## 4 Discussion
50 kg CO2/tonne DK will be assumed, based on typical figures reported for the energy usage linked to the drying and grinding of other recycled cementitious materials [49] and road transport [50]. Assuming a 10-15% replacement of Portland cement with DK, which aligns with the optimal mechanical performance range observed in most reviewed papers in the literature, 10-15% lower PC contents could be used. Considering that the average CO2 emission from manufacturing 1 tonne of Portland cement is about 850-900 kg CO2, a 10-15% DK use would potentially translate to an estimated saving of 80-130 kg CO2 for every tonne of cement replaced. However, if DK requires post-processing such as neutralisation with lime or NaOH to mitigate acidity before its use in reinforced concrete applications, additional emissions must be considered. Line production, for instance, is reported to emit >1.0 per tonne of lime produced (1.0-1.8 tonnes of CO2, depending on the kiln type [51]); thus, even small additions for pH adjustment (e.g., 5-10 wt.%) can increase embodied carbon of DK by >50-100 kg CO2 per tonne. On the other hand, if washing is employed as an alternative treatment, it may not introduce additional environmental burden, since DK is typically washed prior to disposal in landfills [20]. In such cases, redirecting DK to concrete applications could offer a path towards more sustainable use without requiring extra water or processing beyond what is already standard practice. Nevertheless, a full life cycle assessment (LCA) is required to confirm the net environmental benefit of using DK in cement systems for concrete. In summary, DK presents a promising, underutilised material with demonstrated reactivity and performance benefits in both SCMs for PC and AAC systems. However, to enable broader adoption, further research is needed to define optimum mix designs, understand long-term durability, establish reactivity benchmarks, and assess environmental impacts. Addressing these challenges through coordinated research will be key to unlocking the full potential of DK in SCM and AAC systems for mortar and concrete. \begin{table}
\begin{tabular}{l l l l} \hline
**Ref.** & **Precursor** & **Mix Design** & \multicolumn{2}{c}{**Key Findings**} \\ \hline \multirow{8}{*}{[36]} & \multirow{8}{*}{Cement + Two types} & Cement/Concrete + Two Types & - & Improved compressive strengths (CS) or remaining unchanged at both 7 and 28 days. \\  & & of DK: & - & The flow is reduced because of the DK’s fine particle size. \\  & & DK replacement by 10\% and 15\%; & - & 10\% DK mortar complies with 52.5R strength requirement EN 197-1. \\  & & \(w/b\): 0.43; & - & Penetration of water under pressure was less than 20 mm. \\  & & Superplasticizer: 1\% to 1.4\% & - & No workability loss was recorded. \\ \cline{1-1}  & & & \\ \end{tabular}
\end{table} 
Table 1: Mix Design and key findings of DK utilised as SCM in Portland cement systems. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Ref. ** & **Precursor** & **Mix Design** & \multicolumn{2}{c}{**Key Findings**} \\ \hline \multirow{8}{*}{[16]} & & \begin{tabular}{c} Cement Mortar with: \\ \(w/b\) ratio: 0.43; \\ Sand to binder ratio: 2.25; \\ DK replacement by 5\%, 10\%, 15\%, \\ and 20\%; \\ \end{tabular}  & - & \begin{tabular}{c} DK has a very high surface area 20 m\({}^{2}\)/g. \\ - \\ NC improves CS for all mixes by approximately 3\%. \\ - \\ \end{tabular}  \\  & \begin{tabular}{c} Cement + DK + \\ Nano Carbon \\ referred to as NCDK \\ to 3\%; \\ Water-reducing agent is used. \\ \end{tabular}  & \begin{tabular}{c} Sand to binder ratio: 2.25; \\ DK replacement by 5\%, 10\%, 15\%, \\ and 20\%; \\ \end{tabular}  & - & \begin{tabular}{c} 10\% NCDK mortar increases CS by 14\%. \\ - \\ - \\ - \\ - \\ - \\ - \\ \end{tabular}  \\  & \begin{tabular}{c} The DK is replaced by NC: 1\% \\ Water-reducing agent is used. \\ \end{tabular}  & - & \begin{tabular}{c} Setting time is decreased by up to 50\% with \\ more NCDK. \\ - \\ \end{tabular}  \\ \hline \multirow{8}{*}{[18]} & & \