## 3 Analysis and Discussion of Results
### Thermal Conductivity  
The thermal conductivity is a specific characteristic of each material, and strongly depends on the temperature of the material. Eurocode 2 proposes two equations that estimate the thermal conductivity (\(\lambda\)), as a function of temperature (\(\theta\)), for standard density concrete. They define an upper and lower limit of \(\lambda\) (Equations (1) and (2), respectively). \[\lambda=2-0.2451\times(\theta/100)+0.0107\times(\theta/100)^{2}\;(\mathrm{W/m.K }),\mathrm{for20\leq\theta<1200} \tag{1}\]  
\[\lambda=1.36-0.136\times(\theta/100)+0.0057\times(\theta/100)^{2}\;(\mathrm{W/ m.K}),\mathrm{for20\leq\theta<1200} \tag{2}\]  
Hence, according to Eurocode 2, the \(\lambda\) of a standard density concrete, for \(\theta\) = 20 \({}^{\circ}\)C, varies between \(\lambda\) = 1.33 W/m.K and \(\lambda\) =1.95 W/m.K. According to [PERSON] et al. [50], the \(\lambda\) of concrete mixes varies between \(\lambda\) = 1.4 W/m.K and \(\lambda\) = 3.6 W/m.K. In this investigation, a value of \(\lambda\) = 2.08 W/m.K was obtained for NAC. This value is similar to the upper limit computed by means of the Eurocode 2 equation. Table 11 and Figure 8 show the results yielded by the various mixes with respect to this property. As was mentioned, each result presented in Table 11 was obtained by averaging at least four results. There, it is found that the replacement of NA with RA reduces the \(\lambda\)-values of the concrete mixes, therefore improving their thermal and energetic performance for use in building applications where insulation is important. This improvement is quite significant in concrete mixes in which 100% of the coarse and fine NA are replaced, reaching decreases in \(\lambda\) of 42% and 23%, respectively. The extent of this decrease varies significantly depending on the constitution of the RA used. The different types of RA have quite varied compositions, with different contents of concrete, ceramics, glass, wood, among other materials. Considering that these wastes have different thermal conductivity, concrete mixes with different thermal behaviour were expected. Table 11 also shows that the concrete mixes with FRA from Vimajas displayed the best results, i.e., the lowest values of \(\lambda\), obtaining a decrease of 42%, relatively to the NAC. [PERSON] et al. [12] evaluated the thermal performance of concrete mixes made with CRA consisting mainly of natural stone and mortar in equal proportions. The authors obtained decreases in \(\lambda\)-values of up to 9% with the integral replacement of the coarse NA with CRA. The difference between these values and the results obtained in our study are probably due to the significant variation of the composition of the RA used in the two investigations. Figure 7: Variation of the compressive strength at 28 days in concrete mixes with FRA or CRA from different studies. Reproduced with permission from [[http://www.tandfonline.com/doi/abs/10.1080/19648189.2014.974831](http://www.tandfonline.com/doi/abs/10.1080/19648189.2014.974831)]([http://www.tandfonline.com/doi/abs/10.1080/19648189.2014.974831](http://www.tandfonline.com/doi/abs/10.1080/19648189.2014.974831)) [49]. Abbreviations: f\({}_{\text{cm-RAC}}\), average compressive strength values of the concrete mixes with recycled aggregates; f\({}_{\text{cm-Control}}\), average compressive strength values of the concrete mixes with natural aggregates. Figure 9 shows the values of R\({}^{2}\) obtained in the linear regressions (black lines shown in the Figure) made to the variation of the thermal conductivity with the increase of CRA and FRA in the concrete mixes (R\({}^{2}\) = 0.97 in both cases). The high values of R\({}^{2}\) obtained show that the reduction of \(\lambda\) with the replacement of NA with RA is linear.