Alkali-etched heated clay: Microstructure and physical/mechanical properties

• Zeolites P and hydro-sodalite crystallized in cured alkali-activated heated clay.
• The geopolymer was almost absent in the adopted curing conditions.
• Based on the stability diagram of zeolites, hydro-sodalite was a metastable phase.
• The experimental factors and physical/mechanical properties were well correlated by using RSM.
• The increase of the experimental factors had detrimental effects on the measured properties.

The microstructure of alkali-etched heated kaolinitic-illitic clay, which was cured in wet atmosphere at different temperatures and times, was investigated by using X-ray diffraction, Fourier-transform infrared spectroscopy, thermal analysis and scanning electron microscope. The flexural strength, density and water absorption of the cured samples were correlated to curing time, ageing temperature and alkaline solution concentration by using the response surface methodology. It was found that hydro-sodalite and zeolite P were essentially formed from metakaolinite, the products of chlorite and illite. The geopolymer was almost absent mainly because of excessive hydration. The assemblage of neoformed zeolites differed partially from that predicted using the stability diagram Na2O–SiO2–Al2O3. Polynomial models described well the change of the physical/mechanical properties against the experimental factors. Based on the results of the models, the increase of any factor among the studied ones had an adverse effect on the mechanical strength and contributed to the increase of water absorption. The weight of the effect of the factors on density was less significant. These results were discussed in relation to the microstructure investigation.