## 1 Introduction  
The concept of self-healing can be classified by its mechanism and the healing system applied. Principally, the method of self-healing involves healing agents that trigger and promote self-healing in the deteriorations and cracks of cement-based composites. Figure 1 summarizes the self-healing mechanisms in cement-based applications, which are autogenous and autonomous mechanisms as agreed by most researchers. The distinction between autogenous and autonomous is that the latter approach introduces components that normally are not found in cement-based components, whilst the former depends on the ability of the concrete's mixture without any special additions. In the case of the autogenous mechanism, the processes can be associated with materials such as pozzolan and expansive additives,which are added directly into the cement-based materials [1]. In these circumstances, the healing process involves crack blocking by hydration product of the unreacted cementitious material or sealing by the expansion of hydrated expansive materials in the crack flanks [1,2]. Most researchers found that the autogenous healing mechanism showed good results in the healing of microcracks on the surface of the concrete. Yet, the literature also indicates that the ability of the autogenous healing mechanism is limited to the finer cracks where the crack widths are less than 0.1 mm [3,4]. Furthermore, the healing ability by pozzolan gradually becomes limited at the later stage, as the reduction of calcium oxide content due to the decrease of the total amount of cement affects the availability of calcium hydroxide (Ca(OH)\({}_{2}\)) for pozzolanic chemical reactivity in producing calcium silicate hydrate (CSH). For the use of expansive additive, the effect of expansive additive is significantly influenced by its exposure condition and hydrated compound production, which results in uncontrol expansion, which consequently induces cracks [1]. The most common pozzolan used in ordinary Portland cement (OPC) concrete for improving the autogenous self-healing performance are fly ash (FA), ground granulated blast furnace slag (GGBS), and silica fume (SF) [5, 6, 7]. The two former pozzolans are usually used due to its content of calcium oxide for the cementitious chemical reactivity in producing the hydration product, CSH, for enhancing autogenous healing. Furthermore, it is also common to use these pozzolans, particularly FA and GGBS, in combination with the OPC concrete system for improving autogenous healing. Nevertheless, [PERSON] et al. [8] concluded that the healing performance of concrete with silica fume was less pronounced compared to concrete without silica fume. This could be associated with the reduction of Ca(OH)\({}_{2}\) availability, which affects the continuous pozzolanic reactivity of silica fume particles within the cement matrix of the concrete. In addition, [PERSON] et al. [9] suggested that the silica fume may decrease the self-healing potential of concrete if an insufficient amount of water was used, which may result in a large amount of un-hydrated cement particles. Conversely, previous studies identify that the autonomous mechanism is a more effective method in restoring the mechanical properties of cement-based composites. Literature indicates that the chemical healing using sodium silicate had a great potential of strength recovery of cement-based composites [10, 11, 12, 13, 14, 15]. Sodium silicate reacts with Ca(OH)\({}_{2}\) available within cement paste and with the presence of water to produce CSH gel which is the main product responsible for strength properties. Regarding the healing agent delivery, the encapsulation method is found to be the best approach for the autonomous healing mechanism. The use of the encapsulation system in the concrete application has been discovered in the early 1990s [16]. Currently, there are various types of carriers and encapsulation  
Figure 1: Self-healing mechanisms categorization.