# The Role of an Industrial Alkaline Wastewater in the Alkali Activation of Biomass Fly Ash
## 4 Conclusions  
This work shows the possibility of using an alkaline industrial wastewater, from a Portuguese paper and pulp industry, as a partial replacement of the commercial sodium hydroxide in the synthesis of biomass fly ash alkali-activated materials. The use of the wastewater was found to decrease the workability of the pastes as well as the initial and final setting times, and this feature limited the maximum incorporation content to 25 vol.% (compared with the NaOH). The lower workability and faster setting were associated with the higher Ca dissolution coupled with the lower Al and Si dissolution observed when using the effluent, as demonstrated by the dissolution assays. The results show that the use of the industrial wastewater (content \(\geq\) 20 vol.%) induced a lower earlier strength compared with the reference due to the lower alkalinity of the effluent, which hinders the dissolution of the reactive fraction in the precursors. Despite the lower early strength, the composition containing the highest amount of effluent (25 vol.%) reached a compressive strength on the 28 th day slightly superior (22.7 MPa) to the value observed for the reference (20.0 MPa); this demonstrates the possibility of using an industrial alkaline waste as a partial replacement of commercial activators without compromising the compressive strength of the specimens. Future work should be carried out to maximise the wastewater incorporation volumes using superplasticisers or by tuning the mixture composition.  
Figure 6: Compressive strength for the various compositions measured at the 1 st and 28 th day.  
**Author Contributions:** Conceptualisation, R.M.N. and J.A.L.; methodology, C.C.N., L.S. and R.M.N.; validation, C.C.N., L.S. and R.M.N.; formal analysis, C.C.N., L.S. and R.M.N.; investigation, C.C.N. and L.S.; resources, M.P.S., R.M.N. and J.A.L.; writing--original draft preparation, C.C.N., R.M.N. and J.A.L.; writing--review and editing, C.C.N., L.S., M.P.S., R.M.N. and J.A.L.; supervision, R.M.N. and J.A.L.; funding acquisition, R.M.N. and J.A.L. All authors have read and agreed to the published version of the manuscript.  
The authors would like to acknowledge the project ERA-MIN/0001/2019 (SMART-G--Smart Geopolymers). [PERSON] also acknowledges the FCT support under the grant (2020.01135.CEECIND). This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, UIDB/50011/2020, UIDP/50011/2020 and LA/P/0006/2020, financed by national funds through the FCT/MEC (PIDDAC).  
Not applicable.  
This work was presented at the workshop \"Engineering and circular economy: the road to sustainability\" funded as a part of the ECO-MET-AL Project (PID2019-109520 RB-I00), \"Can industrial and mining metalliferous wastes produce green lightweight aggregates? Applying the Circular Economy\" funded by the Spanish Ministry of Science, Innovation and Universities and ERDF funds, framed in the \"Grants for \"R&D&I Projects\" in the framework of the State Programmes for the Generation of Knowledge and Scientific and Technological Strengthening of the R&D&I System and R&D&I oriented to the Challenges of Society, Call 2019. The authors would like to acknowledge [PERSON] for performing the atomic emission tests.  
The authors declare no conflict of interest.