Effect of Alkaline-Earth Metal Oxide on Compressive Strength and Flexural Strength of Alkali-Activated Slag Cement Mortar

The effect of CaO and MgO content on the compressive strength (CS) and flexural strength (FS) of alkali-activated slag mortar (ASM), which was activated by NaOH (Na2O% = 4%), was investigated. The pore structure of alkali-activated slag paste was investigated via mercury intrusion porosimetry (MIP). The morphology of ASM was observed by field emission scanning electron microscope (FSEM). The study shows that the CS and FS of ASM, which was activated by NaOH, increase first and then decrease with the improvement of CaO and MgO content. The optimal CaO and MgO content for CS are both 9% of the slag mass. The CS of ASM with optimal CaO and MgO content can achieve 46.54MPa and 47.79MPa, respectively. The optimal CaO and MgO content for FS are 6% and 3% of the slag mass, respectively. The FS of ASM with optimal CaO and MgO content can achieve 11.23MPa and 11.11MPa, respectively. The comprehensive mechanics performance of ASM are the best, when the CaO content is 6%.


Introduction
Alkali-activated slag (AAS) cement not only can make full use of slag, reduce energy consumption and greenhouse gas CO2 emission, but also has high strength and corrosion resistance. Its frost resistance, impermeability and fire resistance are all superior to those of traditional Portland cement [1][2][3]. The 2014 Environmental Life Cycle Assessment [3] indicated that compared with ordinary Portland cement concrete with similar compressive strength (CS), AAS concrete reduced 73% of greenhouse gas emission, 43% of energy consumption and 25% of water consumption. Therefore, AAS cement is regarded as the most potential cementitious material in the 21st century [4].
CaO and MgO can improve the mechanical properties of alkali-activated slag cement mortar (ASM). Chen et al. [5] added 2-6% CaO into ASM and found that the 28d CS of ASM increased with the improvement of CaO content. Based on experimental results, Jin et al. [6] concluded that when 0-7.5% MgO was added into the AAS paste, the 1-90d CS of this paste generally increased with the improvement of MgO content. Further, Gawwad et al. [7] showed that when 5% MgO was added, the 1-90d CS of the AAS paste increased, but when the added content exceeded 10%, its 1-90d CS decreased with the improvement of MgO content.
Different expansion characteristics of CaO and MgO and their different hydration products generated when added into AAS cement have different effects on the CS and flexural strength (FS) of ASM. Currently, however, there are no comparative studies on the effects of CaO and MgO content on

Mix Proportion
The mix proportion of AAS cement is shown in

Experimental Methods
The CS and FS of mortar specimens at curing ages of 3d, 7d, 28d and 90d were measured, respectively, according to the specification GB/T17671-1999 [9]. The PoreMaster-60 automatic mercury injection apparatus (Canta company, USA) was used to test the pore structure of the paste specimen. The microstructure of mortar specimens was observed by a Nova NanoSEM type 230 field emission scanning electron microscope (FSEM) produced by Czech FEI Company. Figures 1 and 2 show the CS and FS of ASM after addition of CaO. In figure 1, except for C12, the CS of ASM at each age increased with the improvement of CaO content. At 28d, the CS of C9 (42.36 MPa) exceeded that of C12 (41.53 MPa). At 90d, the CS of C3 (43.01 MPa) and C6 (45.13 MPa) also exceeded that of C12 (41.53 MPa). In terms of the CS of ASM, the optimal content of CaO is 9% of the slag mass.

CS and FS of ASM after Addition of CaO
In figure 2, the 3d FS of ASM increased with the improvement of CaO content. In 7d, the FS of C9 (10.03 MPa) exceeded that of C12 (9.66 MPa). At 28d, the FS of C6 (11.59 MPa) exceeded that of C9 (11.11 MPa), and that of C3 (10.70 MPa) and C6 (11.59 MPa) exceeded that of C12 (10.41 MPa). At 90d, the FS of C3 (10.76 MPa) exceeded that of C9 (10.41 MPa), and the FS of N4 (9.33 MPa) exceeded that of C12 (9.02 MPa). In terms of the FS of ASM, the optimal content of CaO is 6% of the slag mass.    Figures 3 and 4 show the CS and FS of ASM after addition of MgO. In figure 3, the 3d CS of ASM increased with the improvement of MgO content. At 7d, the CS of M9 (37.32 MPa) exceeded that of M12 (36.76 MPa). At 90d, the CS of M12 (44.82 MPa) was lower than that of M6 (46.77 MPa) and close to that of M3 (44.08 MPa). Therefore, adding MgO into ASM is beneficial to improve its early CS. In terms of the CS of ASM, the optimal content of MgO is 9% of the slag mass.

