Experimental

Cement pastes with a water-cement mass ratio (w/c of 0.40 were prepared by mixing Cement and Concrete Reference Laboratory (CCRL) proficiency cement sample 140¹² with water at 20 ºC, using a high speed blender.  The mixing water was either distilled water or a solution of alkalis, prepared by adding the appropriate compounds (see Table 1) to distilled water and stirring with a glass rod until complete dissolution.  Cement 140 is a low-alkali cement, containing only 0.093 % Na₂O and 0.186 % K₂O per unit mass of cement.¹² The additional alkalis prepared for each mixture are listed in Table 1; their masses were selected to provide the same number of moles of additional cations in each mixture.  After mixing, cast cylindrical wafers (≈5 g) of the paste were placed in sealed plastic vials.  A small quantity of lime-saturated 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.

Isothermal calorimetry using a differential scanning calorimeter was conducted over the course of 24 h for a separately prepared paste of each of the five mixtures in Table 1. For the isothermal calorimetry mixtures, mixture proportions were reduced to 10 g of cement and 4 g of water (with the appropriate additions of alkalis per gram of cement to match the levels in Table 1) and mixing was conducted in a small glass beaker using a metal spatula. 100 mg to 150 mg of each paste were placed in a sealed stainless steel pan that was then placed in the calorimeter cell. By requesting an isothermal scan for 24 h at 19 ºC, an average temperature of 20.3 ºC with a maximum standard deviation of 0.03 ºC was achieved over the course of a typical 24 h run.

Degrees of hydration of the cement pastes were further assessed using loss-on-ignition (LOI) analysis to measure the non-evaporable water content, w_n, as that quantity removed from the specimens between 105 ºC and 1000 ºC, corrected for the LOI of the initial cement powder.¹³ Previously, the expanded uncertainty in the calculated wn had been estimated to be 0.001 g/g cement.¹³ These w_n 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 hydrated cement clinker phases.¹⁴ Based on a propagation of error analysis, the estimated uncertainty in the calculated degree of hydration was 0.004.

Small pieces of the hydrated cement pastes were also utilised in low temperature calorimetry (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, ¹⁵ 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 DSC manufacturer has specified a constant calorimetric sensitivity of ± 2.5 %, with a root-mean-square baseline noise of 1.5 µW. The peaks observed in a plot of heat flow 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, a larger isolated water-filled pore will not freeze until the water in the smaller entryway pores surrounding it first freezes. 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 contents are complicated by the change in freezing point depression due to the variable ionic concentration of the (freezing) pore solution. For these experiments, the initial dosages of added alkalis in the cement paste mixtures would be expected to depress the freezing point of bulk water between about 1 ºC and 3 ºC.16 As the alkali concentration of the pore solution becomes more concentrated during continuing hydration, this freezing point could be further depressed.