## 4 Conclusions  
The aim of this study was to investigate the feasibility of producing sustainable alkali activated GGBFS composites cured at ambient temperatures using NaOH solution as an activating agent, without addition of any silica reactive source. The effect of molarities of NaOH solution ranging from 5 to 15 M; the liquid to solid ratio (from 15% to 30%); the curing age (7, 28, 56 and 90 days) and durability on the mechanical strength was investigated. Lastly, the optimum synthesised alkali activated GGBFS composites were evaluated for microstructural performance and potential environmental impacts using TCLP. Based on the findings of this research paper, the following conclusions can be drawn:  
1. The mechanical strength of the alkali activated GGBFS composites is highly dependent on the concentration of the alkaline activator, curing period and the L/S ratio. 2. The most favourable condition to synthesize alkali activated GGBFS composites was a concentration of 15 M NaOH solution at a L/S ration of 15%, cured for 90 days. 3. The long-term curing significantly improved the mechanical strength of the composites by achieving the highest UCS of 61. 43 MPa which is 27% higher compared to composites cured for a short term (sample M10). \begin{table}
\begin{tabular}{c c} \hline
**90 Days Ambient Cured Alkali Activated Sample** & \\ \hline Mass of cast (g) & 278 \\ Mass of cast after 30 days soak (g) & 285 \\ UCS before soak (MPa) & 61.4 \\ UCS after soak (MPa) & 57.4 \\ \% water absorption & 2.64 \\ \% reduction in UCS & 6.92 \\ Open porosity & 0.079 \\ Volume of permeable pores (\%) & 9 \\ \hline \end{tabular}
\end{table} 
Table 4: 30 days-soaked alkali activated specimen properties. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{4}{*}{**Constituents**} & \multirow{4}{*}{
\begin{tabular}{c} **Raw GGBFS** \\ **Concentration** \\ \end{tabular} } & \multicolumn{4}{c}{**USEPA Maximum**} & \multirow{4}{*}{**7 Days**} & \multirow{4}{*}{**14 Days**} & \multirow{4}{*}{**28 Days**} & \multirow{4}{*}{**56 Days**} & \multirow{4}{*}{**90 Days**} \\  & & & & & & & \\ \cline{2-2} \cline{5-8}  & **(ppm)** & **(ppm)** & **(ppm)** & **(ppm)** & **(ppm)** & **(ppm)** \\ \hline Cu & 0.37 & 5 & 0.24 & 0.16 & 0.09 & 0.08 & 0.06 \\ Fe & 0.04 & 10 & 0.03 & 0.03 & 0.021 & 0.019 & 0.01 \\ Mn & 0.06 & 5 & 0.04 & 0.03 & 0.03 & 0.03 & 0.02 \\ Zn & 0.01 & 0.1 & 0.01 & 0.01 & 0.01 & 0.01 & 0 \\ Ni & 0.01 & 0.2 & 0.01 & 0 & 0 & 0 \\ Cr & 0.09 & 5 & 0.09 & 0.08 & 0.08 & 0.07 & 0.02 \\ Pb & 0.12 & 5 & 0.09 & 0.79 & 0.06 & 0.05 & 0.03 \\ \hline \end{tabular}
\end{table} 
Table 5: Leachability of heavy metals concentration before and after alkaline activation. 4. The synthesized composites did not have any potential environmental impacts as the leached heavy metal concentrations of all composites were insignificant and within the allowed USEPA limits. 5. The study also shows that alkali activated cementitious material can be synthesized by just using a sole alkaline activator without addition of a silica reactive source. 6. Using the mechanical strength criterion, the developed composites meet ASTM specifications for different applications in building and construction such as facing and solid masonry brick and pavement block and can be used as an alternative binding material. 7. Further studies on using the synthesized alkali activated composite should be tested as a binder to develop building and construction materials. Supervision, writing--original draft preparation, conceptualization, methodology, validation: T.S. ; investigation, data curation. writing--original draft preparation: N.T. ; co-supervision, conceptualization, writing--review and editing: T.M. All authors have read and agreed to the published version of the manuscript. This research was funded by the National research foundation (ZA), grant number TTK180412320124 and the APC was funded by the University of Johannesburg. Data available within the text of this research paper. The authors declare no conflict of interest.