CS and FS of ASM after Addition of MgO
In figure 4, the 3d FS of ASM increased with the improvement of MgO content. At 7d, the FS of M9 (9.93 MPa) and M6 (9.75 MPa) exceeded that of M12 (9.38 MPa). At 28d, the FS of M12 (9.94 MPa) was lower than that of N4 (10.09 MPa), and that of M9 (11.05 MPa) was lower than that of M3 (11.28 MPa). Therefore, adding MgO into ASM is beneficial to improve its early FS. In terms of the FS of ASM, the optimal content of MgO is 3% of the slag mass.     In figure 5, when CaO and MgO had the same content, the CS of ASM with MgO was higher than that with CaO. Figure 6 shows that, in 90d, when the alkaline-earth metal oxide content was 3%, the ASM FS of group M was 3.25% higher than that of group C; when the content was 6%, the ASM FS of group M was 2.14% lower than that of group C; when the content was 9%, the ASM FS of group M was 3.75% lower than that of group C; when the content was 12%, the ASM FS of group M was 4.88% lower than that of group C. Table 6 shows the total porosity, average pore diameter and pore size distribution of AAS paste in groups N4, C12 and M12. Academician Wu [10] divided the pores of cementitious materials into four  Table 6 shows that, in 28d, compared with group N4, the total porosity of the paste in group C12 decreased by 6.46%, the average pore diameter decreased by 29.62%, and the porosity of the pore > 50 nm (harmful pores and multi-harm pores) decreased by 58.83%. Compared with group N4, the total porosity of the paste in group M12 decreased by 9.07%, the average pore diameter decreased by 40.10%, and the porosity of the pore > 50 nm (harmful pores and multi-harm pores) decreased by 79.20%. Compared with group C12, the total porosity of the paste in group M12 decreased by 2.78%, the average pore diameter decreased by 14.90%, and the porosity of the pore > 50 nm (harmful pores and multi-harm pores) decreased by 49.47%.
(a) Images of group C9 at 28d. (b) Images of group C12 at 28d.  In figures 7a-7d, the width of three cracks was randomly measured and the mean value was calculated, respectively. Figure 7 shows that, at 28d, the mean width of ASM microcracks in group  C9 (0.48 μm). The mean width of ASM microcracks in group M12 (5.53 μm) was greater than that in group M9 (1.10 μm). It indicates that ASM microcracks increase when the content of CaO and MgO increases from 9% to 12%. The ASM microcracks in group M9 (M12) were greater than those in group C9 (C12), suggesting that the ASM microcracks at 28d formed when added with 9% and 12% MgO were greater than those generated with the same content of CaO, respectively.  [11][12][13]. When Ca 2+ in the solution reaches saturation and achieves dynamic equilibrium with C-(A-)S-H, adding more CaO will precipitate Ca(OH)2 crystals and lead to volume expansion. When the number of Ca(OH)2 crystals is small, its main function is to fill the pores, reduce the total porosity, refine the pore structure, decrease the average pore diameter, reduce harmful and multi-harm pores (table 6), and improve the CS and FS of ASM (figures 1 and 2). However, when there are excessive Ca(OH)2 crystals, the hardened cement paste limiting its expansion will crack (figure 7), thereby reducing the CS and FS of ASM (figures 1 and 2). At curing ages of 28-90d, the CS of ASM increases while its FS decreases (figures 1 and 2), because the latter is more sensitive to microcracks [14].

Influence Mechanism of the Content of Alkaline-Earth
When the MgO content is low, the production of hydrotalcite-like compounds (HTLCs) increases with the improvement of MgO content, and the filling of pores in the paste with HTLCs makes the paste denser (table 6), thus improving the CS and FS of ASM (figures 3 and 4). When Al in the solution is exhausted and MgO continues to increase, Mg(OH)2 is generated and volume expansion occurs to fill the pores of the paste, further improving the compaction of the paste (table 6)  The CaO content in the study of Chen et al. [5] ranges from 2% to 6%, and Jin et al. [6] found that highly active MgO content ranges from 0% to 7.5%. In their experiments, the CS of the specimens increased with the improvement of the content of alkaline-earth metal oxides. This is because the content of these metal oxides in their test group did not reach the optimal content. The experimental results of Gawwad et al. [7] are similar to those in figure 3, that is, there exists an optimal MgO content (5%). The optimal content of alkaline-earth metal oxides is related to the activator type, the activator content and the water-binder ratio.

Influence Mechanism of the Types of Alkaline-Earth Metal Oxides on the CS and FS of ASM
On the one hand, MgO has a larger specific surface area than CaO, making its contact area with water larger than CaO. This leads to the early generation of more Mg(OH)2 crystals, expansion, and filling of pores, and the reduction of multi-harm and less harm pores (table 6). Thus, MgO improves the CS and FS of ASM more effectively than CaO, especially before 7d (figures 5 and 6). On the other hand, the addition of CaO makes the Ca 2+ in the solution reach saturation and achieve dynamic equilibrium with C-(A-)S-H before the precipitation of Ca(OH)2 crystals, that is, C-(A-)S-H will consume part of Ca 2+ . While the addition of MgO directly generates HTLCs and Mg(OH)2 crystals to fill the pores. Therefore, compared with CaO, MgO can more effectively reduce the total porosity, refine the pore structure, decrease the average pore diameter, and reduce harmful and multi-harm pores (table 6). Meanwhile, it can also make the expansion fissure of ASM develop faster (figure 7). Since the FS is more sensitive to microcracks than the CS [14], when the content of alkaline-earth metal oxides reaches more than 6%, the 90d FS of ASM in group M is lower than that in group C (figure 6).