## 2 Materials and Methods  
### Lightweight Concrete (LWC) Mixtures  
The complete set of experimental processes to fabricate the different samples is schematized in Figure 1a-d. First, zinc oxide nanoparticles were mixed with a dry mixture. Consequently, water was added, and the saturated course aggregate was incorporated to fabricate fresh LWC (Figure 1a). After that, the molds were filled with fresh LWC, and the specimens were unmolded after 24 h at room temperature (Figure 1b). Following this, the samples were immersed in water at 20 \({}^{\circ}\)C for 35 days during the curing process (Figure 1c). Finally, the specimens were tested for compression until they failed after 35 days (Figure 1d). The reference mixture, identified as \"LWC_control\", is a structural LWC made of concrete, water, fine aggregate, and lightweight aggregates combining two types of expanded clay aggregate. Cement type II A 42.5, silica sand with particle size from 0 to 2 mm and a  
Figure 1: Schematic picture of the complete experimental process: (**a**) fabrication of fresh LWC, (**b**) molds filled with fresh LWC and unmolded after 24 h at room temperature, (**c**) immersion of the specimens in water at 20 \({}^{\circ}\)C for 35 days to cure them, and (**d**) testing of specimens by compressive strength tests. density of 1600 kg/m\({}^{3}\) was used in the mixtures. The water-to-cement ratio used in the mixtures was 0.4. The coarse aggregate was lightweight expanded clay aggregate (LECA). Two sizes of lightweight aggregate were combined, including \"LECA-S\", with a particle size ranging from 1 to 5 mm, and \"LECA-Dur\", with a particle size varying from 4 to 12.5 mm. Several percentages of ZnO nanoparticles were incorporated to assess the influence of these nanoparticles in the innovative SLWC. The percentages of ZnO nanoparticles added were 0, 0.5, 1, 1.5, and 2.0 wt% through two methods of incorporation including (i) nanoparticle addition and (ii) nanoparticles as a cement substitute. The mixes with added nanoparticles are identified with \"A-%\", indicating the percentage of ZnO added. The mixes with ZnO as a cement substitute are identified as \"S-%\". Nine samples were manufactured in this work including the reference LWC and eight mixes of LWC with nanoparticles. To ensure the reliability of the results, six repetitions of each type of LWC were performed. The total procedure included six specimens of structural LWC with no nanoparticles; twenty-four samples of LWC with ZnO nanoparticles as an additive in four percentages (0.5%, 1%, 1.5%, and 2%); and twenty-four samples of LWC with ZnO nanoparticle as a substitute for cement in four percentages (0.5%, 1%, 1.5%, and 2%). So, to assess the effect of ZnO nanoparticles on LWC, fifty-four specimens were manufactured. The specimens were cube-shaped and 50 mm \(\times\) 50 mm \(\times\) 50 mm in size following ASTM C-109 [25]. The LWC mixtures were designed based on previous works by the authors of [26]. Table 1 shows the proportions of each type of LWC. The chemical and physical properties of the used ZnO nanoparticles are shown in Tables 2 and 3. The manufacturing process of the LWC specimens was designed by the GICONSIME-Uniovi Research Group and mixed in their laboratories using an automatic planetary mixer. The equipment is programmable for the automatic mixing of mortars and similar materials, as shown in Figure 2.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Mix**} & \multirow{2}{*}{**Cement (g)**} & \multirow{2}{*}{**Sand (g)**} & \multicolumn{2}{c}{**Coarse**} & \multirow{2}{*}{**Coarse**} & \multirow{2}{*}{**Water (g)**} & \multirow{2}{*}{**ZnO (g)**} \\  & & & & & & \\  & & & & & & \\  & & & & & & \\ \hline LWC\_control & 338.43 & 425.49 & 61.57 & 63.78 & 135.37 & 0 \\ Zn-S-0.5 & 336.74 & 42  
The mixing process was developed to obtain structural LWC with a compressive strength higher than 20 MPa. The authors developed a mixing method consisting of the following steps:  
* The coarse aggregate was saturated with water for 24 h before being mixed.