Cement pastes with a w/c of 0.40 were prepared by mixing Cement and Concrete Reference Laboratory (CCRL) proficiency cement sample 140 [6] with water at 20 ºC, using a high speed blender. The mixing water was either distilled water or a solution of alkalis (sulfates or hydroxides), prepared by adding the appropriate sodium and potassium compounds to distilled water and stirring with a glass rod until complete dissolution. Cement 140 contains 0.093 % Na2O and 0.186 % K2O per unit mass of cement [6]. To prepare the cement paste with additional alkali sulfates, 0.76 % Na2SO4 and 0.93 % K2SO4 per unit mass of cement were added to the mixing water. For the alkali hydroxides, to maintain the same molar addition rates of the sodium and potassium, 0.43 % NaOH and 0.60 % KOH per unit mass of cement were added, being sure to account for the quoted 89 % purity of the commercially available KOH pellets. After mixing, cast wafers (≈5 g) of the paste were placed in sealed plastic vials. A small quantity of distilled water was added to the top of the wafers to maintain saturated curing conditions. The capped vials were placed in a walk-in environmental chamber maintained at 20 ºC. At various ages, specimens of the pastes were removed from the vials for further analysis.
Degrees of hydration of the cement pastes were assessed using LOI analysis to measure the non-evaporable water content as that quantity removed from the specimens between 105 ºC and 1000 ºC, corrected for the LOI of the initial cement powder [7]. Previously, the expanded uncertainty in the calculated WN had been estimated to be 0.001 g/g cement, assuming a coverage factor of 2 [7]. WN values were converted to estimated degrees of hydration based on the phase composition of the cement and published coefficients for the non-evaporable water contents of the various hydrated cement clinker phases [8]. Based on a propagation of error analysis, the estimated uncertainty in the calculated degree of hydration was 0.004. For a smaller subset of the specimens, thermogravimetric analysis (TGA) was conducted between 100 ºC and 1000 ºC at a scan rate of 10 ºC/min, with a sample size of about 30 mg.
Small pieces of the hydrated cement pastes were also utilized in 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 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 [9], 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. 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). 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 critical information concerning the characteristic sizes of the "percolated" (connected) water-filled pores in the microstructure of the hydrating cement pastes. One advantage of LTC over mercury intrusion porosimetry and other techniques for assessing pore size and connectivity is that the specimens may be evaluated without any drying that might damage the pore structure. However, LTC studies with variable alkali content are complicated somewhat by the change in freezing point depression due to the variable ionic concentration of the (freezing) pore solution, and this effect must be considered as well. The initial dosages of alkali sulfates or hydroxides in the cement pastes with additional alkalis would be expected to depress the freezing point of bulk water between 1 ºC and 2 ºC [10].