To further our understanding of the nature of alkali-aggregate reaction products, hydrous alkali calcium silicates were prepared and studied. N,K-C-S-H products were synthesised by the reaction of alkali silicate solutions with lime or hme dispersed in sodium or potassium hydroxide solution at room temperature. The products were examined by chemical analysis, X-ray powder diffraction, infra-red spectroscopy, SEM and energy dispersive X-ray analysis. The state of the suspensions and composition of the solutions were also examined. In the N-C-S-H system, the nature of the solid-liquid system formed varied with the silica to soda ratio of the reaction mixture. Conditions which were highly favourable for flocculation in the N-C-S-H system (S/N = 1.4, H/N = 100) gave rise to colloidal dispersions in the corresponding K-C-S-H system. (N+K)/S ratios of lime-poor, mixed-alkah gels were close to 0.2, as observed previously for sodium-containing gels by Kalousek (1944). However, alkalis in lime-rich, mixed-alkali gels were more readily replaced by calcium than in their sodium analogues. Products possessing C/S ratios less than 0.6 were amorphous when formed in reaction mixtures in which the concentration of soda was 0.35-0.5 M, while products with higher ratios gave a C-S-H(I) X-ray diffraction pattern with or without a basal peak. When the soda concentration in the mixtures was 2 M, all products gave a C-S-H(I) pattern. The implications of the results in relation to the formation of products observed in alkali-aggregate reaction studies are discussed.