## 4 Conclusions  
This work assesses the mechanical performance of LWC with ZnO nanoparticles, as well as its morphological and chemical characteristics, using two methods of incorporation and different percentages. The conclusions of this work are the following:  
Figure 9: SEM images and EDX of ZnO nanoparticles (**a**), samples with ZnO incorporated by addition (**b**), and ZnO incorporated as a partial substitute for cement (**c**).  
The manufacturing process developed in this work provided a very homogeneous distribution of nanoparticles in LWC.  
The use of ZnO nanoparticles delays the curing process by several days. Seven extra days are needed in this process to reach structural mechanical properties.  
The incorporation of nanoparticles as a cement substitute significantly reduces the compressive strength of LWC. Percentages of ZnO nanoparticles above 1 wt% cannot be classified as structural LWC.  
ZnO incorporation by addition provides structural LWC with a compressive strength above 20 MPa when added in percentages up to 1.5 wt%.  
There is no morphological difference between the two methods of incorporation of ZnO nanoparticles. The morphological characteristics are very similar and independent of the method of incorporation and the ratio of nanoparticle/cement.  
Results of this work reveal that ZnO nanoparticles reduce the compressive strength of LWC. These nanoparticles delay the hydration, hardening, and curing process of LWC, leading to changes in the mechanical performance of LWC. Future research must focus on the incorporation of a water reducer or superplasticizer to reduce the delay of cement hydration. The effect of ZnO in lightweight concrete has a great influence on hardening, leading to a lower resistance material. The micro- and macro porosity of hardened samples of LWC are also interesting properties to determine in future work.  
The results of this study reveal that incorporating ZnO nanoparticles may improve the LWC durability of structures. The incorporation of ZnO delays the hydration, hardening, and curing processes. So, it is expected that the natural carbonation process will also be delayed. Creating more durable materials with good structural performance and lightweight may be very interesting for applications such as port infrastructures or aggressive environment structures, such as offshore structures.  
Conceptualization, M.A.M. and J.J.d.C.-D.; methodology, M.A.M.; software, L.T.S.K.; validation, L.T.S.K., A.S.G.G. and V.V.M.; formal analysis, M.A.M.; investigation, A.S.G.G.; resources, J.J.d.C.-D.; data curation, L.T.S.K.; writing--original draft preparation, A.S.G.G.G.; writing--review and editing, M.A.M.; visualization, V.V.M.; supervision, J.J.d.C.-D.; project administration, M.A.M.; funding acquisition, M.A.M. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Spanish Ministry of Science and Innovation, TED2021-131976B-I00 and PID2021-128056 OA-I00.  
Not applicable.  
The raw data supporting the conclusions of this article will be made available by the authors on request.  
The authors thank the company Asturiana de Zinc from Glencore for their support of this research by providing ZnO nanoparticles as one of their industrial by-products.  
The authors declare no conflicts of interest.