MiraFlex Membrane C 7038 43 lb Natural Gray Cementitious Membrane Powder Part 1. MiraFlex Membrane C is a state-of-the-art, two-component, polymer-modified, highly flexible, cementitious membrane for the waterproofing and protection of concrete, masonry and many other types of construction materials and substrates. Designed for exterior or interior applications, above or below grade, MiraFlex Membrane C can be used and applied to horizontal, vertical and overhead surfaces.