# One-Part Alkali-Activated Pastes and Mortars Prepared with Metakaolin and Biomass Ash
## 3 Results and Discussion
### Characterization of Mortars  
By increasing the K/Al molar ratio, thus increasing the biomass ash quantity in place of metakaolin, the workability of one-part alkali-activated mortars increased considerably. Therefore, it was not possible to maintain the w/s ratio constant at 0.65 for mortars with a K/Al molar ratio above 1.5 and when K/Al molar ratios equal to 1.5 and 2.5 were adopted, the w/s ratio was lowered up to 0.49. This effect was found also by [PERSON] et al. [34], who prepared two-part alkali-activated mortars by partially replacing metakaolin with forest biomass ash. They suggested that the increased workability of mixtures containing a higher content of biomass ash was caused by the higher dissolution rate of metakaolin related to the increased alkalinity of the mixture. This process was confirmed by calorimetric analysis; in fact, by increasing the K/Al molar ratio of pastes also the dissolution rate increased (Figure 5). On top of that, metakaolinite needed more water to obtain workability than for instance fly ash or a blast furnace slag. The dissolution of metakaolin, together with the spherical shape of biomass ash particles used in this study, and their lower water uptake, caused the increase in workability of the studied one-part AAMs. Results obtained by one-part alkali-activated mortars in terms of hardened density and compressive strength are reported in Table 5. In general, all manufactured mortars showed similar \(\rho\) values, which range around 1.57 and 1.63 g/cm\({}^{3}\) regardless of their composition. In the literature [34], it is reported that by maintaining the w/b ratio, curing method, curing time, and drying conditions constant, the increased replacement level of metakaolin with biomass ash causes an increased porosity; however, in the compositions prepared in this experimentation, this effect was not so evident. Figure 6: Relation between K/Al molar ratio and reaction enthalpy of mixtures after 60 h of testing. Concerning the compressive strength of one-part alkali-activated mortars manufactured with the unheated biomass ash, it was quite evident that R\({}_{\rm c}\) values were very low. The mortar manufactured with the lower K/Al molar ratio equal to 0.5 had a R\({}_{\rm c}\) value equal to 1.6 MPa. The increase in K/Al molar ratio to 1.0 and to 1.5 caused a slight increase in compressive strength to 1.7 and 1.8 MPa, which were 6% and 13% higher than 1.6 MPa. Even though the mortar manufactured with K/Al molar ratio of 1.5 was less dense, thus more porous, than that with a K/Al ratio of 0.5, it showed a higher compressive strength, meaning that the extent of reaction in dissolution and polymerization of materials was slightly higher. By decreasing the w/s ratio from 0.65 to 0.49 at the same K/Al molar ratio of 1.5, the compressive strength of one-part alkali-activated mortars increased up to 3.4 MPa, which was 89% higher than 1.8 MPa. Moreover, a further increase in compressive strength was registered at the same w/s ratio of 0.49 by increasing the K/Al molar ratio from 1.5 to 2.5. In fact, the ASH_MK_2.5b mortar reached a R\({}_{\rm c}\) value of 3.7 MPa, which was 9% higher than 3.4 MPa. In traditional cement-based materials, the mechanical strength increases by decreasing the water/cement ratio, whereas in two-part alkali-activated matrices the mechanical strength increases both by decreasing the w/s ratio [11] and by increasing the alkalinity of the activating solution [35; 36]. As for two-part AAMs, in one-part alkali-activated matrices, the reduced w/s ratio, at the same K/Al molar ratio, reduced the porosity of mortars. Moreover, the increased alkalinity given by the increase in K/Al molar ratio, thus the biomass ash content, at the same w/s ratio, increased the amount of reactive material and contributed to a higher densification of the matrix and reduction of porosity. This result was also found by [PERSON] et al. [24] who reported an increase in inorganic polymers' compressive strength by modifying the ash/metakaolin mass ratio from 0.3 to 0.9.