Measurement of the water retentivity of cement mortars

This paper describes techniques to measure how well a soft mortar mix retains water when it is in contact with a masonry substrate. We show that the water retentivity may be characterised by a well-defined and reproducible parameter the desorptivity. Two methods of measurement are investigated: a capillary suction technique and a pressure cell technique. The pressure cell technique proved to be the better method, capable of measuring the water desorption behaviour of mortars over the full range of pressures which represent the suctions of typical masonry materials. Values of desorptivities of cement/sand, lime/sand and cement/lime/sand mortars are presented. The empirical test to assess the water retentivity of cement mortars included in BS 4551 is evaluated.