## 3 Results and Discussion  
### Thermal Properties  
Results of thermal conductivity tests carried out on the FTB mixtures in the two previously described curing conditions are given in Table 8. Listed values were calculated as the average of the results obtained in individual holes drilled in the specimens.  
The results displayed in Table 8 clearly indicate that cement content has a significant effect on thermal conductivity. In fact, by increasing cement content from 50 to 100 kg/m\({}^{3}\), a corresponding non-negligible increase of k was obtained in both curing conditions and for mixtures containing both types of sludge. Such a variation is due to the thermal conductivity of the products formed during the hydration process of cement and to the porosity reduction induced by making use of higher cement dosages [14; 18; 19; 56; 57].  
By analyzing the data in Table 8, effects related to the type of sludge can also be appreciated, especially at the lowest cement content, as a function of the different chemical composition and quartz content of its constituent. In particular, it can be observed that FTB mixtures prepared with sludge B, which has a higher quartz content that sludge A, showed enhanced thermal properties. Such an outcome is consistent with the improved thermal characteristics of quartz [11] and may also be attributed to the different thermal conductivities and concentration of heavy metals contained in this sludge.  
As a result of residual moisture loss, a decrease of k was recorded for all FTB mixtures when passing from the conditions associated to 18 days of curing to those reached by means of laboratory drying. By comparing the two sets of data, it is possible to assess the so-called thermal stability of the considered mixtures. This consists in their aptitude to maintain a sufficiently high value of k, even in the presence of long-term dry out phenomena. All mixtures exhibited a residual thermal conductivity (i.e., in lab-dried conditions) greater that 50% of the reference value corresponding to 18 days of curing. The percentages of residual thermal conductivity were similar, of the order of 66-67%, for the two FTB mixtures prepared with higher cement content (100 kg/m\({}^{3}\)) regardless of sludge type. On the contrary, when considering the FTB mixtures with the lower cement content (50 kg/m\({}^{3}\)), the residual thermal conductivity recorded for the material containing sludge B (equal to 72%) was higher than that of the mixture containing sludge A (equal to 52%). Such a result is consistent with the general finding that sludge B, as a result of its composition, leads to enhanced thermal characteristics.  
In order to perform an overall assessment of the effectiveness of employing sawing sludge in FTB mixtures, obtained results were compared to typical k values reported in the literature for backfilling materials. Although significant variations may occur as a function of particle mineralogy, water content, and degree of compaction, values considered in ampacity calculations for standard backfilling soils, in the absence of any dry-out phenomena, are comprised in the 0.5-1.0 W/(m\(\cdot\)K) range [2]. As indicated in Table 8, all considered FTB mixtures higher values, comprised between 0.97 and 1.70 W/(m\(\cdot\)K). Furthermore, thermal conductivity values recorded in lab-dried conditions were consistent with those reported by [PERSON] et al. [13; 14] for similar FTB mixtures containing various types of fine fractions. It can be thereby postulated that the remarkable thermal properties of the considered FTB mixtures derive from the distribution throughout their structure of sludge particles that possess a significant heat transfer capacity due to their silicate nature and to the presence of heavy metals.