## 9 Effect of Parameters on Performance of Cementitious Self-Healing Agents  
Autonomously healing concrete can be achieved via the crystalline admixtures. The hydrophilic properties of crystalline admixtures cause the water permeability of the concrete to decrease, while enhancing the mechanical properties. The corrosion of steel is overcome by the alkalinity and the stiffness of the concrete is enhanced due to the incorporation of the crystalline admixtures [95]. The permeability of the concrete is diminished due to the needle-shaped structures that arise from the crystalline admixture when it reacts with water, acting as a waterproofing layer that fills in the formed cracks. Thus, the capacityto turn concrete into a water barrier makes its use effective in water tanks, dams, and other hydraulic structures. Subsequent sections highlight various studies that combined concrete with crystalline admixtures to achieve improved traits effective for functional applications. The influence of crystalline admixtures on the autonomously healed concrete has been reported [96], wherein the concrete specimen was subjected to various exposures and loading-induced pre-cracks. The propagation of the crack was halted through the use of steel fibers. The compressive and splitting tensile strengths were assessed. Standard concrete showed about 14% less CS than concrete with crystalline admixes. The values of CS and TS of the specimens were improved by 13.98% and 35%, respectively, by incorporating calcium sulphate-hydrate and calcite (Table 5). The compatibility of crystalline admixtures as a healing agent in the autonomously healing concrete was examined [97], in which distinct environmental conditions were used for the crystalline admixtures and fiber-reinforced concrete mixes. The healing performance was measured via the optical microscopy and permeability assessment. The crack width was remarkably reduced due to the immersion in water. The self-healing efficacy of the concrete mixed with crystalline admixtures was also examined [98]. The efficacy of the autonomous healing property was evaluated via the crack closing and permeability assessments, wherein two mixes were prepared one with 275 kg/m\({}^{3}\) of cement and water-to-cement ratio (w/c) of 0.60 and another with 350 kg/m\({}^{3}\) cement and w/c ratio of 0.45. The CS of standard concrete was found to be 15% lower than precast concrete that contained crystalline admixtures. Improved healing was demonstrated at 30 \({}^{\circ}\)C when the samples were immersed in water followed by the incorporation of crystalline admixtures. The cracks formation in the autonomously healing mortar surface via the expansive crystalline additives (CEA) in the concrete were investigated [99]. The healing process was enhanced by placing the pre-cracked samples in water for 28 days where the surface cracks widths were approximately 100-400 mm. The surface cracks were found to fill due to the incorporation of CEA. The crystalline admixture and calcium sulfo-aluminate content was 1.5% and 10% of the cement weight, respectively. The large surface crack of 400 mm was healed in its entirety by such autonomous healing substances. Moreover, in 28 days, an absolute drop in the healing occurred because of water penetration. It was concluded that a substantial enhancement in crack closing capability can be triggered by the incorporation of calcium sulfo-aluminate and crystalline admixtures. The calcium carbonate precipitation was crucial and was positively affected by the quantity of calcium precipitated from the concrete mix. In addition, conventional concrete was observed to release significantly fewer calcium ions than concrete incorporated with CEA. The autonomous healing performance of concrete mixed with crystalline additives was examined [95,100] to determine the nature of the healing rate and its impact under accelerated and natural curing conditions. The natural environment conditions were created by immersing the samples in water and air.