# The Role of an Industrial Alkaline Wastewater in the Alkali Activation of Biomass Fly Ash
###### Abstract  
Alkali-activated materials are generally considered a more sustainable alternative to Portland cement binders. This derives not only from the use of solid wastes as precursors, but also from the low temperatures required for their synthesis. However, to increase the environmental advantages of these materials, alternative activators should be explored, as the common route involves the use of commercial activators such as sodium silicate or sodium hydroxide solutions. In this work, the possibility of using an alkaline industrial wastewater, coming from a Portuguese paper and pulp industry, as a partial replacement of the commercial sodium hydroxide solution was studied. The results show that the use of the industrial wastewater decreased the workability of the pastes and their setting times, higher incorporations inducing a stronger reduction. Despite this, the results demonstrate the feasibility of replacing up to 25 vol.% of the sodium hydroxide solution with the industrial wastewater without compromising the mechanical performance of the binder. The compressive strength of this composition reached 22.7 MPa, this being slightly higher than the value seen in the reference (20.0 MPa). The use of a waste-containing activator, as reported here, might be a key driver to foster the wider use of this technology. e 202220220220220220220220220220220220220220220220222022202  
Alkali-activated materials are obtained by the alkali activation of alumina- and silica-containing precursors [11]. The use of waste-based solid precursors is a common approach. However, this is not the case for the activators, as most of the studies to date use commercial alkaline reagents, such as alkali hydroxides and/or alkali silicates where the alkali cation is usually potassium or sodium [1; 12; 13]. The use of commercial alkaline activators in the mixtures raises environmental and economic concerns [14; 15]. For example, it has been reported that the production of 1 kg of sodium hydroxide generates between 0.3 [16] and 1.1 kg of CO\({}_{2}\)[1]. In addition, the use of highly alkaline solutions also raises concerns regarding the safety of the operators during in situ applications, but this issue is less relevant in precasting [8]. Considering that the alkaline activators are the main contributors to the environmental footprint and cost of these materials, alternative activators must be explored as this would contribute to the large-scale adoption of this emergent technology. A promising strategy could be the use of alkaline industrial effluents, but this route has barely been considered [17] and this is demonstrated in recent reviews on the topic [1; 16; 18]. Most studies to date evaluated the use of silica-rich wastes as activators such as bottom ash, rice husk ash, and waste glass [18]. For example, rice husk ash, ground granulated blast furnace, and palm oil fuel ash have been studied [19]. The extraction of the reactive silica is performed by using fusion [20], hydrothermal, or thermochemical routes [21]. Despite being an interesting approach, these processes typically require high thermal treatments (e.g., 550 \({}^{\circ}\)C [20]; 150-330 \({}^{\circ}\)C [21]) and have associated costs. An alternative could be the use of industrial alkaline effluents that can be directly employed as activators, as proposed here, but this strategy is uncommon. One of the few exceptions is the use of a waste aluminium anodising etching solution in [22] and the use of an alkaline solution generated during the mould washing step in the aluminium casting industry [23]. Another rare example is the use of Bayer process liquor as a source of Na\({}_{2}\)O and Al\({}_{2}\)O\({}_{3}\) in the synthesis of alkali-activated fly ash [24]. In the present work, and for the first time, the possibility of using an alkaline effluent generated by the pulp and paper industry is studied. It is used as a partial replacement of sodium hydroxide in the synthesis of alkali-activated materials derived from biomass fly ash.