Potential of Scoria, Pumice, and RHA as Supplementary Cementitious Materials for Reducing Setting Time and Improving Early Strength of Pozzolan Blended Composite Cement

Tanzania has huge deposits of scoria (S-N) and pumice (P-N) minerals that can be used as supplementary cementing materials (SCMs) in cement factories to cut down the cost of cement and its pollution effect to the environment. Besides this, agricultural wastes such as rice husk produce rice husk ash (RHA) having high silica content that can be used with cement to reduce the cost of cement and its impact to environment. Performance indicators of mortar and concrete such as slump, flow, permeability, shrinkage, modulus of rupture, compressive and tensile splitting strength were tested with different proportions of SCMs. It was found out that in addition to cutting the CO 2 emissions, SCMs reduce energy bills and that they confer extra strength and resistance to mortar and concrete. This work only examined the properties of scoria (S-N) and pumice (P-N) and rice husk ash (RHA) as supplementary cementing materials (SCMs) for Portland cement. The investigation considered these materials in binary and ternary module. X-ray fluorescence, X-ray diffraction, and pozzolanic activity index (PAI) tests confirmed the suitability of these materials as potential SCMs. Initial and final mean setting times observed for a binary composite cement were 166 and 285 min respectively. The setting times were longer than those of Ordinary Portland cement (OPC) but shorter when compared to Portland pozzolana cement. Characteristic and target mean strengths of 30 and 38.2 MPa were considered. The ultimate mean compressive strengths achieved at 28 days of curing were 42.5, 44.8, and 43.0 MPa for S-N, P-N, and RHA respectively indicating the potentials of these materials as SCMs. Further observation show that, the 28-days maximum compressive strength achieved by the blended cement concrete (with 10% replacement of SCMs) were 44.2 and 43.1 MPa for S-N 10 and P-N 10, respectively. The modulus of rupture decreased with an increase in the amount of S-N. On the other hand with P-N, a maximum of 8.0 MPa at 20% replacement was observed but then dropped to a minimum value of 6.4 MPa at 40% replacement level. This indicated potentially a superior ability of the P-N concrete to endure more sustained stress such as those caused by tremors and earthquakes and impact-related stresses. The residual compressive strength of P-N blended cement concrete samples, after subjection to a high temperatures of 600 ºC, was higher compared to S-N blended cement indicating the superior resistance of P-N to higher temperatures. S-N 10, S-N 20 and S-N 30 gave coefficients of permeability, (K), of 5.2526E-08, 5.20833E-08, and 4.9741E-08 m/s, in that order. This low permeability was attested by their dense microstructure, with implied reduced chemical attack, less carbonation, improved steel protection against corrosion, and enhanced durability of the reinforced concrete. The maximum compressive strength of ternary materials of 53.8 MPa was attained at 10/20% P-N/RHA replacement level. The 28 days Strength Activity Index (SAI) of S-N/RHA blended cement at 30/0, 10/20, 5/25 and 0/30% S-N/RHA were above 75% recommended by ASTM. On the other hand the SAI of P-N/RHA blended cement were higher than the ASTM recommended value at all replacement levels. Therefore, 10% of S-N, P-N or RHA is recommended as the optimum replacement for Portland cement in binary materials and 10/20% P-N/RHA for ternary materials to enhance performance of cement.