Quasi-Static and High Strain Rate Behavior of Cementitious Syntactic Foam

Structural materials with low density and high strength are of great interest to material scientists. Most of the currently available lightweight cementitious composites have a cellular structure and tend to show poor mechanical performance (1 -- 10 MPa compressive strengths) especially at lower densities (0.80 -- 1.80 g/cm3). Previous studies on the brittle materials show a direct relation between the size of the largest voids (D90) and the composite strength. Therefore, tailoring the microstructure of brittle composites presents an opportunity for improvements in their mechanical performance. Particulate composite materials called syntactic foams have recently gained significant interest from material scientists, showing great potential for tailoring the mechanical properties through their microstructure, which includes selecting hollow particle fillers of appropriate size, density and surface treatment for incorporation in metals and polymeric materials. However, the research conducted on cementitious syntactic foams (CSF) is limited but has great potential in civil engineering applications. This study shows that CSFs with hollow glass microspheres (HGM) can be manufactured for tailored low densities (1.15 -- 1.80 g/cm 3) that are comparable to cellular composites while they can provide significantly superior mechanical properties (30 -- 90 MPa compressive and ~5 MPa flexural strengths). These results are further supported by micro-computed tomography assisted investigations on the microstructural failure mechanisms, which showed that the density and the size distribution of HGMs play an important role in the mechanical performance of CSFs. Within the scope of this study, the highest specific strengths are achieved with HGM particles with ≥0.38 g/cm3 density with the smallest average particle sizes. Mechanical investigations are then further extended into high-strain rate (HSR) conditions (10-5 -- 103 s -1), where CSFs showed significant strain rate sensitivity (i.e. higher dynamic compressive strengths). This strain rate sensitivity of CSFs is investigated through crack propagation patterns and failure mode on the macro scale. The surface area of cracks was found to be significantly higher in specimens that were tested under HSR conditions compared to the quasi-statically tested specimens.;The HGMs, given their soda lime-borosilicate (amorphous silica) chemistry, were considered as inclusions with a potential for deleterious alkali silica reaction (ASR) in cementitious matrix materials. This study also shows that the HMGs, thanks to their hollow geometry, do not lead to an expansion of the composite material and do not have deleterious effects on the mechanical properties in the long-term. In addition to its chemical stability, CSFs also showed low permeability which implies improved durability, especially with corrosive reinforcements. In summary, the results of the study show that CSFs can outperform conventional concrete while providing densities as low as half that of conventional concrete.