## 1 Introduction  
Hydraulic calcium silicate cements have successfully replaced the conventionally available materials in dentistry since their introduction due to their properties such as biocompatibility, bioactivity, radiopacity, their ability to stimulate the repair of tissues and the remineralization of dentin [1; 2]. These materials require water to set and do not deteriorate in a wet environment, such as in the presence of blood, tissue fluid, saliva or water [3]. They are broadly used in various procedures such as direct and indirect pulp capping, the restoration of carious teeth, apexification, perforation, root repair, root end filling and endodontic surgeries [2; 4]. MTA, biodentine, bio aggregate, Theracal, calcium-enriched mixture cements and endo sequence root repair materials are some of the few calcium silicate cements, but the principal focus is on the material Biodentine [5].  
Biodentine is acknowledged as a prominent representative of calcium-silicate-based cements, as it is radiopaque, antimicrobial and biocompatible with good sealing ability and marginal integrity. It has the shortest setting time, does not discolor and possesses good physio-mechanical properties [6; 7]. The powder contains traclcium silicate as the main core material, calcium carbonate as the filler and zirconium oxide acting as a radio pacifier. The liquid consists of calcium chloride, water and hydrosoluble polymer [8]. It sets via a hydration reaction and releases calcium hydroxide once set, which initiates the secondary dentin formation when placed above the pulp, and hence this material is popularly known as the \"dentine substitute\" [9]. All the aforementioned properties make it a suitable material for numerous clinical conditions, such as for pulp capping, pulpotomy, furcation or perforation repair, root end or root canal filling and as a dentin tissue replacement material [10].  
Zirconia, a metal oxide with a natural white color and high toughness, is favorably used in dentistry due to its superior physio-mechanical and biological properties. It is biocompatible and bioinert with steady chemical features, radiopacity and good corrosion resistance [11]. Nano zirconia is extensively used to repair dental tissues. Nano-zirconia-reinforced groups of polymethyl methacrylate denture base significantly increase surface hardness, flexural strength and impact strength compared with un-reinforced groups [11; 12]. Similarly, adding nano zirconia to GIC increases its surface microhardness, compressive strength and flexural strength, resulting in overall mechanical properties [13; 14].  
Microhardness, a significant material property, especially when placed for posterior restoration, is the resistance of restoration to permanent deformation. Vickers microhardness testers are used to determine hardness by marking three indentations on the surface and calculating the value of the diagonals [12]. Water sorption is the phenomenon of absorption and adsorption, in which water is absorbed into the body of a material and adsorbed on the surface. Solubility is the amount of the substance, the solute, that dissolves to form a solution in a given amount of solvent [6].  
Several studies have been conducted on adding nano zirconia to different materials in dentistry [14; 15; 16; 17], but there are no data available regarding the enforcement of nano zirconia to biodentine. Hence, this study was conducted to determine the microhardness, water sorption and solubility of nano-zirconia-reinforced groups in different percentages with biodentine.