Evaluation of mechanical properties and microstructure of high-performance mortars with superabsorbent polymers and metakaolin by means of X-ray computed microtomography

Superabsorbent polymers (SAP) may provide internal curing in cementitious materials, consequently, they reduce autogenous shrinkage caused by low water/cement ratios and autodessecation of pores. This paper aimed to analyze the effect of Superabsorbent polymers in high-strength mortars, using metakaolin as supplementary cementitious material (SCM). A X-ray microtomography test was carried out on two mixtures of mortars, a reference mix (MKREF) and a mix containing 0.30% superabsorbent polymer (MKSAP30), and the acquired images enabled the evaluation of mortar phases, anhydrate cement and void content. Also, a compressive strength test was carried out at 3 ages for both mixtures. The superabsorbent polymer was added still dry in the mixture, and no extra water was used for SAP absorption, hence, all the mixtures had the same total water/cement ratios. The results point to an increase in mechanical properties on mortars modified with SAP of 51% at the age of 28 days. Also, a higher porosity was observed in MKSAP30 samples, however, the pores were of small diameters, showing that SAP may provide refinement in pores of the cement matrix. In addition, a higher pore connectivity was detected, and an increase in the number of pores accounted using microtomographic images and the software called Fiji/ImageJ software for MKSAP30. The SAP modified mortar also showed more pores tortuosity. Finally, a lower percentage of anhydrate cement remaining in the mixture with SAP was shown, pointing to an optimization of the hydration degree, corroborating to the improvement of mechanical properties.