# Magnesium Oxychloride Cement: Development, Opportunities and Challenges
## 4 Influence of SCMs on the Properties of MOC
### Effect of GGBFS on MOC  
GGBFS is an industrial byproduct generated from the iron and steel industries and is widely used as an OPC replacement in concrete. It is believed that approximately 530 million tons of GGBFS are produced worldwide, out of which only 65% are used by the building sector [69]. The use of GGBFS in cement mortar can enhance the strength, durability, and resistance to sulfate attack due to its pozzolanic activity [70; 71; 72]. In high-performance cementitious composites, it has been demonstrated that partial substitution of cement with GGBFS can increase workability; decrease hydration heat; improve sulfate attack resistance; decrease shrinkage, permeability, and chloride-ion diffusion into a com  
Figure 2: Strength retention coefficient of FA-modified MOC after water immersion. (Note—CM: control mix, HDTMS: hexadecyltrimethoxysilane, and SF: silica fume) [19; 20; 22; 66; 68].  
posite; reduce the risk of degradation due to alkali-silica reaction; and improve composite durability [73].  
Considering the high potential of GGBFS, its influence on MOC cement has also been investigated. [PERSON] et al. [74] investigated the influence of GGBFS on the strength and water resistance behavior of MOC and found positive results. They reported that by replacing MgO with GGBFS ranging from 5 to 30%, the compressive strength of MOC increased, whereas no significant change in flexural strength was reported. Further, it was found that the addition of GGBFS significantly improves the water repellency, and the softening coefficient of MOC was even higher than 0.82 with a maximum of 1.07. [PERSON] et al. [75] incorporated GGBFS as a partial substitute of MgO in MOC cement paste at various substitutional levels ranging from 10% to 30% and studied its impact on the fresh, mechanical, and water resistance properties of MOC cement. They found that the water resistance of MOC increased with the inclusion of GGBFS. The 28 d strength retention coefficient after water immersion of MOC paste without GGBFS was found to be 0.31, which increased to approximately 0.42 when 30% GGBFS was incorporated. Moreover, it was found that the addition of GGBFS increased the fluidity and setting time of the mix but decreased the compressive strength. The compressive strength of MOC cement pastes reduced from 126 MPa to approximately 101 MPa when 30% of GGBFS was incorporated.  
[PERSON] et al. [76] substituted MgO with GGBFS to develop an MOC cement-based composite and revealed mostly promising findings regarding the development of compressive strength. The compressive strength increased progressively over time, reaching approximately 48 MPa at 1 d of curing, followed by 51 MPa and 57 MPa, respectively, at 14 d and 28 d curing age. [PERSON] et al. [77] studied the behavior of GGBFS-incorporated MOC-solidified waste sludge and reported that the GGBFS-MOC mixture is effective in solidifying sludge. Further, they stated that the addition of GGBFS prevents the strength retraction of MOC-solidified sludge and develops C-S-H gels upon alkaline activation, thereby causing an enhancement in the long-term strength and durability. They stated that the combined action of GGBFS-MOC can be considered as an efficient and eco-friendly approach for disposing of urban sludge. In conclusion, the existing observations show that GGBFS seems compatible with MOC, and its addition has the potential to enhance the overall performance of MOC.