## 3 Results and Discussion
### Cement Sludge Fines (CSF)  
The chemical composition of the CSF, OPC (CEM I42.5 N) and soil used are shown in Table 6, while their particle size distribution in comparison to that of CEM I42.5 is shown in Figure 12. CSF consists mostly of hardened cement paste and fine limestone aggregates; therefore, its CaO and SiO\({}_{2}\) contents were not expected to be reactive. The high value of loss on ignition can be attributed to the bonded water proportion and CO\({}_{2}\) quantity that was emitted from the sample, while the DTA-TG results (Figure 13) indicated a small amount portlandite (0.18 wt %) and the presence of a high amount of CaCO\({}_{3}\) (67.77 wt %).  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline
**(wt \%)** & **Na\({}_{2}\)O** & **K\({}_{2}\)O** & **CaO** & **MgO** & **Fe\({}_{2}\)O\({}_{3}\)** & **Al\({}_{2}\)O\({}_{3}\)** & **SiO\({}_{2}\)** & **L.1\%** & **Cl\({}^{-}\)** & **NO\({}_{3}\)\({}^{-}\)** & **SO\({}_{3}\)** \\ \hline CSF & 0.19 & 0.21 & 32.09 & 1.05 & 1.26 & 2.45 & 36.51 & 25.22 & 0.01 & \textless{}0.01 & 0.05 \\ CEM I42.5 N & 0.57 & 1.08 & 66.84 & 3.91 & 2.40 & 3.74 & 19.55 & 1.91 & 0.03 & 0.02 & 1.49 \\ Soil & 0.90 & 0.98 & 8.67 & 5.39 & 3.79 & 6.88 & 63.14 & 10.25 & 0.08 & 0.02 & 0.02 \\ \hline \end{tabular}
\end{table}
Table 6: Chemical composition of CSF and cement I42.5 N.  
Figure 11: Drying shrinkage of test concretes.  
The XRD diagram (Figure 14) shows that the mineralogical composition of CSF was mainly calcite. Also, an absence of C\({}_{3}\)S and traces of C\({}_{2}\)S peaks were observed. The latter may indicate full hydration of sludge. Also, a strength development test with lime according to ASTM C593-95 showed minimal strength development at 28 days (<0.50 MPa), which implies that there is no significant pozzolanic activity from CSF. The results of the analytical tests imply that CSF is probably a fine material without any hydraulic or pozzolanic properties. The fact that it can be easily crushed and sieved through the 75 \(\upmu\)m implies that CSF could be used as a filler material. Regarding handling and processing, it would be preferable to use CSF in its original wet condition in order to avoid the drying process; however, further research is required in this direction.  
Figure 12: Particle size distribution of CSF and cement.  
Figure 13: Differential thermal–thermogravimetric analysis (DTA-TGA) analysis of CSF and quantification of calcite content as a percentage by mass of sample.  
3.2.1. CSF as a Cement Replacement in Mortars  
The results of compressive strength testing are displayed in Figure 15. Mortar with a 10% replacement of cement by CSF reached almost the same strength levels as the reference. The early strength was slightly lower than the reference; however, the 28-day strength was slightly higher compared to reference. The mortar with 20% CSF showed a 7% reduction of strength compared to the reference. These results can be attributed to the filler effect [39], while the XRD results (Figure 16) on 28-day mortar samples with 20% CSF confirm the absence of ettringite. When a filler is added in a certain percentage, it influences the pH value of the binders in the pore solution. If the pH is reduced, this affects the hydration and as a result the compressive strength [40].  
Figure 14. XRD diagram of CSF, where C: calcite.  
Figure 15. Compressive strength development of mortars.  
The decrease of compressive strength when 20% CSF was added can be related to the calcite content of CSF and its behavior as filler. Similar results have been observed by [PERSON] et al. when limestone filler was used for cement substitution in mortars [41]. The porosity of mortar with 10% CSF was 6.6% and of mortar with 20% CSF was 10.3%, showing an increase compared to the reference mortar with no CSF inclusion, which had 3.5% porosity. This increase can be attributed to the porous nature of the hydrated cement mortar present in CSF, in a similar manner to the porous nature of fine recycled aggregates.