Analysis of the abstraction of water from freshly mixed jointing mortars in masonry construction

Sharp front theory is used to derive an equation describing the time taken for a wet mortar joint to be dewatered by an absorbent substrate. The equation shows that the time taken to dewater a mortar joint is directly proportional to its thickness and inversely proportional to the transfer sorptivity, itself a function of the sorptivity of the substrate and the desorptivity of the wet mix. Experimental data are presented which validate this equation and the results show that the time to dewater a mortar depends on both the hydraulicity and the initial water content of the mix. For the practically important case of jointing mortars applied in 10 mm layers to a typical facing brick we show that dewatering times range from 20 s to 1 min for Portland cement and NHL 2 mortars, respectively, and that 40-60% of the original mix water is lost in the process.