Alternating current impedance spectroscopy (ACIS) is used to monitor in situ the kinetics of geopolymer formation by alkali silicate activation of metakaolin at 40 degrees C. The evolution of conductivity and resistivity as a function of time are found to correlate very well with existing models of geopolymerisation kinetics. The dissolution of metakaolin gives a rapid increase in conductivity, with a subsequent decrease due to gelation and gel ageing. After a period of curing, the resistivity of the gel network is seen to be a sensitive probe of the development of a well-formed gel structure. This enables investigation of the changes in geopolymer formation kinetics and gel structure caused by addition of various salts of interest in nuclear waste management, in particular the nitrate, sulphate and hydroxide salts of caesium and strontium. Each of the six salts studied is shown to have specific effects on the kinetics of geopolymer formation and the pore structure of the final geopolymer gel. Caesium nitrate and caesium sulphate have very similar effects. with a slight retardation of gelation followed by significant disruption of gel structure while caesium hydroxide accelerates gel formation markedly. Strontium nitrate at high concentrations interferes significantly with the pore network development, while strontium sulphate is converted gradually to carbonate salts by atmospheric carbonation. (c) 2008 Elsevier Ltd. All rights reserved.