# An Investigation on the Synthesis of Alkali Activated Materials from Thermally Modified Clays
## 1 Introduction
Natural and artificial pozzolans (e.g., volcanic ashes [25], pumice [26], and metakaolin [27]) and industrial by-products such as waste glass [28], bauxite residues [29; 30], non-ferrous metallurgical slags [17; 31; 32], and coal and biomass fly ashes [33; 34; 35; 36] have been examined as potential precursors, while silicates, hydroxides, carbonates, sulfates, and mixtures thereof can be used as activators. Metakaolin is the most well-established and technological mature precursor used for AAM production. Metakaolin has attracted attention as a precursor due to its purity and consistent composition as well as its outstanding mechanical properties and the thermal stability of metakaolin-based binders. Technical challenges include the high specific surface area of metakaolin, which often results in poor workability and demand for high water dosages that later result in considerable shrinkage and cracking [37]. Several shrinkage mitigation strategies have been recently described (e.g., heat curing, shrinkage reducing admixtures) and can be used to counteract such deleterious phenomena [32; 38]. However, the commercial exploitation of metakaolin-based AAMs can be limited by local shortages and the excessive cost of high-grade kaolinite clays. The sustainability profiles and green credentials of building materials will be compromised if long transport distances are considered, and the use of endogenous materials as precursors should be favored to further future-proof alkali activation as a scalable, environmentally friendly, and cost-competitive technology. In this view, the interest in using low- and medium-grade clays (kaolinite and otherwise) abundant in many locations around the globe as metakaolin surrogates is increasing [39; 40; 41; 42; 43]. Portugal has plenteous clay resources including kaolinite clays. Kaolin deposits are distributed mostly in Portugal's northwest region comprising the Lusitanian MesoCenozoic basin and a narrow northwest strip located between Viana do Castelo and Aveiro districts [44]. In 2016, Portugal ranked 12 th in kaolin production in Europe [45], and although abundant, most kaolin resources are consumed by the local ceramic industry and other high-end markets such as paper, paint, rubber, and agrochemical production. Therefore, highly pure kaolinite clays (i.e., without significant levels of \"industrial contaminants\" such as quartz, mica, montmorillonite, iron, and titanium oxides) are highly disputed resources that are unlikely to be available for the construction market. Affordable alternatives for AAM manufacturing are needed, and several common clays might have the potential to be used in (partial) lieu of metakaolin. Many of these clays contain kaolinite minerals, but not in the desired quantities for metakaolin commercialization. High iron contents can also be allowed in AAM production and will not constitute a limiting factor as previously demonstrated [17; 46; 47]. Common clays are considerably cheaper, are available in significant volumes in Portugal, and are even often discharged as industrial waste in activities such as dredging, sand-washing, and kaolin extraction. Therefore, this research focused on the use of a common clay found in central Portugal as precursors, taken as a representative case study of a broad group of currently underutilized clay resources that could potentially be valorized via alkali activation. The influence of various synthesis factors on the fundamental properties of metakaolin systems was examined and taken as the baseline to describe the effects of replacing metakaolin by calcined common clays. Mortar specimens were also produced with progressive contents of common clay to investigate the key fresh and hardened-state properties.