The estimated degrees of hydration of the plain portland cement pastes made from the T-1H and T-1D cements as a function of age, determined by LOI and SEM point-counting, are listed in Table 2. The Δ column in the table stands for the difference between those two measurements (SEM point counting - LOI), divided by the LOI results and multiplied by 100 %. The results are consistent with each other, as the deviations for both cement pastes are within ± 10 %.
| Table 2. Degree of Hydration of Plain Cement Pastes by LOI and point-counting | ||||||
|---|---|---|---|---|---|---|
| Age | T-1H w/c=0.4 | T-1D w/c=0.4 | ||||
| α % SEM | α % LOI | Δ, % | α % SEM | α % LOI | Δ, % | |
| 3 d | 52.2 | 53.9 | −3.1 | 50.8 | 51.4 | −1.1 |
| 7 d | 54.8 | 60.0 | −8.7 | 53.8 | 59.8 | −9.9 |
| 14 d | 62.9 | 64.3 | −2.2 | 57.8 | 62.4 | −7.3 |
| 28 d | 64.2 | 66.8 | −4.0 | 59.9 | 66.1 | −9.4 |
| 60 d | 68.7 | 68.6 | 0.2 | 66.8 | 71.3 | −6.3 |
| 90 d | 67.2 | 71.6 | −6.1 | 68.8 | 68.8 | 0 |
One should note, in the Δ columns, that the value of Δ is almost always negative. This indicates that the SEM point counting results for the degree of hydration are systematically lower than the LOI measurements. This may be partly due to other materials releasing mass in the temperature range 105 ºC to 1000 ºC, like calcium carbonate, which releases carbon dioxide, which would tend to make the LOI results higher than the point-counting results. The calcium carbonate could come from partial carbonation of the CH present.
Figure 4 shows the regression analysis between the data obtained by LOI and SEM point-counting for the two plain cement pastes. The values of R2 for these regressions are 0.90 for the T-1H cement and 0.85 for the T-1D cement, with the regression lines forced to go through the origin. Note that both slopes are close to and a little larger than unity. The fact that the point counting measurements seem to be systematically lower than the LOI measurements could be due to a multiplicative factor or to an additive factor. This kind of regression fit indicates a multiplicative factor. If the intercepts are not forced to be 0, then the regression equations become:
T-1H cement paste: α % (LOI) = 0.91 x α % (SEM) + 0.08,
R2 = 0.91
T-1D cement paste: α % (LOI) = 0.94 x α % (SEM) + 0.07,
R2 = 0.86
The R2 factors are now somewhat closer to 1, and the slopes are now a little less than one. The positive intercepts are slightly larger than 0, which means that the degree of cement hydration determined by LOI is a little larger than that determined by SEM due to an additive factor. This is reasonable, because some of the very fine particles of the cement clinker may be miscounted due to the limited resolution, and because of the overall uncertainty in phase identification. Figure 4 shows the actual standard deviations in the degree of cement hydration as determined by point-counting calculated according to Equations (7) and (8). The standard deviation in the point counting determination of the degree of hydration ranged from ± 1.5 % to ± 1.8 %.
All of the above analyses suggest that the results from SEM point-counting are close to those from the traditional LOI method, and the differences are within a reasonable range. However, the SEM point counting results are systematically lower than the LOI measurements. The uncertainties of the LOI measurements are much lower than those for the point counting procedures and would not show up on the scale used in Fig. 4. Thus SEM point-counting, carefully used, can be an effective technique to determine the degree of hydration of plain cement paste. Having tested this procedure on plain portland cement pastes, attention is now turned to its much more important use on blended cement pastes, as LOI measurements are not meaningful at present for blended cements.
Figure 4. Regression analysis relating SEM point counting and LOI determinations of α for the plain portland cement pastes (error bars represent point counting standard deviations).