The water retaining characteristics of lime mortar

Desorptivity (R), which defines the water retaining ability of mortars in the freshly-mixed state, has been determined for hydrated lime (CL90) mortars using a modified American petroleum institute pressure cell. CL90 mortar (i.e. non-hydraulic pure lime mortar) is dramatically more water retaining than any of the hydraulic mortars. This paper demonstrates that influencing the strong water retaining characteristics of CL90 mortars is possible and suggests underlying causes of behaviour. The results show that hydrated lime mortars can become less water retaining by the use of simple ionic solutions as mix water over a range of capillary pressures typically exerted by absorbent masonry substrates such as brick. The effect of temperature is also reported. This ability to control desorptivity could lead to mortar/substrate optimization or ‘matching’ to overcome the inherent problems of either very high or very low suction substrate materials.