## 4 Discussion  
This review has synthesised the current state of research on dealuminated metakaolin (DK) as both a supplementary cementitious material (SCM) and a precursor in alkali-activated cement (AAC) systems. While the number of studies remains limited, the evidence demonstrates that DK is a potentially viable material with unique characteristics. Its high amorphous silica content, moderate alumina, and high specific surface area make it a reactive material capable of contributing to both pozzolanic and geopolymerisation reactions. In terms of innovation, DK offers an opportunity to valorise a by-product of alum production that would otherwise be landfilled, thus contributing to circular-economy goals. Despite these promising results, several challenges remain. First, the current number of studies is very limited to give confidence in the industrial deployment of this material. These studies often are not exhaustive, and therefore very considerable gaps remain regarding the behaviour of DK in mortar and concrete mixes and the optimal mixes designs for the industrial usage of cement systems with DK; for example, in the reviewed literature, a number of studies relied on single alkali activator dosages or curing regimes, limiting our understanding of the behaviour of DK under varied processing conditions. Moreover, few studies exist to validate long-term durability and performance under real-environment conditions. Furthermore, systematic comparisons between preparation methods and different DK sources are lacking, making it difficult to establish general design guidelines, a challenge commonly encountered when attempting to valorise waste materials for their use in concrete applications. The body of research remains fragmented and lacks depth in key insights of underlying processes. For instance, the variability in the effects of DK on setting time, workability, and hydration reactions reflects an incomplete understanding of the interaction of DK with cementitious matrices. Few studies only (e.g., [26; 29; 45]) attempted to explore the underlying chemical processes, such as how DK alters gel formation, ion mobility, or pore connectivity, which are crucial for performance prediction and mix design. This review has tried to offer plausible interpretations, but dedicated material analysis of the cement systems with DK will be required to confirm the underlying processes and their effect on the concrete and mortar properties. The above factors contribute to the current absence of DK from international cement standards such as EN 197-1, which currently excludes materials without robust performance data and standardised testing methods. This is a clear barrier to the industrial adoption of DK in cement systems for concrete and intensive work is required to overcome this obstacle to commercialisation. Additionally, available production and usage data for DK are regionally concentrated and often inconsistent, complicating global extrapolation and creating uncertainties about supply in quantities required for concrete, the second most used material in construction after water. Moreover, an overlooked aspect in the literature and a potential limitation for the use of DK in structural concrete involves the residual acidity of DK, due to acid-washingduring alum production. Thus, if used in high proportions, DK can lower pore solution pH, compromising the protective passive layer on steel reinforcement (requiring a pore solution pH > 12.5) and increasing corrosion risk. Therefore, DK may require neutralisation for use in concrete systems, including concrete with AAC. From an environmental perspective, the potential for CO2 reduction through partial cement replacement with DK remains underreported in the literature. Considering that DK is a by-product of the alum industry, its environmental burden would be small, as its footprint would be mostly linked to post-processing (e.g., drying, grinding, transport, etc.). For the sake of illustration, an estimated CO2 emission of ca.