Abstract: An improved flexible cementitious material is prepared by mixing an aqueous dispersion or latex of elastomeric polymer (e.g. butyl rubber) with cement (e.g. Portland), allowing the mixture to harden typically for 28 days under conditions of high relative humidity, and thereafter compressing the hardened material (e.g. by passing the material between rollers) to reduce the brittleness and to increase the flexibility thereof. The initial cementitious mixture preferably contains additional components such as a dessicant (e.g. lime). The resulting flexible material is useful as a joint filler between concrete structures to take-up deformation therebetween.