Quantitative characterization of the interfacial transition zone around lightweight and normal aggregates in cement mortars at different water-to-binder ratios

The interfacial transition zone (ITZ) between aggregates and cement binders has long been known as the most vulnerable to crack resistance among the three phases in heterogeneous concretes [i.e., aggregate, ITZ, cement binders] owing to its porous microstructure. The characteristics of the ITZ are determined by the aggregate types, water-to-binder (W/B) ratios, and supplementary cementitious materials. Furthermore, the correlations between the physicochemical properties of the interface in the vicinity of saturated lightweight aggregates (LWAs) and the mechanical properties of cement concretes are still controversial because of the rough surface and ambiguous water desorption behavior of LWAs. To clarify such correlations, in this study we systemically investigated the effect of different water desorption behaviors of LWAs in cement mortars for various W/B ratios ranging from 0.2 to 0.5 on the mechanical properties of cement mortars via isothermal calorimetry tests, scanning electron microscopy, and nanoindentation grid tests. In addition, the experimental results were compared to those of mortars containing fine normal-weight aggregates (NWAs). These investigations clarified the effect of mechanical properties and porosity of the ITZ on the inherent strength of cement mortars with different aggregate types and W/B ratios. An improvement in the ITZ morphology around the LWAs was observed in relation to that around the NWAs for W/B > 0.3. Conversely, the porosity increased significantly on the LWA surface with respect to that on the NWA surface for W/B = 0.2. Nanoindentation tests revealed the deterioration of the elastic moduli and hardness of mortars containing LWAs at all investigated W/B ratios.