Hybrid cements made of engineered CaO-Al2O3-FeOx-SiO2 slags

Jorn Van De Sande,  Roberto E. Murillo Alarcón, Yiannis Pontikes, Hubert Rahier


Synthetic slags with a compositional window relevant to the non-ferrous metallurgical industry, i.e. FeO = 40-65 wt.%, SiO2 = 22-40 wt%, CaO = 4-20 wt%, Al2O3 = 5-12 wt%, were investigated for their reactivity in alkali-activated blended cement, also known as hybrid cements. Slags were made by melting analytical grade oxides and metals at 100 ◦C above the liquidus temperature, followed by water quenching. Hybrid cement pastes were subsequently synthesized by mixing 80 wt% of the above slag, after milling to a Blaine surface of 4000 cm2/g, with 20 wt% ordinary portland cement (OPC), activated with solid Na2SO4 and water. The water over binder mass ratio (W/B) was equal to 0.4 (B = Slag+OPC+Na2SO4) and the Na2SO4 concentration was 0.75 M (1.5 N). The kinetics of cement hydration and reactions of the slag were investigated using isothermal calorimetry operating at 10 and 20 ◦C. From these data, the pre-exponential factor A and the apparent activation energy Ea of the cement hydration and slag reaction for the different slags were calculated. The strength development was monitored by compressive tests on cubes made of those pastes, after 2, 7 and 28 days. The reaction products were identified by XRD. The measured strength was correlated with the heat flow, cumulative heat, A and Ea. By mapping the chemical composition and mineralogy, reactivity and compressive strength, an optimal slag chemistry is proposed for the Na2SO4-activated hybrid cements. This could lead to the upcycling of non-ferrous metallurgy slags and reducing the environmental impact of cementitious materials.