The influence of grain size and calcination conditions on the compressive strength of the Mahina lime- rich brick clays

Lime-rich clays from Mahina, Northwest part of Tanzania were used in this work. Chemical analysis showed that Mahina clays contain considerable amounts of CaO (16.25%) and CaCO3 (25.41%). Mineralogical characterization of raw materials was done by XRD and DTA both of which revealed the presence of kaolinite, montimorillonite, quartz, calcite, feldspar and minor accessories. Bricks constructed from different grain size (63, 125, 300, 450 and 600 μm) were each calcined at 950 oC for 30 and 60 minutes. New phases identified were Gehlenite, wollastonite, anorthite and calcium aluminate. Courser (600 μm) and finer (63 μm) particles gave lower values of compressive values, while the 450 µm samples showed maximum compressive strength. Bricks of 450 μm size were heated for 60, 30, 90, and 120 minutes at temperatures between 650 and 950 oC. The compressive strength increased with the calcination temperature and with a soak time of 120 minutes. The maximum strength (325 kg/cm2) was obtained from materials calcined for 90 minutes at 900 oC. Calcination duration had an influence on compressive strength. For instance samples calcined at 950 oC had strengths of 131 and 278 kg/cm2 for calcination period of 5 and 90 minutes, respectively. However, there was reduction of compressive strength at higher temperatures i.e. >900. The results showed that when grain size and calcination conditions are properly chosen for lime rich clays, carbonate affects the quality of the products such as bricks positively.