# Performance of Pozzolan-Based Reactive Magnesia Cement Mixes against Sulphate Attack
## 1 Introduction  
The production of Portland cement accounts for 5-10% of global anthropogenic CO\({}_{2}\) emissions, with it being manufactured at a calcination temperature of approximately 1450 \({}^{\circ}\)C [1]. In turn, reactive magnesia cement (RMC) is made in a temperature range of 700-1000 \({}^{\circ}\)C [2]. Cement hydrates gain compressive strength by being cured in water [3], whereas Mg-based products acquire strength by being stored in a CO\({}_{2}\)-rich environment [4]. RMC is a potential alternative to cement, which is the second most used material after water [5]. Mg-based products can significantly isolate CO\({}_{2}\), and their wide application in construction can reduce their carbon footprint. Brucite (i.e., Mg(OH)\({}_{2}\)) forms as a result of mixing RMC with water, and it rapidly carbonates in a CO\({}_{2}\)-rich environment, in turn resulting in the production of hydrated magnesium carbonate (HMC). Dypingite (4 MgCO\({}_{3}\).Mg(OH)\({}_{2}\).5H\({}_{2}\)O), hydromagnesite (4 MgCO\({}_{3}\).Mg(OH)\({}_{2}\).4H\({}_{2}\)O), artinite (MgCO\({}_{3}\).Mg(OH)\({}_{2}\).3H\({}_{2}\)O), and usequheonite (MgCO\({}_{3}\).3H\({}_{2}\)O) are different forms of HMC and contribute to the development of compressive strength in RMC-based mixes [6]. They have round, prismatic, and rosette morphologies [7]. RMC particles are not very soluble in water, but they still turn into a strong binder during the formation of HMCs [8]. The carbonation of brucite is much slower in an ambient environment, and it should therefore be cured in an accelerated carbonation regime. This carbonation mechanism is also dependent on other factors, such as temperature, relative humidity, and CO\({}_{2}\) concentrations [9]. Pozzolanic materials are widely used for the modification of concrete properties. Fly ash and slag are solid waste materials obtained from thermal power plants and steel manufacturers, respectively. Their use in large volumes, as a replacement for cement,significantly reduces environmental impacts and material costs. Both these fine materials cause a filling effect, and also aid in the formation of secondary products in concrete matrices due to their pozzolanic activity [10]. This chemical reaction allows blended cement mixes to attain a compressive strength greater than that of control mixes at later ages [11]. Fly ash particles have a spherical morphology, and their incorporation improves workability and long-term durability [12]. However, gains in strength at initial ages are very weak due to the much slower hydration kinetics caused by them. The use of low-calcium fly ash (as a cement replacement) in large volumes reduces concrete degradation in aggressive aqueous environments [13], but such a concrete modification is not desirable under freeze-thaw conditions [14]. Slag is a by-product of the steel industry, and when used in concrete mixes, it has shown numerous performance benefits. Previous studies have shown that the use of slag as a replacement for cement (up to 55%) has resulted in an increase in compressive strength [15]. Slag-based concrete mixes with reduced permeability have improved chloride binding capacities, leading to the significant production of Friedel salts [16]. The formation of calcium aluminosilicate hydrate (C-A-S-H) and hydrotalcite in conjunction with calcium silicate hydrate gel (C-S-H) reduces the pore volume [17] and strengthens the cement matrix [18]. Magnesium silicate hydrate (M-S-H) and hydrotalcite form in RMC-based mixes modified with fly ash and slag. They transform into HMC and magnesium calcite in a CO\({}_{2}\)-rich environment. This uptake of CO\({}_{2}\) alters the morphology of stable carbonates and facilitates their agglomeration [19], in turn leading to the densification of the microstructure of the binder matrix [20]. The use of fly ash in carbonated RMC mixes facilitates carbonate agglomeration and improves morphology due to the formation of HMC and magnesium calcite (Mg-CaCO\({}_{3}\)) [21].