Pozzolanicity, Chloride Ingress and Compressive Strength of Laboratory made Kenya Clay- Portland Cement Blends
Marangu, Joseph Mwiti
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The study investigated ingress of Cl- ions and their effects on the compressive strength development of a pozzolana based cement. The cement was made by blending Ordinary Portland Cement (OPC) with selected calcined clays. X-Ray Fluorescence (XRF), Atomic Absorption Spectroscopy (AAS) and flame photometry chemical analysis were carried out as a preliminary test to determine the chemical composition of the sampled clays. It was found that only one of the sampled clays had a sum of Al2O3, SiO2 and Fe2O3 above 70 percent. Sampled clays were activated at a temperature of 700 0C for one and two hours separately and also at 800 0C for one hour. Pozzolanicity test was carried out on the sampled clays using the conductometric method. Clays calcined at 800 0C for one hour exhibited the highest level of pozzolanicity. The calcined clay at 800 0C for one hour was blended with OPC at replacement levels of 25, 30, 35, 40, 45 and 50 percent by weight of the clay to make test cements labeled PCC25, PCC30, PCC35, PCC40, PCC45 and PCC50 for the purposes of this study. Mortar prisms measuring 100 mm x 40 mm were cast using the PCC at w/b ratio of 0.4, 0.55 and 0.63. For comparison purposes, commercial OPC and Portland Pozzolana cement (PPC) were also used to make mortar at the said w/b. Nine mortar prisms were made for each cement category. The mortar prisms were cured for twenty eight days at humidity above 90 percent. The cured mortar prisms were subjected to chloride media under an electric acceleration of 12.0 ± 0.1 V for a period of thirty six hours. The mortar prisms were then subjected to compressive strength, chloride profiling and diffusion coefficient analysis. The results showed that OPC recorded the highest compressive strength for each w/b ratio at the twenty eighth day compared to the pozzolanic cements. Pozzolanic cements showed higher percentage gain in compressive strength development when exposed to 3.5 percent sodium chloride solution than OPC. Compressive strengths of PCC25, PCC30, PCC35 and PPC were not significantly different at each w/b ratio. There was appreciable increase in compressive strength for PCC with increasing replacement levels from 25 to 35 percent. Compressive strength development decreased with increase in replacement of OPC from 35 to 50 percent. Pozzolanic cements showed a decrease in chloride ingress than OPC as w/b and depth of ingress increased. Blended cements showed lower apparent chloride diffusion coefficients (Dapp) compared to OPC. PPC, PCC30 and PCC35 exhibited similar performance in terms of Cl- ingress, Dapp and strength development before and after exposure to Cl- media. The test cements, PCC30 and PCC35 can thus be used in similar environments as commercial PPC.