Statistical significance of curing variables on geopolymerization of mining by-product untreated clay

The focus of this research is to investigate of the possibility of reusing mineral wastes and by-products from mining processes to produce more sustainable binders in large amounts. A mining by-product consisting of halloysite (approximately 48 wt%) was used to produce dense alkali activated solids. Attention was paid to the influence of temperature on the geopolymerisation process in terms of the microstructural characteristics of the samples obtained. The challenge was to alkali activate the clay as received without any firing pre-treatment. The fresh paste of untreated clay was cured at 50% relative humidity (RH%) at different temperatures: 40, 60, and 70 °C. The halloysite-bearing powder (HC) was then mixed with a low-quartz sand, a second by-product of the mining industry, to achieve higher chemical stability. The results showed a clear difference in chemical stability for samples containing sand compared to those without sand. Low percentages of metakaolin (5–15%) were also added to the same formulations to improve the chemical and physical properties of the samples and to reduce the curing time. For mixtures consisting of untreated clay and sand only NaOH was added as alkaline activator. The effect of curing temperature, curing time and addition of metakaolin (MK) on the microstructure of the geopolymers was analysed by different techniques: measurement of pH and ionic conductivity of the eluate of the chemical stability test in water, X-ray diffraction (XRD), scanning electron microscopy (SEM), bulk density, and compressive strength. Finally, a statistical approach was adopted to rationalise the effect of the curing conditions on the consolidation of the fresh pastes of samples with the untreated HC, sand and metakaolin. To test whether the parameters of the curing process, as well as the MK addition had an influence on the values of the ionic conductivity of the eluate from water sinking and/or the density of the solid final product, a 3-way ANOVA followed by Tukey-Kramer post hoc tests (p < 0.05) were performed. To further investigate the interaction between preparation parameters and the material properties, we also built a Generalized Linear model that provided an equation to predict the final results. Overall, we found that the addition of MK significantly reduced the conductivity while only marginally increasing the density of the material. This effect was extremely temperature-dependent, and it disappeared at 70 °C. In conclusion, it has been demonstrated how the addition of fillers, an easily controllable industrial step, can maximise the performance of the consolidated material, rather than controlling the curing time and temperature conditions.