Workability, physical & mechanical properties of the cement mortars strengthened with metakaolin and steel/basalt fibers exposed to freezing-thawing periods

Mortar samples' strength and durability properties can be improved by adding different pozzolanic materials and fibers to their mixtures against freeze–thaw (F-T) periods. For this purpose, metakaolin (MK) was used as a pozzolanic material in mortar mixtures by replacing 10% with cement. In the blends, hooked-end macro-steel (ST) fibers with 30 mm and 24 mm macro-basalt (BA) fibers were used in a single form and hybrid form with micro-basalt BA fibers of 6 mm in length. The study results showed that MK improved the strength and durability properties of the samples after the F-T periods. The primary reason is that MK filled the voids with the filler effect and formed calcium-silicate-hydrate (C-S-H) gels in the matrix due to its high pozzolanic activity. In addition, ST and BA fibers contributed positively to the improvement of the strength and durability properties of the samples after the F-T periods. This contribution is because ST and BA fibers effectively restrict the formation and development of micro- and macro-cracks originating from the F-T periods at the weak fiber/matrix interfacial zones. Besides, ST fibers improved samples' strength and durability properties in both single and hybrid use compared to BA fibers. In addition, using ST and BA fibers in hybrid form increases the samples' strength and durability properties slightly more than their single use. The most important reason is that micro-BA fibers play a crucial role in preventing micro-cracks formation in the inner structure due to F-T periods and their transformation into larger macro-cracks.