Capillary imbibition is a transport phenomenon occurring in concrete structures exposed to weathering, frequently in direct connection with the resistance against different deterioration processes. This property depends on the volume and connectivity of pores. For ground granulated blast-furnace slag (GGBFS) and fly ash blended concrete mixes, the pore refining action of these supplementary cementitious materials plays a positive role in the disconnection of the capillary porosity and consequent reduction of the capillary imbibition rate. Moreover, for this particular transport process, primary and secondary transport rates can be defined in connection with different driving mechanisms. This allows a complementary description of the pore structure of concrete. In this paper, blended concrete mixes were prepared by substituting 20, 40 and 60 % of OPC by GGBFS, and 20, 30 and 40 % of OPC by fly ash. The pore structure of these concretes is assessed by water absorption under vacuum and mercury intrusion porosimetry after curing periods of 28 and 90 days. Long-term capillary imbibition tests were also performed and primary and secondary imbibition rates are computed by a novel approach that considers their linear evolution with the fourth root of time. Results show the refinement action of GGBFS and fly ash by a softening in the transition stage between the primary and secondary imbibition periods. A low water flow rate is consequently correlated with the increased tortuosity of samples.
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