## 3 Results  
### Ucs  
The MT was initially activated with a 12.5 molal sodium hydroxide solution, without FA (paste P0, Table 2), in order to assess the reactivity of the tailings. It immediately became clear, based on the structural integrity of these specimens, that chemical reactions did occur and generated some type of binding gel. This was confirmed by the UCS value of 1.8 MPa obtained. A parametric analysis was then developed to evaluate the influence of different parameters on compressive strength, namely, the ash/tailings weight ratio (FA/MT), the sodium hydroxide concentration (SH conc. ), and the silicate/hydroxide weight ratio (SS/SH). Regarding the ash/tailings ratio (with constant NaOH concentration and SS/SH ratio of 7.5 molal and 0.5, respectively), the results presented in Figure 4 show that an increase in the ash content produced an increase in compressive strength (UCS). The exception to this general trend was the paste with the lowest FA/MT ratio. This is not surprising, given that with a reduction in fly ash, the UCS tends to stabilise. Other authors [18], when studying the simultaneous activation of tungsten mining mud and red clay brick waste (precursor), using an activator solution based on SS and SH, obtained a compressive strength for the blends with a precursor content of 50% that was 3 times higher than that for the blends with only 10%. These authors were also able to prove that an increase in precursor content promoted the dissolution of some phases of the tungsten MT, with a positive effect on strength development. Given its higher strength among the P1 set, paste P1A was chosen for further analyses. The addition of the sandy aggregate to the paste produced a clearly positive influence on the mechanical response for every FA/MT ratio, representing a strength increase of between 14% (lowest FA/MT ratio of 0.25) and 42% (FA/MT = 0.33). Regarding the influence of the NaOH concentration (Figure 5), it became apparent that this variable was responsible for the most significant impact on UCS. An increase in concentration was responsible for an increase in UCS by favouring alkalinity levels which facilitate the dissolution of the species needed to form the final structural matrix. [PERSON] et al. [16], when investigating the effect of the activator composition on the strength of high-sulphide tailings, obtained UCS values very similar to those here presented (between 1 and 2 MPa after 28 days, and between 2.5 and 4.0 MPa after 1 year). These authors also tested the use of Portland cement, which resulted in a compressive strength of 1.5 MPa after one year of curing. [PERSON] et al. [17], also focusing on NaOH properties for the activation of phosphate MT and metakaolin blends, reached a similar conclusion, i.e., an increase in NaOH concentration produces denser and stronger pastes. Although paste P2C reached the highest UCS among the P2 set, paste P2B was chosen for further analyses since its strength/cost compromise was considered more effective, i.e., the strength showed by paste P2B is enough for situations where this material might be used (e.g., structural embankments, pavement bases and sub-bases), and its cost is approximately 90% that of P2C. The influence of the aggregate phase on UCS was also positive. However, it is noticeable that the strength increase due to the addition of sand was more significant than that registered when analysing the FA/MT ratio, varying between 42% (lowest concentration of 7.5 molal) and 75% (12.5 molal). Of the three factors considered, the SS/SH weight ratio appears to be the factor producing the lowest level of influence on strength development (Figure 6). Even though this analysis is not straightforward, since there are other variables changing besides the SS/SH (i.e., the Ac/Sol and Na\({}_{2}\)O/SiO\({}_{2}\) ratios and the water content), the results clearly show that a decrease in the SS/SH ratio did not produce a decrease in UCS.