Factors affecting the water retaining characteristics of lime and cement mortars in the freshly-mixed state

Desorptivity, the parameter which quantifies water retaining ability, has been determined for freshly-mixed natural hydraulic lime and cement mortars using a modified American Petroleum Institute pressure cell. The results show how greatly this ability depends on hydraulicity. Mix composition is also highly significant: smaller proportion of sand, larger grain size of sand and higher proportion of mix water all decrease the water retaining ability. In practice water is abstracted due to the capillary pressure of the masonry unit and the results presented are discussed in terms of typical capillary pressures exerted by clay facing brick. Changes in desorptivity also occur with elapsed time from mixing. The most hydraulic mortars become less water retaining for about 90 min after mixing and then, gradually more water retaining.