## 1 Introduction  
Concrete is the second most used material and the most manufactured material in the world; for this reason, its production is taxing to the environment not only in terms of water usage and mining/extraction of non-renewable raw materials and the related CO\({}_{2}\) emissions, but also due to the large quantities of cement binder used in its composition. According to studies on sustainable economics, the cement industry is considered one of the major culprits in terms of release of CO\({}_{2}\) and other gases associated with the burning of fossil fuels. Namely, the global cement industry was responsible for emitting 0.576 billiontonnes of CO\({}_{2}\) in 1990 but in the subsequent 24 years, these emissions experienced a significant increase, surpassing a threefold rise to reach 2.083 billion tonnes in 2014 [1]. By 2050, global CO\({}_{2}\) emissions from cement plants alone are expected to reach approximately 2.34 billion tonnes if no mitigation strategies are implemented [2]. Currently, the cement industry contributes approximately to 8% of the global CO\({}_{2}\) emissions [3], which play a critical role in global warming and climate change. The cement industry also affects soil, air, water, and human health. Hazardous heavy metals accumulate in the soil near cement plants, while CO\({}_{2}\), SO\({}_{2}\), NOx\({}_{X}\), and fine dust particles cause health risks [4]. Additionally, cement production increases water contamination, leading to elevated pH, phosphate, nitrate, as well as total dissolved and suspended solids [5]. Given the environmental impact of the cement industry, there is a growing interest in finding adequate alternative materials that can either partially replace cement or complete substitute it, as is the case of alkali-activated or geopolymers cements (binders formed by the activation of aluminosilicate materials using various alkalis) [6]. The aim of this effort is to provide effective eco-friendly Portland cement (PC) alternatives with a low carbon footprint, while improving concrete properties in comparison to PC. Improvements could be linked to a rapid strength development, increase in compressive strength, reduced permeability, and high durability [3; 6; 7; 8; 9]. For example, studies have demonstrated that geopolymers can reduce embodied carbon by up to 80% when utilised as cement replacement [8], with the additional advantage that they can incorporate the use of waste aluminosilicate materials originating from other manufacturing processes (e.g., metakaolin (MK), water treatment sludge (WTS), silica fume (SF), ground granulated blast-furnace slag (GGBS), and electric arc furnace slag (EAFS), making them a promising solution for reducing the environmental impact of construction materials. In this context, this paper studies dealuminated metakaolin (DK), as a potential material used as a precursor in alkali-activated cements (AAC)/geopolymers or as a supplementary cementitious material (SCM) for the partial replacement of PC. DK is a by-product of the aluminium sulphate (alum) industry, considered one of the most important industries, as alum is used in a wide range of applications like water treatment, manufacturing of paper, dyeing cloths, and others [10; 11]. DK is formed during the process of acid leaching of metakaolin to produce the alum and is considered a hazardous material due to its low pH, which increases environmental risks when disposed in landfills, as well as due to its fine particles, which contribute to air pollution [5]. This paper conducts a thorough review of all relevant scientific literature on DK to establish a comprehensive understanding of its characteristics, formation, and potential applications with focus on its use in cement and concrete.