Factors Affecting Transfer Sorptivity of Lime and Cement Mortars in the Freshly-Mixed State

The sorptivity, S, is a measure of the ability of a porous material to absorb and transmit water by capillary action. Transfer sorptivity, A, is a measure of the ability of an absorbent substrate to withdraw water from a wet mortar mix. Desorptivity, R, on the other hand, is the parameter that quantifies water retaining ability of freshly-mixed natural hydraulic lime and cement mortars. The measurement of transfer sorptivity is essentially identical to that of sorptivity except that the brick is placed in contact with a wet mortar mix rather than with water. This paper concerns the combination of wet mortar and brick substrate and the interaction between them. The purpose of this paper is to investigate the influence of hydraulicity, mix water content and sand grain size on the transfer sorptivity of a range of freshly mixed hydraulic lime andPC mortars and to investigate the relationship between transfer sorptivity and desorptivity for the case a substrate material of constant sorptivity. The measurement of transfer sorptivity is not straight forward. Before such measurements could be carried out, it was necessary to confirm that the substrate material to be used was highly homogeneous. To this end, two types of bricks are first investigated to establish their suitability as substrate materials for these measurements. A methodology for the measurement of transfer sorptivity is developed. The results have been shown that the transfer sorptivity increases systematically with increasing hydraulicity; that the transfer sorptivity increases systematically as the water content of a mix is increased; and that the transfer sorptivity of a given mortar increases as the sand grain size is increased. These parameters have important practical consequences, not only in the initial adhesion of the mortar to the substrate but also in the strength of the set material. The ability to monitor the water retaining properties can also allow efficiency in masonry construction.