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Use of cement clinker as aggregate

Berger [19] has investigated the use of cement clinker aggregate in concrete. He attributed the increased strengths of these systems to an improved cement paste-aggregate bond. Since the clinker aggregate will hydrate on its surface, the one-sided growth mechanism should be eliminated. Additionally, since cement clinker is generally porous, it should affect ITZ microstructure in a manner similar to the lightweight aggregates discussed in the previous section. Once again, simulation provides a convenient means for studying these systems.

To model cement clinker aggregate, the aggregate is assigned a phase identifier of C₃S so that it is eligible for dissolution and reaction just like the smaller cement particles. Because of its small surface-to-volume ratio, only the exterior will undergo significant hydration as shown in Fig. 6a for an absorptive clinker aggregate (12% absorption by mass as measured on a typical dry clinker in our laboratory) with w/c = 0.39 and 75% hydration for the bulk cement paste. From the phase fractions plotted in Fig. 6b, it can be seen that the distribution of porosity and of C₃S+CSH are now nearly homogeneous throughout the system suggesting a superior paste-aggregate bond. Due to the high cement content at the aggregate surface, porosity decreases slightly and C₃S + CSH increases within a few pixels of the aggregate. Not only has the one-sided growth mechanism been eliminated, but now the aggregate side of the local volume is composed entirely of cement.

Figure 6: Hydration of absorptive clinker aggregate concrete (a) hydrated microstructure (top), and (b) quantitative phase analysis (bottom).

Non-absorptive clinker aggregate can also be easily simulated. In this case, improvements in ITZ microstructure are not as drastic but are substantial as seen in Fig. 7 for a fully hydrated system with a w/c ratio of 0.47. Like the plots shown in Fig. 3 for conventional concrete, the porosity increases as the aggregate is approached from a distance, but a decrease is observed in the immediate vicinity of the aggregate due to the now beneficial effects of the aggregate-side growth. Similar trends are also observed for the C₃S + CSH phase fraction. Uisng cement clinker as aggregate produces a superior ITZ microstructure because both the one-sided growth mechanism and wall effect can be addressed, depending on the clinker's absorption characteristics.

Figure 7: Quantitative phase analysis for non-absorptive clinker aggregate concrete.