Figure 9 shows results obtained using the standard prisms specimens in ASTM C 1012 (25.4 mm × 25.4 mm × 279 mm). The ranking of the resistance to sulfate attack is the same as for the smaller cement paste samples, e.g., the specimens with Class C fly ash expand more than the controls during the time of the measurements.
Fig. 9. Average expansion of large specimens versus time. The standard deviations of the three specimens are shown as uncertainty bars. For FA-C, the uncertainty bars are smaller than the data symbol.
As is done in ASTM C 1012 [2, 3], the time to reach an expansion of 0.1 % was taken as the critical time to classify the performance of the cements. Table 3 shows the results. The small cement paste specimens ranked the cements in about 20 d to 30 d, while the large specimen required twice the time while ranking the control as worse or identical to the cement with fly ash. If the testing stopped at this point, the conclusions reached with the large prisms would not be a valid assessment of the resistance of these cements, as it is known that addition of FA-C is deleterious for sulfate resistance of cement paste. In Table 3 we also show the expansion reached at an arbitrary later date, 30 d for small specimens and 100 d for the larger specimens. We could not select the same date for both types of specimens because 1) the large specimens at 30 d were below 0.1 % expansion; 2) the small specimens were not longer measurable after 30 d and 3) at 120 d the FA-C large specimen were not longer measurable. Therefore, from Figs. 8B and 9 and from Table 2, it could be inferred that the FA-C cement is less sulfate resistant than the control. The results for small specimens mirror those for larger specimens and provide a result in at least a third to a fifth of the time for the cements shown here that were highly reactive. A similar reduction was seen in preliminary tests (not shown here) for cements that were not highly reactive, including hydraulic cements conforming to ASTM C 1157 [4].
| Table 3. Time in days to reach 0.1 % expansion and expansion at an arbitrary date | ||||
|---|---|---|---|---|
| Time in days to reach 0.1 % expansion | Expansion | |||
| Small sample method (d) | ASTM C 1012 (d) |
Small sample at 30 d (%) |
ASTM C 1012 at 100 d (%) | |
| Control | 15 | 49 | 0.2 | 0.3 |
| Class C Fly ash | 10 | 55 | 0.3 | 0.7 |
Obviously, these preliminary observations of the relative behavior of the two types of specimen should be verified with further testing of various cements.