This paper intends to analyze the performance of mortars with reactive MgO, as a sustainable alternative to cement. Six different MgOs from Australia, Canada, and Spain were used in the production of mortars as partial substitutes for cement, namely 5%, 10%, 15%, 20%, and 25% (by weight). MgOs with different levels of reactivity were used to analyze its influence on the performance of MgO mortars. In order to evaluate the mechanical performance of these mortars, compressive strength, flexural strength, dynamic modulus of elasticity, and ultrasonic pulse velocity tests were performed. Compressive strength tests showed that the use of 25% reactive MgO can cause a decrease of this property of between 28% and 49%. The use of reactive MgO affected the other mechanical properties less. This paper also intends to analyze the durability performance of mortars with reactive MgO. To that effect, water absorption by capillarity was assessed. In this research, the effect of using MgO on the shrinkage was also analyzed. It was found that shrinkage may decrease by more than a half in some cases.
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The world population is estimated to reach 9.7 billion in 2050 and 10.9 billion in 2100. Population growth directly affects the construction sector, as this sector is responsible for providing the necessary housing conditions through the development of infrastructure [1,2,3]. Currently, the construction sector is responsible for the emission of 11% of the total carbon dioxide (CO2) expelled to the atmosphere per year [4], with the cement industry emitting approximately 5.7 billion tons of CO2 in 2018 [5]. Therefore, one of the alternatives to mitigate this problem in the construction sector is the incorporation, to the detriment of ordinary Portland cement, of sustainable materials such as fly ash (FA), active silica, slag, metakaolin, reactive MgO, among others.
As mentioned, an alternative to reduce the CO2 emission caused by the Portland cement production process is the use of MgO in the production of new concrete. This reactive MgO can be produced by the calcination of magnesite (MgCO3) at much lower temperatures (700 to 1000 °C) than those used in the production of Portland cement [6]. In addition, concrete produced with MgO can allow permanent sequestration of CO2 over its lifetime, thus partly offsetting the CO2 emitted in its production phase [7]. As well as this environmental advantage, concrete with reactive MgO may behave better in some aspects, e.g., in terms of shrinkage [8].
The physical and chemical properties of MgO are directly related to the process of calcination of magnesite (MgCO3 → MgO + CO2). Thus, the MgO obtained by magnesite calcination is currently divided into four types, depending on the calcination temperature used in the process, which can vary between 800 °C and 2800 °C. The first type of MgO is called light-burned or caustic-calcined and results from the process of calcining magnesite at temperatures between 700 °C and 1000 °C. In this process, a MgO (purity) content of more than 85% is obtained, and it presents a higher reactivity and a larger specific surface area. MgO is called hard-burned MgO when it is calcined at temperatures ranging from 1000 °C to 1500 °C and has a lower reactive degree and specific surface than the previous one. MgO is designated dead-burned MgO or periclase when it is produced at temperatures between 1400 °C and 2000 °C. Finally, MgO is called fused MgO when it is calcined at 2800 °C, producing an MgO with practically zero reactivity that is mainly used in the steel industry [9,10].
MgO additions to cementitious materials can be made mainly through two methods. The first is to add an MgO quantity to the clinker, producing high magnesium cements. These are used in refractory concrete (Al2O3–MgO system), where the reaction of magnesia with alumina (from 1000 °C) results in the formation of spinel in situ, which has a higher resistance to penetration by slag than equivalent compositions containing preformed spinel [11,12]. The second method consists of preparing MgO from the calcination of magnesite (MgCO3) and then incorporating MgO into concrete as a binder [11]. The use of MgO as a binder results in the formation of Mg(OH)2 (Equation (1)) and its subsequent carbonation (Equation (2)), giving rise to hydrated magnesium carbonates. This type of binder was designed to replace Portland cement in large quantities, obtaining environmental benefits in relation to CO2 emissions [13].
MgO + H2O → Mg(OH)2
(1)
Mg(OH)2+ CO2 + 2H20 → MgCO3·3H2O
(2)
Research carried out by several authors on the incorporation of different reactive MgO contents (with low calcination temperatures) in cement materials has shown that there is a reduction in mechanical properties, such as compressive strength [14,15,16,17,18,19,20,21,22], flexural strength [17,18,21,23,24], and tensile strength [25].
Liu et al. [14] showed that there is a reduction of up to 15% in mixes produced with 3% MgO contents tested at different ages. Mo et al. [18] tested mortars with 8% MgO incorporation with different degrees of reactivity (reactivity values of 50 s, 100 s, 200 s, and 400 s), observing a reduction of approximately 18% in compressive strength at 28 days for the least reactive MgO. This decrease was more marked at advanced ages (14% at 91 days). The authors also observed that the degree of reactivity of MgO did not influence the strength of the mortars.
In another study by Mo et al. [16], where FA and MgO were incorporated in mortars exposed to pressures of 0.55 MPa and 0.10 MPa of CO2 for 3 and 24 h, it was observed that mortars exposed to a higher amount of CO2 had an increase in strength between 7% and 72% compared to mortars exposed to current pressures.
