EFFECT OF INITIAL WATER CURING ON THE HYDRATION OF CEMENTS CONTAININGNATURAL POZZOLAN

This paper deals with the effect of initial curing conditions on the h ydration process of Portland pozzolan cements. Experiments were perfor med on pastes made with Canadian Standards Association (CSA) Type 10 ( ASTM Type I) Portland cement (PC) and Portland pozzolan cement (PPC) c ontaining 30% natural pozzolan by mass. PC pastes are more sensitive t han PPC pastes to the period of initial water curing because the hydra tion of PC occurs more quickly than does the pozzolanic reaction in PP C pastes. The hydration of Portland cement and the pozzolanic reaction continue after specimens are removed to a dry environment (relative h umidity almost-equal-to 20%). The presence of natural pozzolans retard s the normal hydration of PC before an age of 1 day but accelerates it after 1 day. For the tests reponed here, an excellent correlation exi sts between strength and the quantity of ''hydrate'' water. The measur ement of pore structure shows that die measurable total pore volume is significantly decreased for specimens that are water-cured and then d ried, compared with those that are water-cured only. The period of ini tial water curing does not have a significant effect on total pore vol ume at 28 days (measured by Hg porosimetry), but does affect the pore size distribution. Examination of strength is not a good indicator of the effect of initial water curing.