# The Role of an Industrial Alkaline Wastewater in the Alkali Activation of Biomass Fly Ash
## 2 Materials and Methods
### Alkali-Activated Material Characterisation  
#### 2.3.1 Fresh Paste Characterisation  
1. Consistency and setting time  
Consistency and the setting time of the fresh paste were evaluated at room temperature. The flow table test (to determine consistency) was performed in accordance with the EN 1015-3 (2007). A fixed amount of fresh paste was placed on the mould and then after removing the mould and rotating the handle 15 times, the spread of the paste was measured.  
The initial and final setting times were determined in accordance with EN 196-3, using a Vicat needle. The fresh paste was placed on a semi-conical mould and the needle was placed above the paste. It was released and the needle penetration in the paste was measured at regular intervals of time (5 or 10 min). To measure the final setting time with accuracy, a needle with a ring attached was used and the final time corresponded to the time when no visible ring mark was observed. The setting time was evaluated for the reference, and for the compositions coded as 15_Ef and 25_Ef.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Sample ID**} & \multirow{2}{*}{\begin{tabular}{c} **Metakaolin** \\ **(g)** \\ \end{tabular} } & \multirow{2}{*}{**Fly Ash (g)**} & \multirow{2}{*}{\begin{tabular}{c} **Sodium** \\ **Silicate (g)** \\ \end{tabular} } & \multirow{2}{*}{\
\begin{tabular}{c} **Industrial** \\ **Hydrooxide** \\ **Solution (g)** \\ \end{tabular} } & \multirow{2}{*}{
\begin{tabular}{c} **Effluent** \\ **(ml)** \\ \end{tabular} } \\ \hline Reference & & & & 25.00 & - & 0 \\ (0\_Ef) & & & & 23.75 & 0.97 & 5 \\ \hline
5\_Ef & & & & 22.50 & 1.94 & 10 \\ \hline
15\_Ef & & & & 21.25 & 2.92 & 15 \\ \hline
20\_Ef & & & & 20.00 & 3.89 & 20 \\ \hline
25\_Ef & & & & 18.75 & 4.86 & 25 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Mixture composition of the studied formulations. The amount of effluent used to replace sodium hydroxide is provided in mL, and the corresponding vol.% is given.  
Dissolution assays  
Dissolution tests were performed to measure the dissolution of Ca, Si, and Al from the precursors. In these tests, 0.2 g of the solid precursors (metakaolin and fly ash) was added to 50 mL of the activating solution (10 M NaOH mimicking the reference composition, or NaOH/effluent at a 75:25 volume ratio).  
The solids and the activating solution were magnetically stirred for various contact times, ranging from 5 min to 60 min. After this, the samples were centrifuged, and liquid aliquots were extracted and neutralised prior to the evaluation using inductively coupled plasma-optical emission spectroscopy (ICP-OES, [PERSON], Activa M, Kyoto, Japan). The detection limits for Al, Ca, and Si were 20 \(\upmu\)g/L, 50 \(\upmu\)g/L, and 100 \(\upmu\)g/L, respectively.  
3 Calorimetric characterisation  
To further characterise the impact of the alkaline effluent on the fresh-state properties of the alkali-activated materials, the heat evolution of the pastes upon curing was monitored during 24 h by using a quasi-adiabatic calorimeter [29]. Two compositions were evaluated, the reference (prepared without the effluent) and the composition containing the highest amount of the alkaline effluent (coded as 25_Ef). The tests were performed using a climatic chamber (Fitoclima 300 EP10, Aralab) at 23 \({}^{\circ}\)C and 65% RH. A thermocouple was placed inside the cells and the data acquired with an acquisition device (Data Logger Switch Unit 34970A from Agilent). Four samples per composition were measured. It should be noted that the mixture of the pastes was performed outside the cell, and therefore the initial heat evolution could not be measured.