Improvement of Setting Time and Early Strength Development of Pozzolana Cement through Chemical Activation

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The use of clay pozzolana is being massively promoted in Ghana by CSIR-BRRI because of its technical, economic and to a large extent, environmental advantages. Work done with clay pozzolanas in Ghana indicate that by replacing approximately 30% by mass of ordinary Portland cement with burnt clay pozzolana through intimate mixing, the resulting Portland pozzolana cement (PPC) exhibits compressive strength values good for both load-bearing and non-load-bearing structural application. That notwithstanding, pozzolana cements are noted for their slow strength development resulting in low early strengths and slow nature of setting. This study has investigated the improvement of setting times and early strength development of cement samples containing 10%, 20%, 30% and 40% clay pozzolana by chemically activating the samples with 1% - 4% CaCl2, Na2SO4, and AlCl3. The physical, chemical and mechanical properties of the pozzolana cement at raw and hydrated state were studied. The results indicate that, 30% pozzolana cement activated with 2% Na2SO4, 3% CaCl2 or 4% AlC3 is able to improve early strengths by 28.4%, 28.4%, 26.8% respectively and 28-day strengths by 9.5%, 8.0% and 16.3%. Setting times were also accelerated to equal that of OPC by the addition of 2% Na2SO4, 3% CaCl2 or 4% AlCl3. The study therefore showed that the addition of Na2SO4, CaCl2 or AlCl3 could greatly improve the early strength development and significantly reduce the setting time of pozzolana cement. Water permeability of pozzolana cement decreased as chemical activator concentration increased from 1% - 4%. The economic analysis reveals that chemical activation of pozzolana cements is economically viable as the activation with Na2SO4, CaCl2 or AlCl3 gives similar strengths as OPC. The activation of 30% pozzolana cement with 2% Na2SO4, 3% CaCl2 or 4% AlCl3 slightly increases the unit cost of pozzolana cement but still about 16.7%, 16.2% or 13.1% less expensive than OPC.
A thesis submitted to the School of Graduate Studies, Kwame Nkrumah University of Science and Technology in partial fulfilment of the requirement for the award of Master of Science degree in Environmental Resources Management