# The Role of an Industrial Alkaline Wastewater in the Alkali Activation of Biomass Fly Ash
## 3 Results and Discussion
### Fresh Paste Characterisation  
#### 3.2.1 Consistency and Setting Time  
Figure 2 presents the flow table values measured for the various compositions. The results show that the workability of the pastes decreases with the increase in the effluent amount. The spread values ranged from 272.5 mm (reference) to 140.5 mm (25_Ef). Considering the decrease in the pastes' workability, higher amounts of effluent were not considered, as the objective of the present study was to keep the mixture composition unchanged. Nevertheless, the lowest spread values here reported is superior to that seen in [33] when the slag amount in the alkali-activated pastes was \(\geq\)70 wt.% (~100 mm), being lower than the minimum value reported in [34] (178.8 mm) for alkali-activated pastes produced using a mixture of high-calcium fly ash and slag as solid precursors. Previous works in the literature suggest that for alkali-activated mortars, a spread between 180 and 220 mm typically ensures good workability, compaction, and homogeneity for construction applications [35]. In fact, 160 mm was still considered an acceptable value, while 110 mm was related to a state of unworkability [35]. It should be noted that the value seen for the composition containing 25 vol.% of effluent (140.5 mm) is slightly below the acceptable limit; however, the workability of the pastes can be tuned by modifying the recipe or by adding a suitable superplasticiser. Moreover, the values here reported are similar to those reported for fly-ash-granulated ground blast-furnace slag (GGBS)-based geopolymer mortar (87-189 mm) [36] and for dreg-containing alkali-activated materials (150-280 mm) [27].  
Figure 3 presents the Vicat needle penetration upon curing for the three studied compositions (reference, 15_Ef and 25_Ef). In line with the spread results, increasing the amount of effluent in the compositions decreases the initial and final setting times of the pastes. The initial setting of the reference was 67.5 min, while in the effluent-containing pastes the initial setting was much faster (32.5 min--15_Ef, and 17.5 min--25_Ef paste). The final setting follows the same trend: the setting being 160 min for the reference composition,  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Element** & **Na** & **K** & **Cl** \\ \hline Concentration (mg/L) & 1130 & 11.3 & 653 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Chemical composition measured by ICP of the industrial effluent.  
while for the effluent-containing compositions the setting occurs earlier (105 min and 75 min for the 15_Ef and 25_Ef, respectively).  
The setting time of the alkali-activated materials is usually related to the calcium amount. Studies show that there is an exponential decrease in the setting time with an increase in calcium content [37]. It is also known that the calcium enhances the rate and extent of metakaolin dissolution, causing faster setting [27]. The biomass fly ash used here had ~12.2 wt.% of CaO, which can play a major role in the pastes' setting behaviour. The decrease in the setting time for the effluent-containing compositions suggests differences in the calcium dissolution compared with that seen in the reference, possibly due to the lower pH of the effluent compared with the sodium hydroxide solution. This feature will be further discussed in the following section (dissolution tests). Future work should be carried out to understand the impact of the effluent on the nature (sodium- or calcium-rich gel) and quantity of the produced gel.  
Figure 3: Needle displacement measured with the Vicat method for the studied compositions.  
Figure 2: Influence of the alkaline wastewater content on the workability of the pastes (consistency determined by the flow on a table method).