## 5 Conclusions and Future Directions  
This review consolidated the current understanding of DK, a poorly characterised hazardous by-product of alarm manufacturing, and evaluated its potential for use in cementitious systems. While related materials like MK have received considerable attention, DK remains largely absent from scholarly discourse despite its likely widespread production. This paper addressed this gap by performing a systematic investigation of studies on DK valorisation, with a focus on its use in cement, mortar, and concrete, while identifying key knowledge gaps and barriers to adoption. A structured protocol of filtering existing literature was followed to extract relevant data related to DK, analyse the extracted information, and categorise it. The review began by describing the formation of DK in the aluminium sulphate industry and its hazardous characteristics; namely, high acidity and ultrafine particles, highlighting its environmental and health impacts. It then presented a comprehensive analysis of DK used as an SCM in PC or as a precursor in AAC/geopolymer systems, including hybrid combinations with other aluminosilicate precursors such as metakaolin, slags, and water treatment sludge (WTS). The key contribution of the paper lies in the comprehensive scope of the review and the first-of-its-kind synthesis, integrating DK origin, environmental hazards, physicochemical properties, pozzolanic behaviour, and performance within both SCM and AAC, towards potential commercial valorisation of this hazardous waste. By classifying performance trends across different SCM-PC and AAC binder systems, it provides a comparative framework previously missing in the literature and a foundation for future research. The main findings on the feasibility of using DK in cements for mortars and concrete were that:* 10-15% DK as SCM in cement systems enhances compressive strength by 15% to 30% but causes a dramatic reduction in flowability due to its fineness. Superplasticizers are therefore required in concrete mixes with DK used as SCM. Setting time was variably affected. * DK used as SCM in cement systems led to enhanced concrete durability. However, further studies are still required to consolidate these findings. * In AAC/geopolymer systems, DK performed best with MK or WTS but showed a poorer performance when partially replacing GGBS. With EAFS, late strength gains were observed, particularly after seawater exposure. In general, the available studies on DK lack diversity in mix designs, particularly with respect to AAC systems, and suitable AAC systems are yet to be developed for a consistent performance in terms of strength. * AAC mix designs with modest DK contents were shown to enhance concrete durability, which concurs with findings using DK as SCM and aligns with microstructural observations. Overall, the reviewed literature highlighted the promising technical performance particularly in terms mechanical strength and durability, when DK is incorporated at moderate levels. However, the current body of work remains fragmented, with incomplete understanding of chemical and physical mechanisms underpinning the behaviour of DK in cements for concrete, which is still subject to ongoing study. More detailed studies are needed to clarify reaction kinetics, gel formation, ion mobility, and pore structure evolution, and how these influence performance across different binder systems. From an environmental perspective, the use of DK has the potential to significantly reduce cement-related CO\({}_{2}\) emissions. Replacing 10-15% of Portland cement with DK could reduce emissions by an estimated 80-130 kg CO\({}_{2}\) per tonne of cement. However, the actual benefit depends on processing assumptions and could be offset if DK requires neutralisation (e.g., with lime or NaOH) to mitigate acidity, particularly for use in reinforced concrete. Detailed life cycle assessments (LCA) are essential and need to be performed to determine the net environmental benefits.