Effects of the experimental factors on the microstructure and the properties of cured alkali-activated heated clay

Abstract Heated kaolinitic-illitic raw clay was etched with NaOH aqueous solution (4–14 M) and cured at different temperatures (30–85 °C) and times (up to 30 h). The microstructure of the cured products was investigated by using X-ray diffraction, Fourier transform infrared spectroscopy, thermal analysis and scanning electron microscope. The effects of the above experimental factors on the mechanical strength, density, water absorption and the electrical conductivity of the cured samples were evaluated by using response surface methodology (RSM) and adopting a polynomial model. The results showed the presence of chabazite together with natrolite, which formed from the amorphous clay products as well as from illite. The neoformed phases differed from those predicted from the Na 2 O-SiO 2 -Al 2 O 3 diagram, and their abundance depended on the magnitudes of the factors. In conjunction with the above zeolitic phases, sodium carbonate formed as a carbonation product. Also, the results pointed out that the adopted model well described the effects of the experimental factors on the studied properties. Temperature and NaOH concentration had positive and marked effects on the mechanical strength and the electrical conductivity, respectively. The increase of one of the three factors reduced water absorption and had an insignificant negative effect on density. The effects of the factors were discussed in relation to the microstructure characterization.