## 1 Introduction
So, more deep research works are needed to develop a clear understanding of the effect of nanoparticles on OPC materials [5; 6]. Some studies provide results indicating the performance of cement-based materials using nano-Al\({}_{2}\)O\({}_{3}\). Diffusivity and permeability are lower than control mixtures with no additives. The compressive strength of mixtures with nano-alumina at 28 days is also discussed. Nano-Al\({}_{2}\)O\({}_{3}\) improves the contact structure in the interface between cement, creating a stronger bond and reducing cracks [7]. Other research works report that nano-Al\({}_{2}\)O\({}_{3}\) modifies the concrete microstructure, mainly the pore structure and interstitial transition zone, increasing the modulus of elasticity and improving compressive and bending strength [8]. The single and combined effects of nanoparticles on cement-based materials were also analyzed by other authors [9]. It was seen that the amount and type of nanoparticles had a significant influence on the fresh and hardened cement mortars studied. The combination of nanoparticles had negative effects on the physical and mechanical properties of the mortars. Titanium oxide (TiO\({}_{2}\)) nanoparticles are well known as a good additive to improve building sustainability and are widely used nano-additives in cement-based materials. Titanium oxide (TiO\({}_{2}\)) nanoparticles exist in three different polymorphs including rutile, anatase, and brookite. Rutile and anatase are commonly used in the construction industry among other industries. Their effect is mainly related to the acceleration of the hydration process, an improvement in resistance, self-cleaning properties, and CO\({}_{2}\) uptake [10; 11]. In this context, several novel nanomaterials have been explored as good additives to improve building sustainability, including nano-ZnO. Nano-ZnO is a by-product material from the zinc manufacturing industry. It is identified as a promising material because of its unique properties such as low cost, environmental benefits, and photocatalytic performance. Thus, some studies show that the incorporation of ZnO resulted in the contrary effect of a remarkable delay in the hydration process [12; 13]. Nano-ZnO has a wide range of applications such as in chemical sensors, the biomedical sector, solar cells, photocatalysis, and the construction field [14]. Recent research has assessed the effect of ZnO on High-Performance Concrete (HPC) [15], concrete block pavements [12], and alkali-activated slag (AAS) [16]. In the construction industry, nano-ZnO has been assessed as a suitable alternative to TiO\({}_{2}\), although a few studies have compared the effectiveness of ZnO and TiO\({}_{2}\)[5; 15; 17]. Both nanoparticles, individually and combined, are used as nano-additives to enhance photocatalytic properties [15]. Previous studies reported the effect of ZnO nanoparticles on the hydration of Portland cement [15; 17]. Even a very low percentage of ZnO incorporation retards the hydration of cement by forming other compounds such as Zn(OH)\({}_{2}\) or Zn\({}_{2}\)Ca(OH)\({}_{6}\cdot\)H\({}_{2}\)O. The formation of Zn(OH)\({}_{2}\) is responsible for the hydration delay and an increment in pH at the initial setting time of regular concrete [18] Zn(OH)\({}_{3}\)\({}^{-}\) and Zn(OH)\({}_{4}\)\({}^{2-}\) do not influence cement hydration, but they contribute to forming Zn silicate, which is transformed into calcium zincate. Tests on normal concrete using ordinary Portland cement (OPC) incorporating percentages of 1% and 3% of ZnO showed a reduction of 30% in compressive strength [19]. However, it was seen that ZnO increased the durability of concrete in marine environments with high Cl\({}^{-}\) concentrations [18]. Recent research studies have analyzed the influence of small additions of ZnO nanoparticles in the mechanical properties of cement-based materials [5; 18; 20]. However, to the knowledge of the authors, there are no studies about the effect of ZnO nanoparticles on lightweight concrete (LWC).