Effect of Temperature on the Composition and Structure of C-S-H Gel Obtained By Hydrating C3S

Abstract

This study explored the effect of temperature (25, 40 and 65 ºC) on the structure and composition of C-S-H gels obtained by hydrating C3S for 1, 14, 28 and 62 days at 100 % RH. The samples were studied with 29Si MAS NMR to determine the mean chain length (MCL) and degree of hydration (HyD) of C-S-H gels. Paste behaviour was also studied with DTA/TG and XRD. Further to the XRD results, all the pastes contained portlandite and anhydrous C3S (T1). No other crystalline compound was detected. The MCL obtained for the C-S-H gels from the 29Si NMR data ranged from 2.5 to 3.8, and was observed to lengthen with rising temperatures. While the degree of sample hydration (according to the NMR data) rose with time, it increased with temperature only up to the 28th day, for in 62-day samples it declined with rising temperature. The Ca/Si ratio of the gels, found by combining the TG results and the NMR degree of hydration data, increased with rising curing temperatures. The longer the hydration time and the higher the temperature, the lower was the H/Si ratio. In samples cured at 25 and 40 ºC, the Ca/Si ratio grew with the HyD to a value of 1.5 at 62 days. In the samples cured at 65 ºC, however, the Ca/Si ratio remained constant at 1.5 regardless of the degree of hydration.