This paper describes a new concrete permeability reducing admixture capable of effectively reducing the movement of water under hydrostatic head pressure (PRAH). The new admixture is an aqueous suspension of nano-sized transition metal polymeric silicate hydrates that catalyze the nucleation of C-S-H in the capillary pores of hydrating cement paste. The crystallization of C-S-H in the capillary pores refines the capillary porosity of the cement paste and significantly increases the resistance to water penetration under pressure, as demonstrated by Environmental Scanning Electron Microscope (ESEM) investigations. The effectiveness of the new admixture was tested on concrete specimens by measuring the water permeability according to European Standard EN 12390-8. The results indicated that the new admixture is highly effective in reducing the penetration of water, compared to a reference concrete with the same W/C and to other PRAH admixtures, working with different mechanisms. The new admixture also increases the early strength development of concrete and can be used in conjunction with WR and HRWR to produce concrete with outstanding mechanical performances and durability.
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