**Maria Stefanidou \*, Eirini-Chrysanthi Tsardaka and Aspasia Karozou**

Laboratory of Building Materials, Aristotle University of Thessaloniki, 54635 Thessaloniki, Greece; extsardaka@gmail.com (E.-C.T.); akarozou@gmail.com (A.K.)

**\*** Correspondence: stefan@civil.auth.gr; Tel.: +30-2310996535

Received: 30 October 2020; Accepted: 20 November 2020; Published: 23 November 2020

**Abstract:** The present study proposes nano-calcium oxide (NC) and nano-silica (NS) particles as healing agents in cement pastes, taking into account the curing conditions. Two series of specimens were treated in water and under wetting-drying cycles. The addition of NC (1.5%wt of binder) triggered early healing since cracks were healed within 14 days in underwater immersion and before 28 days at wetting-drying cycles. Attenuated Total Reflectance (ATR) spectroscopy and SEM analysis revealed that the healing products were mainly aragonite and calcite in water conditions and more amorphous carbonates under wetting-drying cycles. The combination of NS and NC (3.0%wt in total) offered healing under both curing conditions before 28 days. The presence of NS assisted toward porosity refinement and NC increased the carbonates' content. The newly formed material was dense, and its elemental analysis by SEM revealed the C-S-H compounds that were also verified by ATR.

**Keywords:** nano-calcium oxide; nano-silica; self-healing; cement pastes; curing regime
