Experimental Study on the Mechanical Properties of Basalt FRCM Made of Various Matrices: Validation by X-Ray Microtomography

In the field of structural strengthening, the need to replace common composite materials with sustainable ones has been developing in the last decade. Sustainable composite materials, such as a natural fiber-reinforced cementitious matrix (NFRCM), are both eco-friendly regarding the reinforcing fibers and the matrix. For the purpose of this work, in the first part, a type of natural fiber made with basalt was investigated. In particular, various matrices with different compositions and mechanical properties were considered during the laboratory design and production of composite materials. One inorganic and two organic matrices were studied, classifying and analyzing them in terms of compressive strength, flexural strength, and tensile strength. After that, basalt fibers and NFRCM specimens were characterized under tensile stress using the different type of matrix mentioned previously. All the matrices were made with the same mortar and differ because of the resin component. Specifically, the first was the only matrix made of an inorganic resin. The second and the third shared the same resin but had different resin mortar weight ratios. Only the second type had a nonnegligible water content. In particular, tests were conducted on five basalt strips, nine prisms subjected to three-point bending and compression tests (three for each type of matrix) and 15 basalt FRCM composites samples (also with five for each type of matrix). The comparison of the results obtained on the analyzed composites are in accordance with the results found in literature. The second part of the present work consists of identifying through x-ray microtomography a correlation between the voids of the NFRCM and the adhesion between matrix and fiber. In fact, an x-ray microtomography was carried out to analyze the adhesion status between the cementitious matrix and the basalt fibers tested during the experimental campaign. The x-ray microtomography imaging was used to obtain a nondestructive two-dimensional (2D) and three-dimensional (3D) characterization of the specimens and to detect cracks and voids. A complete set of 2D projections was acquired and a 3D map of the x-ray absorption in the volume was mathematically reconstructed. The 3D map permitted the precise position of the voids and damage in the different specimens analyzed. In particular, we show that the percentage of fiber voids in the sample made with the inorganic resin was half that of the sample made with the organic resin, i.e., $1.8\%\pm 0.5\%$ and $3.5\%\pm 0.5\%$ respectively. The number of holes in the matrix of these two samples was comparable. The sample whose resin contains water, on the other hand, turned out to be considerably different. Specifically, at variance with the former samples, we found that the percentage of voids in the matrix of the latter was higher than 3%, Additionally, by examining the anisotropy of the voids in the basalt fibers, it was possible to show the relationship between mechanical stress and the presence of voids in the fibers. According to our findings, the percentage of voids in the fibers parallel to the tensile tension in the inorganic sample was about twice that of the fibers shifted perpendicularly to the tensile stress, i.e., $0.60\%\pm 0.03\%$ and $0.34\%\pm 0.01\%$ respectively: the sample stress can only explain this difference.