Figure 2 presents the CEMHYD3D model and the experimental results for the degree of hydration for cement pastes with and without 20 % limestone substitution, with a "final" w/s=0.435 and cured under saturated conditions. For this higher w/s, the CEMHYD3D model predicts little if any acceleration of the cement hydration by the substitution of limestone and this is what is in fact observed experimentally. At hydration times of 90 d and beyond, there is a slight acceleration of the hydration by the limestone, most likely due to the higher effective w/c (0.544 vs. 0.435) in the filled system. Similar results are displayed for the pastes exposed to sealed curing conditions as shown in Figure 3. Even under sealed conditions, there is sufficient water initially present in the two pastes for hydration to continue at its "nominal" maximum rate.
Figure 2: Experimental and model estimated degrees of hydration for cement 152 with and without 20 % by mass fraction limestone substitution for w/s=0.435, cured under saturated conditions.
Figure 3: Experimental and model estimated degrees of hydration for cement 152 with and without 20 % by mass fraction limestone substitution for w/s=0.435, cured under sealed conditions.
Quite different results, however, are observed for the w/s=0.35 pastes as shown in Figures 4 and 5. For this lower w/s, the additional water (relative to the amount of portland cement, 0.4375 versus 0.35), along with the additional surfaces provided by the limestone for precipitation of reaction products, results in a significant acceleration of the cement hydration in the filled systems. This trend is observed both for saturated (Figure 4) and for sealed (Figure 5) curing conditions, and is consistent with previous observations that lower w/s pastes, mortars, and concretes can achieve equivalent performance with higher levels of limestone substitutions than their higher w/s counterparts [3, 4, 19].
In general, the results in Figures 2 through 5 indicate that the modified CEMHYD3D model provides a good prediction of the influence of limestone substitution on the hydration rates of these blended materials. While the model does underpredict the observed later age hydration for the pastes without fillers cured under sealed conditions, in each of the four cases (two different w/s and two different curing conditions), the relative effects of the limestone substitution on achieved degree of hydration are modeled within the experimental error in the degree of hydration measurements.
Figure 4: Experimental and model estimated degrees of hydration for cement 152 with and without 20 % by mass fraction limestone substitution for w/s=0.35, cured under saturated conditions.
Figure 5: Experimental and model estimated degrees of hydration for cement 152 with and without 20 % by mass fraction limestone substitution for w/s=0.35, cured under sealed conditions.
The CEMHYD3D model was further employed to project the acceleration of cement hydration for a 20 % limestone substituted blend with a w/s=0.3. The results for saturated and sealed curing are shown in Figures 6 and 7, respectively. The experimental results of Bonavetti et al. [3] are shown for comparison. It should not be expected that the CEMHYD3D model results would exactly match these experimental values as a different cement (composition, fineness, interground limestone, etc.) was employed in the studies in [3]. Rather, the experimental results are provided as a benchmark to evaluate the relative acceleration provided by the limestone substitution in the CEMHYD3D model systems. The magnitude of the observed acceleration for the two different curing conditions using the model is quite similar to that observed experimentally [3]. Because "free" water is at a premium when sealed curing conditions are employed, the relative "acceleration" of cement hydration provided by limestone substitution is always greater at later ages in the systems with sealed as opposed to saturated curing.
Figure 6: Model predicted degrees of hydration for cement 152 with and without 20 % by mass fraction limestone substitution for w/s=0.3, cured under saturated conditions. Experimental data for similarly-cured (concrete) systems from Reference [3] are shown for comparison.
Figure 7: Model predicted degrees of hydration for cement 152 with and without 20 % by mass fraction limestone substitution for w/s=0.3, cured under sealed conditions. Experimental data for similarly-cured (paste) systems from Reference [3] are shown for comparison.
Care must be taken to not interpret the accelerated hydration provided by the limestone substitution in low w/s systems as a projected increase in compressive strength. While hydration is indeed accelerated, this increase in the production of cement hydration products must be considered in light of the initial dilution of the active cement component of the mixture by the limestone substitution [3]. A more proper interpretation in terms of projected compressive strengths is provided by considering the gel-space ratio of the two systems. Bonavetti et al. [3] have shown that the gel-space ratio concept of Powers provides an adequate description of the compressive strength development of concretes with and without limestone substitutions. The gel-space ratios of cement pastes with and without limestone, as computed by the CEMHYD3D model for saturated curing conditions, are compared in Figure 8, which provides plots of the ratios of the values for systems with a 20 % limestone filler substitution to those for unfilled systems. At very early ages of less than 1 d, a strength enhancement is projected in the w/s=0.3 and w/s=0.35 systems containing the limestone filler, due to its significant acceleration of the initial cement hydration. However, in the long term, there is about a 5 % to 8 % reduction in the gel-space ratio in the filled systems with w/s=0.3 and w/s=0.35, as the dilution effect of the limestone substitution eventually overcomes the benefits of the accelerated cement hydration. These reductions in gel-space ratio would project to compressive strength reductions of between 15 % and 20 %, in general agreement with experimental observations [3]. The reduction in gel-space ratio is less for the w/s=0.3 system than for the w/s=0.35 one, suggesting once again that the lower the w/s, the higher the limestone substitution that can be made without sacrificing performance. On the other hand, for the higher w/s=0.435 systems, a higher long term strength reduction on the order of 25 % would be projected (with an even greater strength reduction projected at 28 d), so that a 20 % limestone substitution level simply may be too high to maintain equivalent long-term performance in this blended material.
It is not surprising that the acceleration of cement hydration by limestone substitution is strongly influenced by the w/s of the paste. It is well known that, for w/c below about 0.36, there is insufficient (water-filled) space available in the three-dimensional microstructure to allow for complete hydration of the original cement. In this case, some of the cement clinker is acting as inert (and rather expensive) filler. With the advances in the development of high-range water-reducing agents and superplasticizers, and the concurrent movement towards high-performance concrete, the fraction of concretes with w/s < 0.36 being placed is increasing. In the long term, the efficiency of cement usage in such mixtures must be addressed. Limestone substitutions at levels above the 5 % currently permitted in the ASTM C150 standard specification appear to provide an opportunity to economize on cement in these lower w/s concretes. Of course, durability aspects, particularly those relevant to thaumasite formation [20, 21], must be given proper consideration. Still, as summarized by Bonavetti et al. [3], "The use of limestone filler in this (low w/c concrete) mixture is a more rational option from the energy consumption, emission reduction, and economic point of view."
Figure 8: Model ratios of gel-space factors for cement paste with 20 % by mass fraction limestone substitution to cement paste without limestone plotted versus hydration time for saturated curing conditions.