## 1 Introduction  
Alkali-activated materials (AAMs) are binders obtained when aluminosilicate materials (\"precursors\") react chemically with an alkaline activator (normally an alkaline hydroxide or alkaline silicate solution) [1; 2; 3; 4; 5; 6]. As materials with a low environmental impact, AAMs are attracting considerable research as cements for the future, along with limestone calcined clay (LC3) [7; 8] and belite-ye'elimite-ferrite (BYF) cements [9; 10]. One of the major technological challenges to be faced in alkaline activation is to find precursors that are both widely available and technically suitable. The aluminosilicates most commonly used include burnt kaolinite clay and industrial by-products such as fly ash (FA) and ground granulated blast furnace slag (GGBFS) [11; 12; 13]. Output of FA and GGBFS is very high at this time and both will continue to be readily available during the twenty-first century. Nonetheless, the environmental challenges facing humanity herald a decline in future production, and the universal geographic availability for industrial scale AAM manufacture is not guaranteed. Pure kaolinite clay (metakaolin) deposits, in turn, are characterised by limited market availability and competition from other industries (paper, fired clay ceramics), which raise the cost of this raw material. Moreover, metakaolin-based AAMs are subject to workability [14] and drying shrinkage [15] problems. In contrast, clay minerals, outside of very pure kaolinite, constitute an abundant source of aluminosilicates, available worldwide. Studies carried out in recent years on ways to enhance the reactivity of \"low purity\" clay minerals, with a view to AAM [16; 14] and LC3 [7; 8] cement manufacture,have shown that their mineralogy (crystallinity), texture, and particle size [17; 18; 19; 20; 21] affect its reactivity. As a rule, lower crystallinity, greater surface morphism and smaller particle size lead to higher initial reactivity in precursors and hence to binders with greater initial mechanical strength. Most of the studies published described clays with a low calcium oxide content such as kaolinite [6; 22; 23; 24], illite [25; 26; 27], bentonite [28; 29] or even sedimentary [5; 20] materials. The main post-alkali-activation reaction product in precursors with a low CaO content (\(<\)10%) is a three-dimensional alkaline aluminosilicate hydrate gel (Na\({}_{2}\)O-Al\({}_{2}\)O\({}_{3}\)-SiO\({}_{2}\)-H\({}_{2}\)O) commonly named as, N-A-S-H. Conversely, if the CaO content in the precursor is fairly high (30% to 40%, such as in blast furnace slag), the main reaction product is a two-dimensional calcium aluminosilicate hydrate gel (CaO-Al\({}_{2}\)O\({}_{3}\)-SiO\({}_{2}\)-H\({}_{2}\)O), commonly named as, C-A-S-H [11; 12]. These two gels differ both chemically and structurally from the calcium silicate hydrate (CaO-SiO\({}_{2}\)-H\({}_{2}\)O), commonly named as C-S-H, that precipitates as the main hydration product in ordinary Portland cement (OPC) [7]. Activator CaO content and pH, which also play a significant role in the chemical composition and structure of the cementitious gels forming in AAMs, constitute a promising field of research at this time [11; 12]. Many studies have shown [30; 31] that mixing a precursor with moderate (20% to 30%) amounts of calcium in 4 to 5 M caustic solutions favours the solution/precipitation of a C-A-S-H gel at ambient temperature. Higher alkalinity may raise the degree of reaction and Si and Al (although not Ca) solubility, favouring the precipitation of a N-A-S-H gel or a mix of N-A-S-H/C-A-S-H gels that may interact over time [32] to make (N,C)-A-S-H (calcium-bearing N-A-S-H) gel or C-(N)-A-S-H (sodium bearing C-A-S-H) gel. The composition and structure of the aforementioned gel/mix of gels and the parameters affecting their nature and stability (pH, temperature, moisture... ) are studied in the so-called \"hybrid alkaline cements\" obtained in the alkaline activation of blends bearing approximately 20 wt.% of CaO, SiO\({}_{2}\) and Al\({}_{2}\)O\({}_{3}\)[33; 34].