## 6 Conclusions  
An extensive laboratory testing program was carried out to investigate the effectiveness of using Portland cement and compaction energies to evaluate the engineering behaviour of filtered iron ore tailings. The observations and conclusions can be summarized as follows:  
The employment of the porosity/cement index (\(\eta\)/\(C_{\text{ic}}\)) with the purpose of expressing the performance of iron ore tailings combined with the incorporation of Portland cement and densification through compaction, with curing periods varying from 2 to 90 days, can be considered successful. High coefficients of determination were obtained when \(q_{u}\) and \(G_{0}\) results were correlated with this parameter. Based on the dosage equations established in present research for the studied iron ore tailings-Portland cement blends, there are several technical ways of reaching a \(q_{u}\) or a \(G_{0}\) target value for a given project and the best solution  
Figure 9: Stress–axial strain–volumetric strain curves for the consolidated drained triaxial tests of specimens moulded with filtered iron ore tailings and 3% early strength Portland cement mixed with iron ore tailings at a dry unit weight (\(\gamma_{d}\)) of 17 kN/m\({}^{3}\) under confining pressures of 200 kN/m\({}^{3}\). might change from situation to situation depending on the time period available for curing, accessibility to equipment to reach a given porosity and cost of Portland cement. The stress-strain response showed a strength peak for all the samples and a softening following a peak. Also, the increase in effective stress causes an expansive response in volumetric strain. The peak failure envelope leads to a peak angle of shearing resistance (\(\varphi^{\prime}_{peak}\)) of about 34.1\({}^{\circ}\) for both dry unit weights and a peak cohesion intercept of (\(c^{\prime}_{peak}\)) of 80.9-kPa for (\(\gamma_{d}\)) of 17-kN/m\({}^{3}\) and 157.2-kPa for (\(\gamma_{d}\)) of 19-kN/m\({}^{3}\). The increase of the degree of compaction at standard Proctor energy from 89% to 99% did not cause any change in \(\varphi^{\prime}_{peak}\) but almost double \(c^{\prime}_{peak}\). On the other side, the critical state line reaches an angle of shearing resistance at critical state (\(\varphi^{\prime}_{cS}\)) of 36.3\({}^{\circ}\). The use of 3% of Portland cement for triaxial testing represents an intermediate amount of cement studied in this research. The present work has been envisaged as a contribution to the behaviour of compacted iron ore tailings-Portland cement blends to be disposed by stacking. The influence of degree of compaction as well as the amount of Portland cement on strength and stiffness properties was evaluated. The blends studied herein were compacted at optimum moisture content. It might not be possible to do so in the field, especially during rainy seasons. Therefore, the influence of compaction moisture content in the mechanical behaviour requires further study. Another point requiring future research is the development of alternative sustainable binders for stabilization of stacking filtered tailings in order to have a less costly, greener engineering solution. It is also necessary to emphasize that the present study was constrained to the range of low to medium confining pressures, making it attractive to tailings disposal by stacking up to heights of 10-12 m. Other studies are necessary to evaluate changes in the behaviour of the material under higher stackings, when the confining pressure will be greater than the studied range. At last, the addition of a binder to the compacted filtered tailings reduces the volume of hydraulically carried out sediments, thus allowing smaller sedimentation structures downstream of the disposal structure (e.g., ponds and sedimentation dikes). Conceptualization, N.C.C. ; methodology, N.C.C. and A.L. ; validation, N.C.C., H.M.C. and E.B.M. ; formal analysis, N.C.C. and H.C.S.F. ; investigation, A.L., H.M.C. and E.B.M. ; resources, N.C.C. ; data curation, N.C.C., H.C.S.F. and E.B.M.