THE INFLUENCE OF HIGH TEMPERATURE ON THE STRENGTH AND PORE STRUCTURE OF CONCRETES MADE WITH A NATURAL POZZOLAN

This paper presents the results of a study carried out to evaluate the effect of curing temperature on the strength and pore-structure of concrete and mortar containing pozzolanic additions. The natural pozzolan used is a red tropical soil (RTS) which is present in the B and C horizon of deep chemically weathered profiles typical of tropical and subtropical regions. Laboratory specimens were cured at 20 and 45 degrees C. The results are compared with those obtained for similar concretes and mortars made with ordinary Portland cement (OPC) and with addition of pulverised fuel ash (pfa). A model was also developed to quantify the relation between compressive strength and pore structure of concrete. The results indicate that activated RTS pozzolan concrete mixes exhibit higher strengths than the equivalent OPC concrete mix when cured at 20 degrees C. The natural pozzolan mixtures gave values slightly less than the values obtained for the control mix. Curing at 45 degrees C resulted in early high strengths for the pozzolan mixes. When curing time was increased, the difference between the activated RTS mixes and the OPC control decreased. The mixes made with natural pozzolan actually lost strength with time. This decrease is due to the microstructural changes which take place when metastable aluminates are exposed to high temperatures and very moist environments. An exponential decay function describing the relationship between compressive strength and porosity was obtained for each mix. However a general function to relate strength to porosity for all mixes does not exist. For the covering abstract of the conference see IRRD 853062.