A desorptivity-based approach for the assessment of phase separation during extrusion of cementitious materials

A critical criterion for the extrusion of cementitious materials is that the mix used must remain stable and homogeneous without undergoing phase separation. Therefore, it is important to have an assessment of water retention capacity of mortars to be used for such purposes. Although there are empirical methods like bleed test (ASTM C1741- 18 [1]), these methods are single-point measurements, i.e., conducted at a single pressure value for a fixed amount of time. In the current approach, desorptivity is used as a fundamental material parameter to evaluate the water retaining capability of different cement mortars. Desorptivity values were obtained at different constant pressures to obtain a desorptivity-pressure relationship. The influence of various factors on desorptivity such as the addition of superplasticizer, the addition of water retaining admixtures like hydroxypropyl methylcellulose (HPMC), particle packing density, and water content were examined. Finally, the different mixes were examined for phase separation by using a piston-pump-based system. Based on the maximum pressure applied in the piston pump, an index was proposed that distinguishes the mortars with the required water retention capability for the extrusion process.