Phase transformation on heating of an aged cement paste

The standard cement paste (C-43-St) was studied previously by static heating, SH, immediately after 1 month hydration at w/c = 0.4 J. Therm. Anal. Calorim. 69 (2002) 187]. This paste after 5-year ageing (unprotected from contact with air) was subject to thermal analysis in air and in argon (DTA, DTG and TG), to XRD at various temperatures, T, in a high temperature chamber, to mass spectroscopy (MS) and to IR spectroscopy. The aim of this study was to compare the results of SH (fresh paste) and of TG (the aged one), to verify the assumptions made on SH interpretation and to check the change in hydration products with ageing as measured by phase transformation on heating (ΔM versus the final mass). The sorbed water (EV), escaping at 110 °C from the fresh paste, was bound on ageing with a higher energy and escaped at higher temperatures. The joint water content of hydrates and of C-S-H gel increased on ageing by 1–2% in the dense paste C-43-St and did not change in the less compact one C-43-I. C-S-H gel transformed on heating above 600 °C into C₂S and C₃S. Portlandite content did not change on ageing. In the air atmosphere it became partly carbonated, which was accompanied by an increase in mass between 500 and 600 °C. Carbon dioxide and/or carbonate ions to form carbonates, were sorbed during ageing and were present in the aged paste in some form undetectable by XRD (amorphous or crypto-crystalline). Sensitivity to carbonation ΔM(700–800 °C) increased highly with ageing.