The possibility of using dredging sludge in manufacturing cements: Optimization of heat treatment cycle and ratio replacement

• Valorization of dredged sludge based on illite/muscovite minerals.
• The effects of heat treatment and heating rate on the activation of the sludge and on its pozzolanic activity.
• The effect of the mineralogical composition on pozzolanic activity of the sludge.
• Hydration heat and setting time of activated sludge.
• Compressive strength and pozzolanic activity of activated sludge.

This study aims to produce an eco-cement by using heat treated dam’s sludge as an additive to clinker. Different heat treatment cycles were applied to the dam’s sludge: three temperatures selected in the range between the dehydroxylation and decarbonation temperatures (600, 700 and 800 °C) reached by adopting different heating rates (5, 10 and 20 °C/min). The analysis of DRX, DSC–TG, FTIR results show that the heat treatment cycles let to the change of the sludge structure allowing its use as a pozzolanic additive to produce cements. The optimal heat treatment cycle is 600 °C with a holding time of 5 h and a heating rate 20 °C/min. Normalized mortars have been designed using modified cements constituted by 5% of gypsum, different percentage of heat treated sludge (5–10% and 15%) and clinker. The compressive strengths at 28 days lead to the choice of blending 85% clinker, 10% heat treated sludge and 5% gypsum to produce modified cements. The influence of the thermal activation of the dam’s sludge on the technical properties such as normal consistency, fineness, setting time, heat of hydration, compressive strength have been investigated and tests were conducted according European standards. Results reveal that heat treatment at 600 °C with a rate of 20 °C/min conducts to an activated sludge with no emission of carbon dioxide and the higher strength resistance when added to clinker at 10%. However, whatever the fineness or the rate of heat treatment, the percentage of kaolinite (11%) existing in the natural sludge is not sufficient to develop by heat treatment at 600 °C hydration products allowing significant improve of mechanical resistance of modified cements by comparison to CEMI.