The influence of heat on elastic properties of illitic clay Radobica

Abstract

Brick clay from a locality Radobica, Central Slovakia, which was exploited for brick manufacturing in the past, was investigated for its possible reuse in the brick industry. The crystalline phases of the green sample were 48% of quartz, 37% of illite, 13%, of Na-feldspar and 2% of calcite. The measurements of Young's modulus of clay samples were performed during heating up to 1100°C and also at room temperature on samples preheated at temperatures from 100 to 1100°C. It was found during firing that 1) the physically bound water is released in 3 steps (up to 300°C) and reaches ∼2.5 wt %. The thermal expansion is decelerated by setting the crystal closer at low temperatures. Young's modulus increases in its values (∼36%) which is a result of the closer structure that is created via release of the physically bound water. 2) The mass loss during dehydroxylation (450–750°C) is ∼3 wt %. The superposition of dehydroxylation and α → β transformation of quartz creates a step ∼3% of the relative thermal expansion. Young's modulus slightly decreases in its values, the dehydroxylation does not influence this trend. 3) Above 900°C, the intensive contraction due to sintering is observed and a steep increase (250%) of Young's modulus takes place in this temperature interval. The irreversible changes of the Young's modulus measured at room temperature after firings at the temperatures from the interval 100–1100°C give a different picture. Dehydroxylation affects Young's modulus very significantly decreasing its values from 7.8 GPa (at 400°C) to 4.3 GPa (at 700°C). After dehydroxylation, the sintering increases Young's modulus. Since the porosity remains relatively high (∼30%) and a part of the glassy phase in the sample fired at 1100°C is relatively low (25%), the Young's modulus is low even after firing at 1100°C (9.3 GPa).