## 4 Conclusions  
The results of these experiments lead us to conclude there is a key opportunity to perform the technical-economic optimization of the current manufacturing process of pre-casting elements by means of optimizing the material dosage. Thus, we can accurately determine the water balance when the mixture dosage allows the concrete mix to be optimised, reducing the amount of cement needed for similar strength objectives for the concrete. Hence, for these experiments, a reduction of up to 120 kg/m\({}^{3}\) in the cement volume (from 420 to 300 kg/m\({}^{3}\)) can be applied to the current concrete mixtures for similar strength requirements. In consequence, the analysed reductions in the cement volume could result in savings of around EUR 8-10/m\({}^{3}\). Indeed, even an extreme optimization, reducing the cement volume and adjusting W\({}_{\rm ef}\)/C ratio to the standards limits, yields satisfactory strength results.  
Regarding the optimization of additives, the results of these experiments show that the use of low rates of accelerator additive (0.36%) produces a significant improvement in the compressive strength (+12.5 MPa) at 7 and 28 days, without this entailing a substantial increase in the unitary cost of the mixture (EUR +1.5/m\({}^{3}\)). However, the results show that increments of up to double the amount had no greater impact on the strength properties of the concrete, suggesting there exists an optimum low rate for this additive. Regarding the hardening of the concrete, the results show that for very early ages (\(\leq\)24 h), the analysed nanocomponent additive did not have the desire effect, since both mixtures showed the same evolution over the first 3 days. Only a 7% improvement in the hardening rates at 7 days was detected, although compressive strength at 28 days finally shows similar values.  
Finally, the proposals for the optimization of the concrete mixtures may generate savings of between EUR 3 and 10/m\({}^{3}\) on the current mixture costs. Applying this proposal to an industrial type building of 890 m\({}^{2}\), savings of around EUR 2500-3800/m\({}^{3}\) on concrete production can be obtained for similar strength capabilities. Additionally, the replacement of natural aggregates with recycled aggregates obtained from the concrete waste of non-conforming elements are perfectly suitable for manufacturing recycled concrete with C30 strength properties using replacement rates of 50% and 100%.  
The proposal of using recycled concrete may yield savings of around EUR 0.8-3.5/m\({}^{3}\). However, due to the management costs of the waste concrete elements, which is partially avoided when recycled aggregates are reused, the final economic margin could reach savings of about EUR 3950, depending on the type of industrial building. Thus, the use of recycled concrete emerges as a very interesting proposal for companies. In any event, further research is required to corroborate the functional aptitude of manufactured recycled concrete for certain manufacturing procedures and pre-stressed applications.  
**Author Contributions:** Conceptualization, J.M. and J.C.-G.; methodology, J.M.; software, J.M. and J.C.-G.; validation, J.M. and J.C.-G.; formal analysis, J.M. and J.C.-G.; investigation, J.M. and J.C.-G.; resources, J.M. and J.C.-G.; data curation, J.M. and J.C.-G.; writing--original draft preparation, J.M. and J.C.-G.; writing--review and editing, J.M. and J.C.-G.; visualization, J.M. and J.C.-G.; supervision, J.M.; project administration, J.M.; funding acquisition, J.M. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The authors declare no conflict of interest.