Microstructure modification of cement mortars: Effect on capillarity and frost-resistance

• We presented tests on capillarity, frost-resistance and micro-structural parameters.
• Four cement mortars with modified microstructure were tested.
• Superplasticizer, silica fume and an air-entraining agent led to different porosity.
• Modifications resulted in changes in capillarity and frost-resistance of the mortars.
• It has significant influence on durability of composites of this kind.

The paper presents results of tests on capillarity, frost-resistance and micro-structural parameters of four cement mortars. Basic cement mortar, which was the reference recipe I, was modified “option by option”, becoming gradually recipes II, III and IV. In recipe II, a superplasticizer was used, causing a noticeable reduction of the w/c ratio. In recipe III, silica fume was applied. Whereas recipe IV contained an air-entraining agent. Firstly, an experiment on capillary rise in the tested mortars was carried out. The tests for all mortars I, II, III and IV lasted two months and the resulting data on capillary rise were used to determine sorption coefficients. Frost-resistance of each mortar was evaluated on the basis of their mechanical strength after 80 cycles of freezing-thawing of four sample series type I, II, III and IV. Structural parameters were measured using the Mercury Intrusion Porosimetry (MIP). According to the porosimetry test results, it was concluded that application of the selected admixtures and additives distinctly influences pore structures of the mortars. The capillarity experiment indicated that modifications made to the internal structure of the mortars resulted in a different course of the water uptake process. Application of a superplasticizer, silica fume and an air-entraining agent in tested mortars led to decrease of the water sorption coefficient characterizing the rate of capillary suction in the first few hours of contact between water and the material. Another effect of the use of air-entraining agent was revealed in a subsequent phase of the experiment in a significant water absorption. The freeze–thaw test showed that all the undertaken material modifications resulted in an absolute frost-resistance of the mortars II, III, IV after an established number of freeze–thaw cycles, whereas in the reference mortar I more than 60% decrease of compressive strength was noted.