The kinetics of alkali activation of slag is highly dependent on the activator type and its chemical constitution. This paper explores the early-age reaction kinetics of slag activated using powder sodium silicate (which makes handling easier in practice) and compares it with that of waterglass (liquid sodium silicate) activated slag pastes. The influence of Na2O-to-slag ratio and the activator modulus (SiO2-to-Na2O ratio) on the isothermal calorimetric response is presented. The powder sodium silicate activated slag pastes show a calorimetric response similar to NaOH activated slag systems, with both demonstrating single early-age reaction peaks. In contrast, the waterglass activated systems show a calorimetric response similar to those of OPC pastes, with a marked induction period. Kinetic modeling is used to quantify the differences in reaction kinetics. Jander equation is used to model the time-dependent degree of reaction and to relate it to the rate controlling processes when different types of activators are used. (C) 2012 Elsevier B.V. All rights reserved.