Effects of an Illite Clay Substitution on Geopolymer Synthesis as an Alternative to Metakaolin

In this study, a calcined illite clay from Bailén, Jaén, Spain, was valorized as a substitute of metakaolin in the synthesis of new geopolymeric materials. The raw materials, raw clay and commercial kaolin, were pretreated at 750°C (4 h). Several samples (0%–100% by weight of clay) were activated by mixing NaOH solution and sodium silicate solution. The specimens were cured (60°C and 99% relative humidity) for 24 h, then demolded and kept at ambient conditions for 7, 28, and 90 days. The prepared geopolymers were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, and scanning electron microscopy. Physical, mechanical, and thermal properties were determined. The results indicated that the specimens based on the illite raw clay and metakaolin present an amorphous consolidated appearance, characteristic of the polycondensation reactions. The incorporation of up to 50% by weight of raw clay provided geopolymers with higher mechanical strength (39.6 MPa) and bulk density ($\mathrm{1,455}\mathrm{kg}/{\mathrm{m}}^3$), lower apparent porosity (19.6%), and similar although slightly higher thermal conductivity ($0.25\mathrm{W}/\mathrm{mK}$) than control geopolymers containing only metakaolin as a precursor after 28 days of curing. Control geopolymers presented compressive strength, bulk density, apparent porosity, and thermal conductivity of 23 MPa, $\mathrm{1,251}\mathrm{kg}/{\mathrm{m}}^3$, 41.03% and $0.224\mathrm{W}/\mathrm{mk}$, respectively, at the same age of cured geopolymers. The mechanical properties increased with curing time due to a greater advance of the geopolymerization reaction. Therefore, this illite clay can be thermally activated together with metakaolin to obtain geopolymers with suitable technological properties. The results demonstrate that the finished materials can be used for construction applications.