The hydration of two high replacement composite cements (3:1 blast furnace slag:ordinary Portland cement (BFS:OPC), and 3:1 pulverized fuel ash:OPC (PFA:OPC)) with the addition of both SnCl2 and SnCl4 has been investigated and the results from X-ray diffraction (XRD) and scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS) are presented. Adding 5 % or 1 % SnCl(2)(.)2H(2)O or SnCl(4)(.)5H(2)O to the mix water resulted in the formation of Friedel's salt, Ca(3)Al(2)O(6)(.)CaCl(2)(.)10H(2)O, and calcium hydroxo-stannate CaSH(OH)(6), which also involved the consumption of calcium hydroxide. After 90 days hydration at lower levels of addition (i.e., 1 %) there was no longer evidence for CaSn(OH)(6), indicating that it too had been consumed in the pozzolanic reaction due to the lack of calcium hydroxide present. Results from SEM and EDS showed that bright regions between the BFS or PFA grains were tin containing and they were incorporated into the hydrated cement matrix. The tin was, therefore, localized rather than spread throughout and intimately incorporated into the microstructure.