The analysis of hydrated portland cement can be facilitated by the use of thermal analysis techniques. This is especially true with regard to the estimation of the portlandite phase (calcium hydroxide [Ca(OH)2]) which is one of the major phases in set portland cements. There are various references in the literature to the use of thermogravimetric analysis (TG) for the estimation of free Ca(OH)2. Papers by Midgley and Ramachandran have shown it to be reliable. It has been used by Dollimore et al. to estimate the Ca(OH)2 in portland cement (OPC), OPC/pulverized fly ash (PFA), and OPC/PFA/polymer pastes. It is also shown here that the method can be adapted to indicate the relative crystallinity of the Ca(OH)2 present. Taking the investigation into a higher temperature region on the thermal analysis equipment also means that an estimate of the calcium carbonate can also be made. This is important in aged mortars. The method can be extended to cover the quantitative analysis of calcium sulfate if present.