Abstract
The kinetics of dehydroxylation of South African kaolin revealed that both the inner and the surface hydroxyl groups disappear according to first order kinetics, however, the surface group disappeared faster than the inner groups, showing that diffusion control kinetics is also important. The temperature dependent transformations of the kaolin was measure by means of fractional conversion of the ratios between AlO6:AlO4 and Si-O-Al:Si-O-Si, which showed kobs values 0.0168 s-1 and 0.0089 s-1 for the transformation to the spinel phase and values of 0.0165 s-1 and 0.0156 s-1 for the transformation to mullite respectively. The pozzolanic activities of the kaolin calcined at different temperatures showed a maximum pozzolanic activity when the kaolin is calcined at 650?C and the pozzolanic activity for mullite is even less than for the uncalcined kaolin. XPS revealed that the atomic ratio between Si and Al did not change from kaolin to metakaolin (Si:Al = ca. 1.2) however the mullite showed a atomic ratio of Si:Al = 1.52, implying that some deallumination occurred during calcination at high temperatures.
Highlights
The kinetics of dehydroxylation of South African kaolin revealed that both the inner and the surface hydroxyl groups disappear according to first order kinetics, the surface group disappeared faster than the inner groups, showing that diffusion control kinetics is important
The ATR FTIR of the South African kaolin studied is presented in Figure 1, the peak frequencies was allocated in accordance with previously published peak frequency assignments.[20,21]
The kinetics of dehydroxylation of South African kaolin at different temperatures (500–650 °C) was followed on FTIR ATR monitoring the disappearance of the four hydroxyl peaks between 3550 and 3750 cm–1
Summary
The kinetics of dehydroxylation of South African kaolin revealed that both the inner and the surface hydroxyl groups disappear according to first order kinetics, the surface group disappeared faster than the inner groups, showing that diffusion control kinetics is important. The changes in the spectra were studied as the function of fractional conversion of the ratios between AlO6:AlO4 and Si–O–Al:Si–O–Si. to measure the pozzolanic activity of the products that formed at the different thermal treatments, which will be used to determine the optimal calcination temperature for South African kaolin to be used as supplementary cementitious material.
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