Pore Solution Concentrations

Some of the first year efforts in the VCCTL consortium focused on the incorporation of a module into the VCCTL software for estimating the composition of the pore solution during hydration. The new module provides estimates of the solution pH and the concentrations of the following ions: sulfate, calcium, potassium, and sodium. Based on these predicted concentrations, the conductivity of the pore solution can also be easily estimated [18]. To accurately predict the changes in pore solution composition with time, it is necessary, of course, to know both the total alkali oxide and readily soluble alkali oxide compositions of the cement (see Table 4).

 

Table 4: Alkali Contents (Mass Percentages) for Cements 135 and 141 via ASTM C114 [1]

Cement	Total Na2O	Total K2O	Readily soluble Na2O	Readily soluble K2O
135	0.199	0.834	0.120	0.818
141	0.405	1.04	0.250	0.860

Figures 10 and 11 show the predicted and experimental pore solution compositions in terms of the concentrations of sodium and potassium. For potassium, two predictions are provided, considering [19] and neglecting the precipitation of syngenite (potassium calcium sulfate hydrate). Once again, good agreement between experimental observations and model predictions is seen for ages out to 1 d. The deviation between model and experiment for cement 135 at later ages (> 100 h) is likely due to some drying of the sealed specimens, reducing the saturation of the specimen and hence significantly increasing the ionic concentrations.

  

Figure 10: Experimentally measured (data points) and model-predicted (lines) pore solution concentrations for CCRL cement 135 (w/c=0.4, hydrated under sealed conditions at 25 ºC). For potassium predictions, solid lines indicate prediction with no consideration of syngenite precipitation, while dotted line indicates prediction with syngenite precipitation taken into account.

  

Figure 11: Experimentally measured (data points) and model-predicted (lines) pore solution concentrations for CCRL cement 141 (w/c=0.4, hydrated at 20 ºC). For potassium predictions, solid lines indicate prediction with no consideration of syngenite precipitation, while dotted line indicates prediction with syngenite precipitation taken into account.