Influence of firing temperature on ceramic properties of kaolinitic clays from Duthuni, Limpopo Province (South Africa)

Abstract

This study investigated the ceramic suitability of two representative kaolin samples from Duthuni area, subjected to four firing temperatures (800 oC, 900 oC, 1000 oC and 1100 oC).The objectives were to ascertain the effects of the raw clay physical, mineralogical, chemical characteristics on its ceramic potential, elucidate on the thermal evolution and associated mineral phase transformations, and to technologically characterize the raw clays. Physical properties were identified by particle size distribution, consistency limits and clay activity. Mineralogical and chemical properties were investigated by XRD, FTIR and XRF. Firing characteristics were measured by weight loss (WL), bulk density (BD), linear shrinkage (LS), water absorption (WA) and flexural strength (FS). The characteristic mineral assemblage comprised; quartz + kaolinite ± mica + microcline + goethite + hematite + anatase + ilmenite. Quartz occurred as major constituent in both samples, while kaolinite, Goethite and Anatase are present as minor constituents. Visual appraisal of the clay workability chart indicated that D1 and D2 plot within the high shrinkage field, with D1 displaying comparatively higher potential for shrinkage than D2. The potential for high shrinkage suggests possible detrimental effects during clay body formulation (extrusion and molding) as well as firing phases of the ceramic manufacturing process. Interpretations drawn from the Holtz and Kovacs diagram are consistent with the mineral phases identified by XRD. Kaolinite was the main clay mineral in both samples, occurring as minor constituent. The loam to silty texture of D2 and and D1 respectively are consistent with the predominance of quart phases revealed by the XRD and Si – O functional groups observed in the IR. The studied materials are siliceous (SiO2: 38.5–45.5 wt%), aluminous (Al2O3: 23.9–16.7 wt%) and ferruginous (Fe2O3: 23.5–21.3 wt%), with the SiO2/A Al2O3 ratios ranging from 1.61 to 2.71. The geochemistry of the clay samples also shows relatively high amounts of TiO2 (2.91 – 2.71 wt% respectively for D1 and D2). Low kaolinite content and high iron oxide contents equally suggests kaolin impurity and colouring effect on the fired clays. Furthermore, appraisal of the ceramic suitability of D1 and D2 based on their geochemistry indicates suitability for red bodies (D2) and a close outlier for red bodies and porous tiles The DSC curve portrayed three endothermic peaks; 85 oC to 90 oC, 295 oC to 300 oC and 530 oC to 535 oC corresponding to elimination of adsorbed water, goethite dehyroxylation and formation of metakaolinite respectively. A weak exothermic was observed at 945 oC and 955 oC suggesting recrystallization of metakaolinite to spinel or mullite. Small mass loses where observed between 20 oC to 120 oC (2% - 7.2%), 250 oC to 320 oC (1.3% - 3.8%) and 400 oC to 600 oC (5.2% - 6.2%). The relatively low kaolinite dehydroxylation temperature (535 oC) coupled with the generally asymmetric shape of the endothermic peaks suggests a poorly ordered and crystalline structure. The highest percentage of weight loss (WL) was recorded at 1100 oC in D1(12.41%) and at 900 oC in D2 (16.96%). The bulk density (Bd) increased with firing temperature in both samples. The LS in D1 ranged from 10.2 % to 18.59% from 800 oC to 1100 oC. A corresponding increase in LS was also observed in D2 (8.06 % to 11.8%).The FS of D1 increased steadily up to 1000 oC and then experienced a slight decline at 1100 oC. The FS values for D1 ranged from 0.86 to 1.25 MPa, with acceptable strength attained at 1000 oC and 1100 oC. The FS of D2 showed a similar increasing trend and slight decline at 1100 oC. The WA for D1 showed a decrease with firing temperature, ranging from 24.48 % at 800 oC to 9.01% at 1100 oC. A similar trend was observed for D2 (19.39 % to 10.88 %) from 800 oC to 1100 oC respectively. D2 satisfies the conditions for massive bricks, ceramic blocks and roof tiles. The studied clays show moderate potentials for exploitation based on their inherent characteristics (without modification). This, however, limits the exploitation to predominantly face bricks. Beneficiation is thus recommended to increase value and range of structural ceramic applications.