Abstract

Overburden clays are abundant in limestone quarries which are mostly stripped at a high cost to expose the
underlying limestone and carted away to dumpsites. These clays when calcined at specific temperatures can be
used as substitutes for clinker in the production of cement. The substitution of clinker with calcined clays has great
economic and environmental benefits by reducing greenhouse gas emissions and well as cost of burning clinker.
This study investigated the effects of calcination temperature of limestone overburden clay on the flexural
properties of blended mortars, to determine its suitability as a partial replacement material for clinker in cement
production. Mortar samples were cast using Lab cement produced with clinker substituted with limestone
overburden clay calcined at temperatures of 500ºC, 600ºC, 700ºC and 800ºCs, with replacement levels of 30%,
50% and 70% respectively. Mechanical properties were determined at curing ages of 1day, 3days, 7days, 28days,
and 90days respectively. Results were compared to performance of CEM 1 which was the control. 600ºC was
found to be the optimum calcination temperature with flexural strengths of up to 5.9N/mm² at 30% replacement
and 90days curing, compared to CEM 1 which had strengths of 7.8N/mm². The oxide composition of the clay had
a combined sum of SiO2, Al2O3 and Fe3O2 amounting to more than 70% of the total oxides, making it a good
natural pozzolan. Water absorption was on average of 8%, less than 10% which qualifies as good mortar.