Figure 5 shows the degree of hydration of the portland
cement in blended cement pastes, compared with the degree of hydration of plain
cement pastes as determined by point counting only. The incorporation of mineral
admixtures can significantly accelerate the cement hydration in blended systems as
early as 3 d, especially for the slag-blended cement. This effect is more
significant between 7 d and 28 d hydration, and is less significant at later ages.
There are two possible reasons for this acceleration: (1) the relatively higher
w/c ratio in the blended system, and (2) the pozzolanic reaction between the MA
and the calcium hydroxide released from the cement hydration in the fly
ash-blended cement. The standard deviation for the degree of cement hydration in
the blended cement pastes as determined by point counting ranged from ±
1.4 % to ± 2.2 %.
Figure 5. Degree of hydration of cement in plain portland and blended cement systems: (a) T-1H portland cement, T-L blended cement (fly ash added), and (b) T-1D portland cement, T-S blended cement (slag added). Error bars represent point counting standard deviations.
One should note that often the addition of fly ash retards portland cement hydration. This particular fly ash had been used in the manufacture of a fly ash-blended cement by Holcim*, so that a fly ash that retarded cement hydration would not have been so used. Hence, it is not surprising that no apparent retardation with the fly ash present was seen experimentally.