**4. Discussion**

The contribution of nanoparticles NC and NS to the self-healing of cement pastes was tested under two different curing regimes. The results were slightly differentiated depending on both the kind of nanoparticle and the curing conditions.

The characterization of NC-cement pastes cured in water immersion showed that the system could form new material in empty spaces, such as pores and voids in humid conditions, at an early age. SEM-micrographs revealed the deposition of the porous newly formed material. This material assists in satisfactory strength with time. EDS analysis revealed that this material has calcium-silicon-aluminum content. DTG curves revealed a lower portlandite quantity than the reference, indicating its consumption and its participation in other formations. Accordingly, the calcite proportion is greater than the reference and the material shows a more significant strain to carbonation. Nano-calcium oxide corresponded under both curing conditions. At 28 + 14 days, the crack width was reduced by 70.75% compared to

the 28 + 0 days. The small particle size of NC achieved early healing due to the accelerated carbonation of the calcium hydroxide with time.

Under wetting-drying cycles, the optical observation displayed the healing crystals formed at 28 + 28 days, even though the characterization of the microstructure at 28 days showed limited filling of the pores. ATR spectroscopy showed aragonite and calcite presence (Figure 9). The needle-like shaped aragonite has large micro-porosity, according to Shen et al. [31].

Combined nanoparticles decreased the compressive strength of cement pastes by almost 30%, possibly having to do with the increased water demand during the preparation. Additionally, the system depicted high carbonation after water immersion accompanied by dense microstructure with pores slightly closed by the newly formed material. The elemental analysis showed high calcium and magnesium content. The healing of the cracks of this system occurred progressively. At 28 + 14 days, the crack width decreased by 82.10%, and at 28 + 28 days, the crack was closed with solid and compact material. ATR spectroscopy revealed that the healed crack material was calcite and aragonite, while calcium silicate indications were also revealed (Figures 8 and 9).

The mineral admixtures are divided into two categories, depending on their behavior [5]. On the one hand, the expansive additives absorb water and form large-volume products [32]. On the other hand, crystalline additives react with calcium hydroxide and form crystalline products [5,33]. According to this categorization, NC could be considered an expansive agent because it forms calcium hydroxide when absorbs water and leads to calcite precipitation when it is dried. Accordingly, NS could be considered that works as a semi-crystalline agent, as its mechanism of action is to participate in new C-S-H compounds by interacting with available calcium hydroxide [34].

Qureshi et al. [35]indicated that the different expansivemineralsinfluenced the microstructure of healing products added, among quick lime, bentonite, and magnesium oxide. Accordingly, in comparing two different systems with nanoparticles, it was depicted that the healing products were mainly calcite and aragonite. SEM images showed the structure's potential to close its pores with newly formed material of different roughness and micro-porosity during different curing of the same system. Additionally, EDS results and DTG quantifications showed that the systems performed different hydration degrees (according to the available portlandite) and different carbonated species formation (according to available carbonates) during different curing regimes. Consequently, it could be assumed that the different curing conditions of these systems influence the microstructure of the healing products.

The use of nanoparticles seems beneficial for the healing process as they contribute to microstructure changes and the production of calcitic products, promoting the healing of empty spaces. The nanoparticles were used as additives in small percentages and it was proven that even in 1.5%wt of binder, they could be effective. This is important as the cost of nanoparticles is a parameter that should be taken into account when designing such composites. The fact that the cost is reduced in time due to new, low-cost techniques engaged for their production and the low cost of the raw materials used is also important. The fact that nanoparticles were supplied in powder (and not as suspension) is also due to the easy and accurate use of the needed quantity. This fine powder involves the risk of inhalation, as all fine binders, during processing and all the necessary measures for protection (such as mask and gloves) should be taken into account during the preparation of the suspension. The exact impact of the nano-particles' use is not clear due to the lack of available toxicity information [36,37].

The results of this study can be used by materials scientists to improve the development of self-healing cement-based materials and engineers involved in works based on criteria of durable materials with minimum intervention requirements in the future. The system is easy to be prepared, both in a laboratory or in a plant. As demonstrated in the present study, nano-particles can offer early age self-healing, making these systems suitable for applications in humid or wetting-drying environments.

## **5. Conclusions**

Two different nanoparticles; nano-calcium oxide (NC) and nano-silica (NS) were used in cement pastes, in small amounts, as additives. The specimens produced were cured under two different curing regimes; water immersion and wet-dry cycles. Tests were performed at 28 days of curing as well at 90 days. The aim was to record the self-healing procedure and its influence on the microstructure and the mechanical and physical properties of the pastes. Autogenous healing was recorded for neat cement pastes, mainly in samples cured in the water. Nevertheless, the effect of this healing was negligible and did not affect any of the tested properties.

The NC presence in the pastes resulted in early healing and open spaces such as pores and cracks were filled with a newly formed porous material of Ca-Si-Al composition. The healing material had a "sponge-like" form and this fine porosity. Sufficient healing was recorded within 14 days. The role of NC in cycling conditions also had a positive effect but needs more time to be effective. Low porosity and high strength were recorded at 90 days, as drying periods assist the CaCO3 precipitation.

The combination of nanoparticles was misleading, as the total amount of these fine materials was high (3.0%wt total), leading to increased water demand (w/c = 0.30). The strength was reduced due to w/c, but NS assisted on the porosity refinement, and the open spaces into the microstructure were smaller in relation to the reference samples. The curing regime had a small impact on the healing procedure, as in both conditions, the healing was completed within 28 days.

When adding nanoparticles in cement pastes, healing is accelerated, and the presence of water is catalytic. The precipitation of calcite and the presence of aragonite and C-S-H compounds are filling the open spaces and assisting toward low porosity materials and low permeability.

**Author Contributions:** Conceptualization, methodology; validation, editing M.S.; analysis, investigation; data curation; writing, E.-C.T. and A.K. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research is co-funding by Greece and the European Union (European Social Fund—ESF) through the Operational Programme «Human Resources Development, Education and Lifelong Learning 2014–2020» in the context of the project "Use of nanoparticles and inorganic admixtures in producing self-healing cement products" MIS 5047560.

**Acknowledgments:** The authors would like to grant special thanks to the Greece and the European Union (European Social Fund—ESF) that have funded this research through the Operational Programme «Human Resources Development, Education and Lifelong Learning 2014–2020» in the context of the project "Use of nanoparticles and inorganic admixtures in producing self-healing cement products".

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **Acronyms:**


NSLc cement pastes with NC 1.5%wt and NS 1.5%wt, cured in wetting-drying cycles

## **References**


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