Small pieces of the hydrated cement pastes were also used in the LTC experiments. Sample mass was typically between 30 mg and 90 mg. For each LTC experiment, one small piece of the relevant cement paste was surface dried and placed in a small open stainless steel pan. The pan with the sample, along with an empty reference pan of similar mass to the empty sample pan, was placed in the calorimeter cell. Using a protocol developed previously,6 a freezing scan was conducted between 5 ºC and −55 ºC at a scan rate of −0.5 ºC/min. For temperatures between −100 ºC and 500 ºC, the equipment manufacturer has specified a constant calorimetric sensitivity of ± 2.5 % and a root-mean-square baseline noise of 1.5 µW. For comparison, typical measured signals for a freezing scan are on the order of 0.5 mW to 1 mW. The peaks observed in a plot of heat flow (normalized to the mass of the sample) versus temperature correspond to water freezing in pores with various size entryways (pore necks).6 The smaller the pore entryway, the more the freezing peak is depressed. Thus, the presence of, absence of, or change in peaks can be used to infer information concerning the characteristic sizes of the "percolated" (connected) water-filled pores in the microstructure of the hydrating cement pastes. Villadsen has demonstrated that when specimens are first dried equivalently (and then resaturated for the LTC measurements), LTC and mercury intrusion porosimetry (MIP) produce s imilar pore size distributions.12, 13 However, one advantage of LTC over MIP, and most other techniques for assessing pore size and connectivity, is that specimens can be evaluated without any applied external drying that might damage and modify the pore structure. Of course, the LTC technique can only assess the size and connectivity of water-filled pores. For sealed curing conditions, it is assumed that "empty" pores formed due to self-desiccation will not contain any freezable water and thus will not show up on the LTC scans. For this reason, many of the specimens originally cured under sealed conditions were first resaturated for 1 d or more and then analyzed using the LTC technique. It is recognized that the sealed curing and its accompanying internal drying (self-desiccation) can modify the microstructure in a manner analogous to drying.7, 14