**1. Introduction**

The formation of cracks in infrastructures creates potential pathways for aggressive agents to insert into the bulk cementitious materials. This fact can cause significant problems that require high-cost repair solutions. Self-healing can reduce the deterioration rate, extend the structure's service life, and reduce the repair frequency and cost over the life cycle [1]. As a result, self-healing methodologies for cement-based materials were extensively studied in recent decades. Among other methods, incorporating mineral admixtures, such as fly ash or slag, is a well-studied method that can trigger self-healing [2,3]. The primary condition is the homogeneous distribution of the admixtures to react under the presence of humidity [4]. The mineral admixtures absorb and interact with water to penetrate the cracks and lead to product deposition that fills the cracks [5].

Huang et al. [5] suggested no particular method for self-healing, but one self-healing method could be the most suitable for a particular situation. Therefore, mineral admixtures proposed for self-healing should be accompanied by specific parameters and application conditions. The impact of environmental conditions on the self-healing of cement-based materials has been studied mostly by curing in water immersion and wetting-drying cycles, as the humidity and water availability plays a crucial role in the progress of autogenous self-healing and the mineral admixtures mechanism [4]. Water immersion constitutes a very effective curing method, as it brings in contact the calcium cations and the HCOOH- anions that lead to calcite formation and then calcite precipitation [6–8]. This mechanism functions in cement-based materials [9–11] and air lime and lime-pozzolan ones [12]. Encouraging results have also been reported for wetting-drying cycles conditioning. Jiang et al. [13] combined different mineral admixtures and promoted self-healing of cementitious composites under wet-dry cycles and high pH environments. Kan et al. [14] identified autogenous healing before five cycles, and the self-healing products included C-S-H compounds and calcite. On the other hand,

there have been reports that mention no autogenous healing under wet-dry cycles, but only in water submersion, and still, the healing was limited to the surface of the cracks [15]. Also, Roig-Flores et al. [9] reported that the same specimens exhibited different self-healing behavior in different environments, while no self-healing was recorded for constant humidity and constant temperature.

The nanoparticles that have been widely studied in cement-based materials [16,17] have the potential to work as mineral admixtures. Nano-silica (NS) and nano-calcium oxide (NC) are metal oxides with chemical affinity to cement and increased reactivity due to their high specific surface [18,19]. Moreover, small additions could ensure the homogeneous distribution of numerous particles throughout the material's volume, under specific protocols. Huseien et al. [1] referred to nanoparticles as materials produced through chemical synthesis, and therefore, they can generate uniform structured products and nano-sized crystals with "perfect atomic and molecular ordering." So far, the NS and NC nanoparticles have been used to self-healing cement pastes, providing promising results. An effort was made by Stefanidou et al. [20] when 5%wt addition of NC offered self-healing of cracked cement pastes after seven days in water immersion. Accordingly, Stefanidou et al. [21] proved that 1.5%wt NC favored the calcite precipitation and reduced the open porosity of cement pastes in water immersion remarkably.

The present work is an effort to accent the self-healing ability of NS and NC in cement pastes under different curing conditions. A novel approach, by using reactive nano-particles in an efficient amount, in order to promote the healing mechanisms is presented. This procedure is tested under different curing regimes in order to conclude on the optimum conditions affecting self-healing. The optimum curing regime for these nano-particles' best performance on healing has not been investigated before. Cement pastes with 1.5%wt NC and cement pastes with combined 1.5%wt NC and 1.5%wt NS were subjected to water immersion conditioning and wetting-drying cycles. The samples were cured for 90 days to record mechanical and physical properties, as well as the healing process of the pastes. At 28 days, controlled cracks were formed at prismatic samples using the 3-point bending method. That period was chosen in order to avoid early cement hydration. The crack pattern was recorded and the samples were placed again in the different curing regimes to record the healing process in the formed cracks.
