Evaluating the effects of nanosilica on the material properties of lightweight and ultra-lightweight concrete using image-based approaches

This work is aimed at characterizing the effects of nanosilica (NS) on the properties of lightweight aggregate concretes with different densities. Lightweight aggregate concrete (LWAC) and ultra-lightweight aggregate concrete (ULWAC) with targeted oven-dry densities of 850 kg/m³ and 450 kg/m³, respectively, were produced. The mixtures were modified by replacing cement with nanosilica, in concentrations of 1, 2, 5 and 10 wt-%. For comparison purposes, control specimens containing either cement alone or cement with silica fume (SF) were also produced. Their mechanical properties, including flexural and compressive strengths and transport characteristics, were evaluated by measuring the water accessible porosity and water absorption coefficients of the concretes. In addition, the thermal conductivity and drying shrinkage, being important parameters of lightweight concrete, were characterized. The pore structure characteristics of the concretes were assessed using 2D and 3D image analysis techniques; namely, using an automated air void analyser and micro-computed tomography (micro-CT), respectively. The experimental results show that NS has a significant effect on improving the mechanical and transport properties of lightweight concretes and that the efficiency of NS is much higher than that of SF. Moreover, depending on dosage, NS was found to have a negligible or decreasing influence on the drying shrinkage of concrete, after 28 days of curing. Microstructural studies confirmed that NS significantly affects the pore characteristics of concretes, thus resulting in concretes with denser and stronger microstructures.