# One-Part Alkali-Activated Pastes and Mortars Prepared with Metakaolin and Biomass Ash
## 1 Introduction  
Conventional alkali-activated materials (AAMs) can also be referred to as \"two-part\" AAMs. If AAMs are prepared with aluminosilicates with a SiO\({}_{2}\) + Al\({}_{2}\)O\({}_{3}\) content higher than 80 wt.% and a low CaO content, such as class F fly ash and metakaolin, they belong to the sub-class of geopolymers [1]. AAMs are synthesized by activation of a powdered aluminosilicate precursor (e.g., metakaolin, fly ash, and blast furnace slag) and a highly alkaline activator. The alkaline solutions (NaOH, KOH and Na\({}_{x}\)SiO\({}_{(2+\sqrt{2})}\) with x usually between two and four) are categorized as corrosive and must be handled with gloves, masks, and glasses. This issue, together with the formation of a sticky and thick paste, make them not user-friendly and problematic for bulk production if no safety procedures are followed. During the last 20 years, researchers have tried to obtain a novel type of AAM that can be prepared without using dangerous alkaline solutions. They started to refer to them as one-part or \"just add water\" AAMs (or geopolymers), to mimic the preparation of common Portland cement-based materials. In synthesis, they consist of a ready-to-use reactive powder, that can form a solid material with good mechanical properties just by the addition of water. Even though this research field started two decades ago, there are very few studies regarding the preparation and properties of one-part AAMs/geopolymers. Moreover, as for \"two-part\" AAMs,different types of aluminosilicate precursors and activators are used, thus the properties of the final products depend on the characteristics and reactivity of the initial raw materials [2]. The most common precursor is fly ash (FA) from coal combustion either alone or coupled with ground granulated blast furnace slag (GGBFS) [3; 4; 5; 6]. The latter is commonly added as calcium-rich precursor for AAMs to improve the reactivity of low-calcium fly ash [7]. Other aluminosilicate materials used for AAM's preparation are clay minerals. However, as for \"two-part\" AAMs, they require calcination to be reactive in alkali-activation [8; 9]. Moreover the plate-like shape and the high surface area of their particles increase water demand [10], resulting in hardened materials with high total porosity [11]. The most studied aluminosilicate source used for AAM preparation is metakaolin (MK), which is the dehydroxylated form of kaolin. Other clayey materials include metab bentonite [12], albite (a sodium feldspar, often one of the main components in mine tailings) [13], potash feldspar, pyrophyllite, and natural zeolite [14]. Moreover, other natural or by-product aluminosilicate precursors have been used in AAM production, such as volcanic ash [15], red mud [16], geothermal silica [17]; however, the most suitable precursor for one-part mixes depends on materials' local availability [18] since the environmental impact rapidly increases with transportation distance [19]. As well as precursors, different types of activators have also been used in the preparation of one-part AAMs, such as sodium aluminate, sodium silicate, sodium sulfate, sodium carbonate, and sodium silicate pentahydrate [20]. Biomass ash (BA) is a material obtained by thermal treatment of biomasses. It can be generated by burning wood, agricultural wastes, grasses or even algae. Normally, it can be used as fertilizer in agriculture if it does not contain harmful compounds, otherwise it should be landfilled; therefore, a more high-value recovery is necessary. A solution can be the use of BA in one-part AAM preparation. Depending on the starting material, BA can be rich in silicon, calcium or potassium oxides, which makes it a good candidate for one-part AAMs both as a precursor and activator. Obviously, different sources of biomass provide for BA different phases and chemical compositions; in particular, those generated by the thermal treatment of wood are rich in calcium, whereas herbal ashes contain mainly silicon and potassium [21]. [PERSON] et al.