Abstract
Natural silica sand contains about 5 to 30% alkali feldspar. To study the effect of feldspar, synthetic sands of various feldspar contents were made by mixing silica and alkali feldspar both of which were in a similar grain size distribution. The synthetic sands were mixed with 5% bentonite and 6% water, and formed to test pieces of 13φ×30mm. The test pieces were heated for 2 hours at 200°C and air-cooled. Hot compressive strength decreased with increasing the feldspar content until the heating temperature attained about 900°C. Hot deformation was hardly influenced by adding feldspar at temperatures below 1,000°C, while at higher temperatures it was remarkably increased with increasing temperature or feldspar content.
The residual strength of the sand showed a minimum at 1,000°C and increased when the sand was heated above 1,100°C. This increase in the residual strength was caused due to sintering of feldspar grains. Fig. 1 shows the relation between the residual strength and the heating time at temperatures 1,200, 1,300, and 1,400°C. When the sand was heated at 1,100 and 1,200°C, expansion occurred at its maximum one minute after exposure and subsequently shrank rapidly. The amount of shrinkage was increased by adding feldspar, while the amount of initial expansion was slightly decreased by adding it. As a result of microscopical examination, it was observed that sintering did not occur below 1,100°C but it occurred above 1,200°C to produced lumps of sand.
