Compressive Strength of Fired-Clay Brick with Variations in Composition of Rice Husk Ash Compared with Ancient Bricks in Dvāravatī Peroid, Northeast Thailand

Clay raw materials are approved by using stream sediments from a river in the Northeast of Thailand. These sediments at grain sieved for particle size below 75 µm are mixed with black-rice husk ash (RHA) at various ratios of 1:1, 1:2, and 1:3. The clay/RHA mixtures are cast and fired at 900, 1,000, 1,100, and 1,200 °C. A uniaxial compressive test is carried out for all types of specimens acquired from different firing temperatures. The increase of firing temperature increases the overall strength with the exception of the firing temperature of 1,100 °C when RHA is present in the mixture. Results show that inclusive specimens with increased amount of RHA decrease their ultimate compressive strength and elastic modulus. The reduction of the strength with increased RHA is remarkable for specimens fired at 1,100 °C. The compressive strength, elastic modulus of fired-clay blocks shows a linear relationship when compared between two firing temperatures of 900 and 1,200 °C but dramatically fluctuations at firing temperatures between 1,000 and 1,100 °C. The primary conclusion is that the strength of the fired-clay blocks is governed by the quartz-tridymite-cristobalite phase transformation. It is found that cristobalite transforms from tridymite at temperatures as low as 1,100 °C. The strengths and mineral components of the mixed-fired-clay blocks (1:2) at 1,000–1,200 °C can be related to ancient bricks in Fa Daed Song Yang (FDSY), during Dvāravatī period. However, the FDSY bricks show obviously plastic deformation with yield strength of 1.0–1.4 MPa and elastic modulus of 100–230 MPa.