Low temperature calorimetry

When considering the LTC scans for the various cement pastes, three major peaks are observed corresponding to water in (percolated) capillary pores freezing at about −15 ºC, water in open gel pores freezing at about −25 ºC, and water in dense gel pores freezing between −40 ºC and −45 ºC, respectively, employing the naming convention of Snyder and Bentz.¹⁵ Previously, it has been indicated that LTC can be used to assess the percolation (connectivity) state of the capillary pore network in a hydrating cement paste.^{11, 15} The presence or absence of a peak in the LTC scan near −15 ºC indicates a percolated or depercolated capillary pore network, respectively. As Powers first indicated,¹⁷ as a cement paste hydrates, its capillary pores can become disconnected. The point at which this occurs will depend on the paste w/c, with pastes with a lower w/c requiring less hydration to achieve depercolation. For a paste with a w/c of 0.40, Powers estimated that 3 d of saturated curing would be required to disconnect the capillary pores.¹⁷ Here, the influence of the various alkali additions on this percolation will be examined via LTC.

Figure 2 provides the LTC scans obtained on saturated cement pastes of the five mixtures, cured for 2 d. At this early age, each paste still contains a highly percolated capillary pore network, as indicated by the large peaks near −15 ºC for each mixture. Each paste also contains a small peak in the −40 ºC to −45 ºC range, corresponding to the formation of an early age C-S-H gel containing or surrounded by dense gel pores.1⁵ In general, the capillary pore peaks are smaller and the dense gel pore peaks are larger for the four mixtures with the additional alkalis, indicating an enhanced production of C-S-H gel and a concurrent reduction in capillary porosity relative to the low-alkali cement control specimen, in agreement with their higher degrees of hydration at early ages as noted in Figure 1 and Table 2. LiOH, unlike the other three alkali additions that exhibit the expected shift of peak temperatures to the left by 1 ºC to 3 ºC, is observed to shift the peak temperatures slightly to the right. No explanation is available for this observation at this time.

Figure 3 provides the equivalent LTC scans after 8 d of saturated curing. At this age, only the control low-alkali cement paste exhibits a significant peak at −15 ºC, corresponding to a percolated capillary pore network. The four mixtures with additional alkalis each exhibit a depercolated capillary pore network at this age. Figure 3 compares results for the five pastes at an equal age, and while the control and the pastes with sodium and potassium additions do have essentially equal degrees of hydration (Table 2) at an age of 8 d, the two mixtures with lithium additions still have significantly higher degrees of hydration than the control after 8 d of curing. However, their degree of hydration after 7 d of curing is quite similar to those of the other three mixtures after 8 d of curing. Thus, to compare the mixtures on an equal degree of hydration basis, the 7 d LTC scans for the lithium-containing mixtures are provided in Figure 4. Figures 3 and 4 indicate that at roughly equivalent degrees of hydration (about 0.66), the control and the two pastes with lithium additions contain percolated capillary pore networks, while the pastes with potassium and sodium additions do not.

The influence of continuing hydration on the LTC scans is provided in Figures 5, 6, and 7, which provide LTC scans obtained after 14 d, 30 d, and 90 d of saturated curing, respectively. In addition, previously obtained LTC scans for pastes with no additional alkalis and with additional alkali sulphates after over 100 d of saturated curing are provided in Figure 8.¹⁸ After 14 d, as shown in Figure 5, each of the five mixtures exhibits a depercolated capillary pore structure and only a little evidence of a percolated open gel pore structure. In these specimens, the presence of only a peak for the dense gel pores would generally indicate a low permeability cement paste. Normally, it would be expected that beyond this point, further saturated curing would only gradually reduce this dense gel pore peak. ¹⁸ While a reduction in this peak with continuing hydration might seem counterintuitive, it must be remembered that the "water" freezing in this peak includes not only water in dense gel pores but also that in much larger pores (open gel and capillary size) that have been depercolated by a surrounding volume of dense gel pores. The volumes of these latter two "size pores" would be expected to decrease with continued hydration.