Experimental Analysis of Calcined Clay Geopolymers

Sustainable binders of low-energy and low-carbon have become of great importance in the search for alternatives to clinker cements which lack sustainability and durability under certain conditions. This thesis presents a critical literature review on the clay minerals/soils that have been used as a precursor in AAMs and geopolymers. The review extends to understanding the parameters that control the alkali activation reaction process of clay minerals along with reactivity-enhancing processes and other factors affecting their activation. The laboratory work in this thesis investigates the use of pure reference clay minerals (kaolinite, montmorillonite, illite) as well as 4 different natural common clays as precursors for synthesis of geopolymers. To increase their reactivity prior to alkali activation, the pure and common clays were calcined at different temperatures and residence times. This is an attempt to benchmark the optimum calcination conditions that are sufficient for these clays to be reactive. This was evaluated by the dissolution of the calcined clays in alkaline solution as well as the isothermal calorimetry results. The results show that most of the common clays were completely dehydroxylated at temperatures ≥800 °C and for a residence time ≥10 minutes. There were no significant differences between calcining the clays for 10 minutes and 60 minutes, suggesting that longer calcination time has no significant effect on the dehydroxylation of clays.
Alkaline activator type and concentrations are very crucial factors that control the geopolymerization reaction process and the reaction products. Sodium hydroxide and sodium silicates solutions were used throughout the thesis in different concentrations as alkaline activators. Their effect on the geopolymerization reaction and strength development were also studied. Dissolution tests, ultrasonic P-wave velocity analysis, Isothermal calorimetry, and compressive strength, along with other characterization techniques were used in this study. Sufficiently high concentrations of NaOH are required to dissolve Si and Al from the calcined clay structure, but it does not necessarily lead to producing geopolymer products. Soluble silica from the sodium silicates solutions on the other hand, promotes the geopolymerization reaction and products formation as well as the strength development of the produced geopolymers.
Investigating the use of natural common clays and understanding the effect of mineralogical and chemical composition of these clays on the geopolymerization reaction opens new opportunities for the exploitation of these resources to produce sustainable cements. A one-size-fits-all approach for processing and activating clay minerals is not viable. Instead, activation routes need to be tailored according to the clay mineralogy to achieve the binder properties required for key applications.