Comparative study of the effects of a natural pozzolan and an artificial pozzolan on the hydraulic properties of Portland cement mortar

The comparative study of some effects of natural and artificial pozzolans on the hydraulic properties of Portland cement mortar is performed in this work. The natural pozzolan used coming from the Djoungo deposit, Moungo department in the Littoral region (Cameroon) was finely ground and then sieved at 100 μm. The artificial pozzolan used is the alluvial clay of Etoa collected from Yaoundé III District in Mfoundi Division of the Centre Region of Cameroon. It was thermally activated at 700°C after grinding and sieving. The chemical and mineralogical analysis of the raw materials were studied by X-ray fluorescence, infrared spectrometry (FTIR) and X-ray diffraction (XRD) then the specific surfaces and the absolute densities were determined. The formulation of the pastes and mortar was obtained by substituting Portland cement with different pozzolans at different percentages, while the initial and final setting time of the fresh pastes were determined. Linear shrinkage, water absorption, apparent density, flexural strength and compressive strength tests at 7 and 28 days were determined on the hardened mortars. The results obtained show an increasing in the initial and final setting time with the addition of the amount of pozzolan (natural or artificial), the pozzolans played the role of setting retarder in the mixtures, finer particles of the artificial pozzolan speed up the setting time of pastes as compared to that with natural pozzolan. The shrinkage of the hardened mortars increased with the addition of the pozzolans, but it was less than 0.2% in all the samples, the specimens with artificial pozzolans were more affected by the drying shrinkage. The mortars had good mechanical resistance but from 30 to 40% pozzolan, whether artificial or natural, the resistance dropped, the dilution phenomenon caused by the partial replacement of the cement was responsible for the drop in resistance with the addition of pozzolans. The water absorption rate and the apparent density of the specimens decrease when the percentage of pozzolan increases in the mortars. Artificial pozzolan offers a better water absorption rate compared to natural pozzolan due to its higher specific surface. Natural pozzolan makes it possible to obtain a denser mortar.