# Fracture Behaviour of Concrete with Reactive Magnesium Oxide as Alternative Binder
## 1 Introduction  
To reduce the effect of global climate change due to greenhouse gases, many governments around the world have been striving to reduce carbon dioxide's (CO\({}_{2}\)) emission rates as the main cause of this effect. One of the sectors with a large share of CO\({}_{2}\) emissions is the construction industry, which is responsible for 10% of the CO\({}_{2}\) expelled to the atmosphere each year [1], with the cement industry alone emitting about 5.7 billion tonnes of CO\({}_{2}\) in 2018 [2]. A path for CO\({}_{2}\) reduction in the construction industry is the use of supplementary cementitious materials, such as fly ash (FA), silica active, slag, metakaolin, reactive MgO, among others. Among these alternatives, reactive MgO has become a good option as a supplementary material, as non-reactive MgO is already used in the composition of cements for refractory use [3]. Yet, standards restrict the content of MgO in clinker to a maximum of 5% [4]. One of the main advantages of using reactive MgO-based cements is that the production of reactive MgO (calciinated at 700 \({}^{\circ}\)C to 1000 \({}^{\circ}\)C) requires lower temperatures in comparison to the production of clinker (>1400 \({}^{\circ}\)C). MgO is mainly produced by calcining mined magmesetic deposits (MgCO\({}_{3}\)). The temperature and calcination time produce changes in the crystalline structure of MgO. For example, with the increase of temperature and calcination time, MgO suffers a reduction in its surface area increasing the size of the crystalline structure and reducing its reactivity [5; 6; 7]. The calcination process in obtaining MgO can be identified according to the calcination temperature. The EuropeanCommission has defined the grades of MgO as: caustic calcined (the case of reactive MgO), 600-1300 \({}^{\circ}\)C; dead burnt, 1600-2200 \({}^{\circ}\)C; and fused, \(>\)2800 \({}^{\circ}\)C [8]. During mixing, the use of reactive MgO with cement (either in cement production as a small percentage of clinker, or as an addition) results in the formation of Mg(OH)\({}_{2}\) (Equation (1)), which subsequently carbonates to a hydrated magnesium carbonate (Equation (2)) [9; 10; 11]. MgO + H\({}_{2}\)O \(\rightarrow\) Mg(OH)\({}_{2}\) (1)  
Mg(OH)\({}_{2}\)+ CO\({}_{2}\) + 2H\({}_{2}\)O \(\rightarrow\) MgCO\({}_{3}\).3H\({}_{2}\)O  
MgO can also be used as an expansive additive. This expansive character can be controlled with MgO reactivity and fineness. The higher the MgO calcination temperature, the lower the expansion at early ages, and the longer the hydration process lasts. As the hydration of MgO is a gradual and irreversible process, Mg(OH)\({}_{2}\) is stable, and its expansion is stable and not unlimited. The influence on the incorporation of different reactive MgO has shown that, with the use of reactive MgO as binder, there is a reduction of mechanical properties, such as compressive strength [12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22], flexural strength [15; 16; 19; 22; 23; 24], and splitting tensile strength [22; 25]. More specifically, authors such as [PERSON] et al. [18] showed that there is a reduction of compressive strength in ternary concrete with fly ash (FA), metakaolin, and reactive MgO. As the MgO ratio is increased, the reduction was about 30%. The water/binder (w/b) ratio was kept constant (0.55) for all the mixes. [PERSON] et al. [18] attributed this reduction to the fact that MgO increased the porosity and production of magnesium silicate hydrates due to the lack of reaction between FA and Brucite to form the M-S-H gel. In a study by [PERSON] et al. [19], it was observed that the type of curing influences the compressive strength. This property increased by increasing the autoclave time and decreased by increasing the autoclave temperature. The compressive strength values were higher in mixes with 4% MgO, followed by 8% and 12%. [PERSON] and [PERSON] [21] studied the compressive strength under conditions of accelerated carbonation (10% of CO\({}_{2}\)) in concrete blocks with reactive MgO and FA.