Hydration products in two aged cement pastes

Summary

The hydration products in two aged cement pastes (DTA/DTG/TG) were compared with those in fresh ones (static heating, SH) and were also studied by mass spectrometry (MS), IR and thermo XRD-analysis. The products considered here were: the sorbed water, the CSH gel including hydrates, portlandite, calcite, aragonite and vaterite. Except carbonates their content was higher in the stronger paste C-43, than in C-33, but lowered with ageing (only the CSH gel water remained approximately unchanged). The sorbed water content became with time lower and similar in both pastes (it evaporated up to 155-185°C in TG); the escape of the rest moved to higher temperatures (500-700°C). The three DTG peaks at 200-400°C indicated jennite-like phase in the CSH gel; the mass loss (155-460°C) was higher on ageing due to development of organic matter, especially in C-43 (DTA, TG, IR). Portlandite content changed little and carbonate content increased considerably. They decomposed in air at 470 and 720-740°C, in argon at 450 and 680-710°C and in vacuum at 400 and 630°C, respectively (DTG peak, XRD). Between 500 and 700°C the simultaneous evolution of H₂O and CO₂was observed by MS, which is attributed to dehydroxylation of jennite-like phase and/or to decomposition of some carbonate hydrate and/or hydrocarbonate (three peaks on CO₂evolution curve, MS). The d(001) peak of portlandite exceeded the nominal value and will be analyzed separately.