The volume fraction of cement in the paste before hydration can be calculated as:
The volume fraction of unhydrated cement at hydration time t, V(t)cem, is calculated by dividing the number of points counted for this phase by the total number of points counted (3000 in the current study). However, one needs to apply a gypsum correction. All the gypsum contained in the cement is substantially reacted by about 24h after mixing [13]. Therefore, it will not be counted as a cement clinker phase during point counting. The gypsum does not contribute to the cement paste LOI measurement, since it is not present, but it does contribute to the initial volume of cement. Therefore, the initial volume fraction of portland cement needs to be reduced by the amount of gypsum originally present in the portland cement when calculating the degree of hydration of the cement paste at time t:
