Effect of gypsum on the strength development of Portland cement by Moessbauer spectrometry

Portland cement was prepared by adding 1, 3, 5 and 6 wt% gypsum to Egyptian clinker. Each sample was mixed with 25 wt% water and was hydrated at 1, 3, 7, 28 and 90 days. The Moessbauer spectra showed two doublets, one represents the tetrahedral ferric ions and the other the octahedral ferric ions in addition to the iron metal. The degree of hydration was calculated from the spectra. The compressive strength test was performed. The time at which complete hydration takes place and the corresponding value of compressive strength were expected by calculations. The 5 wt% sample showed the highest value of strength at the same degree of hydration. It showed also the lowest degree of hydration at the same value of compressive strength. It could be concluded that the optimum amount of gypsum addition should be 5 wt%, and the rate of hydration didn't affect by the different amount of gypsum. (orig.)