5%, 10%, 20% and 35% replacement levels are tested for effects on hydration and compressive strength. Results show a time dependent variation in heat of hydration which increases for the first 14 hours before decreasing. The variation seems dependent on level of replacement. Compressive strength results show dependence on level of replacement and fineness which was pronounced until 90 days, peaking at 28 days. Increase in replacement level resulted in a general reduction in compressive strength. A compressive strength conversion process that peaks at 10% replacement level is observed in all blended samples leading to compressive strength results at 180 days being lower than those at 90 days of curing. Compressive strength test results, however, sustain the ASTM C618 and BS 3892 minimum requirement for pozzolanic Portland cements indicating that the carbonatites and kamafugites can be gainfully applied in cement production from the resource and energy economy perspectives.