Improvement of hydraulic and mechanical properties of dicalcium silicate by alkaline activation

•Significant increase in the reactivity of C2S by alkalis, KOH being greater than NaOH.•Pectolite is identified in the case of C2S-NaOH and Rhodesite in the case of C2S-KOH.•Na is structurally placed between the SiO4 sheets of Pectolite while K is positioned inside the SiO4 tunnels of Rhodesite.•High mechanical strengths are observed in the case of C2S-KOH pastes at 90 days. The belitic clinker, consisting essentially of large amount of C2S phase, is becoming increasingly important for its ability to reduce the environmental impact by the moderate release of CO2 from the cement plant and for economic reasons linked to its low energy consumption. In this paper we are interested in the mechanism of alkali retention (NaOH or KOH) by Belite dicalcium silicate Ca2SiO4 (ß and α′ forms) during hydration and to the nature of the formed C-S-H. The synthesized powder, from silicon oxide and calcium carbonate, was hydrated with a water/solid ratio = 0.4 by weight. The isothermal calorimetry analysis is perfected by using a microcalorimeter. X-ray diffraction and scanning electron microscopy are used to characterize the mineralogy of hydrated samples during different curing times of 7, 28 and 90 days. The mechanical properties have also been correlated to the structural nature of C-S-H. The results showed that hydration of the Belite in the presence of alkaline activators causes an acceleration of hydration compared to the Belite sample hydrated with water, which also leads to the production of C-S-H phases and hydrates of calcium silicate hydrate containing sodium C-(N)-S-H and potassium C-(K)-S-H. A structural correlation has been involved to establish the retention mechanism of sodium and potassium through the calcium silicate hydrates.