PHASE EVOLUTION AND MICROSTRUCTURE-PROPERTY RELATIONSHIP IN RED CLAY BRICKS

Abstract

Clay brick is one of the oldest and commonly used clay products in construction industry in Khyber Pakhtunkhwa (KP) Pakistan. In spite of its widespread use, studies regarding the manufacturing standards and quality of locally made bricks are much less in comparison to those of technologically advanced countries. It was one of the aims of the present study to investigate the local raw materials as well as locally manufactured bricks and compared the results with literature. Laboratory made brick samples, and coal and wheat husk containing samples were also prepared and examined in detail. In the first step, samples of soil and fired bricks were collected from two representative kilns of district Peshawar (Pakistan). X-ray diffraction of raw materials revealed the presence of illite, quartz, clinochlore and albite while calcite was identified as a minor phase in all the investigated samples. The absence of mullite and cristobalite in SEM results of fired brick-samples indicated that the firing temperature was low (i.e. in the range of ~1000oC). Consequently, the local brick specimens were expected to be more porous and mechanically weak as compared to the standard bricks reported in literature. The non-uniform colour of these bricks visible with naked eye and the presence of pebbles also indicated non-professional processing. Cracks seen in the fired brick samples demonstrated improper heat-treatment and processing. SEM EDS of brick-samples showed the presence of iron which has been reported as the cause of the observed red colour of clay bricks. The use of low-temperature firing cycle, rapid iv cooling, and absence or relatively lower concentration of kaolinite in raw materials may be the main factors leading to bricks with compromised quality. In the second step, phase and microstructural analyses of fired bricks collected from local kilns were performed and compared with those of the laboratory made bricks. X-ray diffraction of raw materials revealed the presence of illite, quartz, clinochlore and albite while calcite was identified as a minor phase in all the investigated samples. Diopside, anorthite and quartz were observed as major phases in the kiln-fired samples, and only diopside and quartz in the laboratory-fired samples. The presence of albite and clinochlore, although in small concentrations, in the kiln-fired samples indicated that the employed temperature was too low to completely dissolve the initial ingredients. Consistent with X-ray diffraction results, semi-quantitative energy dispersive spectroscopy revealed the presence of silicon, aluminum, calcium, magnesium, iron and potassium along with a small amount of sodium in these samples. An increase in firing temperature decreased porosity which resulted in substantial increase in compressive strength and decrease in water absorption. The optimum mechanical properties were achieved at 1000oC. In the third step, clay brick samples containing coal and wheat husk as additives were prepared and characterized. The use of by-products as additives in brick industry is gaining increased research attention due to their effective role in decreasing the total energy needs of industrial furnaces. In addition, these additives leave pores upon burning, causing a decrease in thermal conductivity and affect the mechanical properties of bricks as well. In the present study, various proportions of coal and wheat husk were used as additives in the initial ingredients of clay bricks. Microstructural analysis of the samples v revealed larger voids/pores in coal and wheat husk added samples than the normal bricks when fired at 1000 °C. Thermal conductivity considerably decreased by 27% and even by 68%, with the addition of 5 and 50 wt.% coal additive, respectively. A low coefficient of thermal diffusivity was observed with increasing additives which demonstrated that the investigated samples were suitable for use as thermal insulators. The water absorption of the clay bricks was in the range of 14–35% for coal added samples and 16 %– 37% for wheat husk added samples. The highest porosity was 65% with 50% of wheat husk addition. Compressive strength was observed to decrease from 15 to 4 MPa and 14 to 3 MPa, when coal and wheat husk addition was increased from 5 to 50 wt. % respectively. The densification characteristics of some samples were in good agreement with the international standard for good quality bricks ~20 MPa. Hence clay brick containing 5‒15 wt. % additives showed good results in comparison to the previously reported data.