Regarding flexural strength, Gonçalves et al. [26] obtained decreases of between 27% and 30% in mortars, with MgO contents of up to 20%. In turn, Moradpour et al. [23] studied the influence on flexural strength of the incorporation of nano-MgO in mortars at various contents (up to 5%) and carried out the test at different ages (between 7 days and 90 days). The authors obtained an increase in flexural strength in mortars with nano-MgO, regardless of the age of the test and the content of nano-MgO used. However, it should be noted that these results may be due to a greater compactness of these mortars due to a filler effect. The use of other types of additions in conjunction with reactive MgO has shown positive results, reaching values similar to those of conventional mortars. This was also observed by Wei et al. [27], who produced mortars with different contents of microsilica and MgO and with no ordinary Portland cement, observing results similar to conventional mortars, when the proportion of the mix was 30% microsilica and 70% reactive MgO.
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The modulus of elasticity is also influenced by the incorporation of MgO. Gonçalves et al. [26] studied the modulus of elasticity in mortars with reactive MgO, obtaining a slight reduction (between 9% and 15%) with the use of different types of MgO. The authors justify this reduction with the higher water content of mortars with MgO, which is necessary to maintain the same consistency in all mortars.
Regarding the durability behavior of cementitious materials with addition of MgO, Mavroulidou et al. [20] evaluated the influence of the use of 5% and 10% MgO on the water absorption in mixes with FA and metakaolin. The authors found that the incorporation of 5% MgO decreased water absorption, which was attributed to the better compaction of mixes with higher levels of MgO and metakaolin, due to the greater water requirements in its composition to maintain consistency. With the incorporation of 10% MgO, the authors obtained better results than those of the reference mortars, but worse than those for mortars with 5% MgO.
Moradpour et al. [23] found that the permeability of mortars decreases (7% to 33%) with the use of nano-MgO. According to Dung and Unluer [28], the permeability of cementitious materials with MgO can decrease further (up to 24%) if HCI (hydration agent (HA)) or NaHMP (dispersion agent (DA)) is incorporated simultaneously.
As is well known, CO2 absorption is higher in MgO cementitious materials, increasing their carbonation. Pu and Unluer [25] analyzed the degree of carbonation at 14 days of concrete blocks made with an MgO content of 0% to 10%. The authors found that the degree of carbonation of concrete with MgO is twice that of the reference mix. This increase was also reported by Gonçalves et al. [26], who found that the carbonation depth at 28 days in mortars with 20% MgO increased by between 139% and 483%, depending on the reactive MgO used.
The use of MgO in cementitious mixes greatly affects their shrinkage. This positive aspect is attributed to the fact that the hydration of MgO leads to compensation of shrinkage, as described by Mo et al. [17]. Kabir and Hooton [29] observed a reduction of shrinkage by more than 50% in concrete with 15% reactive MgO content. The authors also found that the use of low reactivity MgO did not cause the same effect and obtained shrinkage similar to that of the reference concrete. Jin et al. [27] also concluded that the influence on shrinkage of the use of MgO depends on the degree of reactivity of the MgO used, having obtained maximum decreases of 26% when using a highly reactive MgO.
This paper presents the experimental results in mortars incorporating six different types of reactive MgO obtained from three manufacturers located in Spain, Australia, and Canada. So far there is not such a comprehensive study in the literature where it is possible to compare the performance in mortars of as many different types of MgO. This extensive analysis is especially important in order to understand the behavior of mortars with reactive MgO, as well as the reasons for it. In other words, by analyzing the incorporation of six different reactive MgOs (with different reactivity and calcination temperatures), it is possible to understand the influence that the variation of the properties of MgO has on the mixes produced with these MgO. On the other hand, in this research, cement substitution contents of 0%, 5%, 10%, 15%, 20%, and 25% were used in order to evaluate whether the behavior of mortars with MgO shows a linear behavior versus the substitution content, or whether, on the contrary, there is a change in trend from a given substitution ratio. All the mortars produced were evaluated in mechanical and durability terms. To determine the mechanical performance of mortars with MgO, tests of compressive strength, flexural strength, dynamic modulus of elasticity, and ultrasonic pulse were performed. In turn, the capillarity water absorption test was carried out to evaluate the durability of the mortars. In addition to these tests, the shrinkage of the mortars was evaluated over 91 days. Thus, it was possible to carry out a fairly complete characterization of the mortars with reactive MgO. This extension of the experimental campaign will make it possible to carry out a global analysis that is not yet available in the literature. Currently, there is still no extensive experimental campaign in the literature that allows an accurate evaluation of the influence of the incorporation of MgO as a binder in cementitious materials. On the other hand, the few existing investigations do not use MgO with different characteristics (reactivity), in order to understand how its reactivity affects the properties of cementitious materials.
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