Effect of lignosulfonate plasticizers on the hydration of C3A

The effect of softwood based calcium- and sodium lignosulfonates on the early hydration of pastes of cubic and orthorhombic C3A with gypsum was investigated using isothermal calorimetry, XRD analysis and thermogravimetry. For XRD paste hydration was stopped with liquid nitrogen and isopropanol after 5, 15, 30 min and 1, 2, 5 h. Additional experiments were performed on Portland cement (PC) pastes for comparison. Calcium lignosulfonate increased the initial heat of hydration in C3A-gypsum pastes while the second hydration peak was retarded and reduced. With sodium lignosulfonate the first heat peak appeared after about 0.7 h. In the PC system no difference in retardation effect was detected between the two lignosulfonates. XRD analysis and thermogravimetry could not detect the difference in performance between calcium and sodium lignosulfonate as observed with calorimetry. It was found that the complete gypsum consumption in pastes was retarded with both lignosulfonates, whereas the early ettringite formation in pastes of C3A and PC seems to be accelerated. Both lignosulfonates appear to stabilize AFm phases, which are not observed in pastes without LS. There are indications of a possible change in the morphology of ettringite.

Author: T.A. Danner, H. Justnes, M.R. Geiker, R.A. Lauten

Section: Science and production

Keywords: C3A, Portland cement, lignosulfonate, retardation