# The Efficiency of Calcium Oxide on Microbial Self-Healing Activity in Alkali-Activated Slag (AAS)
## 1 Introduction  
One of the largest energy-consuming and air-polluting industries is the cement manufacturing process. Twelve to 15% of the overall energy consumed in the industry is directly related to cement production [1]. The annual cement production records 4.6 billion tons, with anticipation of reaching 6 billion tons by the end of 2025 [2; 3]. On the other hand, the cement industry contributes to global greenhouse gas emissions by 12% [4; 5]. Alkali-activated composites, or geopolymers, are introduced and recommended to replace Ordinary Portland Cement (OPC) concrete. They are presented as an environmentally friendly solution that proposes utilizing industrial by-products such as metallurgical slag or fly ash. In addition, they are characterized by their distinguished performance in terms of mechanical and durability properties [6; 7]. Extensive research [8; 9; 10] has been devoted to investigating these sustainable materials. However, a high shrinkage rate is reported in multiple studies as one of the obstacles that hinders the full application of alkali-activated composites. The higher shrinkage rate of alkali-activated materials is associated with the finer pore microstructure and the complex hydration process. The percentage of mesopores in the alkali-activated composite systems is about 74-82%, compared to 24-36% in the OPC pastes [11; 12]. Larger capillary tensile stresses are induced by finer pores, causing shrinkage cracks. The second reason is the formation of two categories of calcium silicate hydrates (C-S-H) gels. The higher surface area of slag leads to the formation of low-densityC-S-H and high-density C-S-H gels. The high percentage of low-density C-S-H gels and the reduced Ca/Si ratio in the alkali-activated systems (Ca/Si = 1.1) compared to that within the OPC (Ca/Si = 2) explain the shrinkage behavior. The shrinkage behavior is activated due to the continuous matrix re-arrangement that takes place at any humidity or moisture condition change [11; 12; 13; 14; 15]. A promising environmental repair approach called \"microbial self-healing\" has been developed and investigated for multiple engineering purposes. The microbial repair technology has shown promising results in healing cracks, and improving the mechanical and durability properties. The methodology of this sustainable application is dependent on the ureolytic action of the added bacteria [5; 16; 17; 18]. The enzymatic hydrolyzation of urea increases the pH value, producing carbonate ions that form calcite precipitate in the presence of calcium ions. The calcite crystals fill the pores and cracks, enhancing the engineering properties [5; 17; 18]. Some studies have recommended incorporating calcium oxide to enhance the concrete's self-healing property, as addressed in Table 1. [PERSON] et al. [19] added CaO by 2.5% and 5% of the total binder to highlight the positive influence of CaO on the crack closure. [PERSON] et al. [4] proposed incorporating calcium oxide in alkali-activated mixtures by 7% of the total binder weight to provide more calcium ions that may motivate the self-healing property of the slag. [PERSON] et al. [20] completely replaced the binder with lime powder, confirming that CaO could facilitate microbe activity in the cementitious matrix. The addition of CaO has attracted the attention of multiple scholars [11; 21; 22; 23; 24; 25; 26; 27; 28] to adjust the Ca/Si ratio and improve the alkali-activated composite properties. However, a contradiction has been realized regarding the influence of CaO addition on the engineering properties of the alkali-activated materials. Table 2 indicates the scattered findings of some studies reporting the influence of CaO on the behavior of the alkali-activated material. Favorable evolution in mechanical properties has been reported in some cases [20]. Other findings have confirmed the negative effect of immobilizing calcium oxide on the mechanical properties of alkali-activated mixtures [19].