The effect of heat treatment and cooling rate on the properties of lightweight aggregates

Abstract

Lightweight aggregates (LWA) were produced from clay in the laboratory. After firing different heat treatments and cooling rates were applied and the resulting material was investigated with respect to strength and microstructure. Fast cooling led to the formation of micro cracks and weakened the material whereas slow cooling enhanced the strength of LWA. The residence time at temperatures between 700°C and 900°C led to differences in average oxidation state of iron in the matrix phase leading to substantial changes in thermal behaviour of the matrix phase. The combination of a highly oxidized shell and a reduced core 2 proved to enhance the strength of LWA. A two hour heat treatment at 800°C in air combined with a subsequent slow cooling rate (0.7°C/min) applied to LWA produced in an industrial rotary kiln led to a strength increase of 114 % compared to material of the normal production without changing any other property.

Influence of burning temperature and cooling methods on strength of high carbon ferrochrome slag lightweight aggregate

“…It can be seen from this figure that, the main difference between them is the middle size pores of 10-40 lm, and this will make some difference than the size of 0-10 lm and 50-450 lm, for they have almost the same quantity pores of 0-10 lm and 50-450 lm. Above all, it can conclude that the particle strength reach a reasonable agreement with the corresponding cooling methods on porosity and pore sizes [24,26].…”

“…Referred to the research of Markus Bernhardt [26], he proposed that the toughening mechanism of the strength was due to the tensile forces induced in the oxidized shell part of the pellet, and there existed an optimum ratio between the thickness of the oxidized shell and the radius of the reduced core. In our experiments a color change caused by the oxidation of Fe 3+ was also found like that the color of slow cooling samples were reddish-brown while the rapid cooling samples were brown.…”

The effect of additives on the properties of lightweight aggregates produced from clay

“…Additionally, T g and the viscosity of the glass phase are significantly influenced by the oxidation state of iron as observed for iron containing silicate melts where an increased amount of Fe 3+ leads to both a higher viscosity and a higher glass transition temperature [22,23]. Investigations on the softening temperatures of the matrix phase of clay based LWA showed that, depending on the oxidation state of iron, the softening temperatures varied from about 700°C for a reduced matrix phase (basically Fe 2+ ) to about 950°C (basically Fe 3+ ) for an oxidized matrix phase [24].…”

“…The resulting enhanced content of iron in a low oxidation state in the centre of the pellet leads to a lower viscosity of the glass phase compared to the reference as indicated by [22][23][24] causing the formation of large pores (cf . Fig 6a)).…”

Use of clay in the manufacture of lightweight aggregate

“…Under these conditions the LWA shell and core may experience different thermal contractions which can induce stress in the shell, producing micro-cracks that reduce strength. It is reported that this effect can be avoided in LWAs manufactured from clay using slow cooling (0.7 °C/min) and this resulted in 114% increase in strength [37]. The influence of cooling on the strength of LWA manufactured from clay and ferrochrome sludge has been reported and a relationship found between the pore structure and cooling rate [38].…”