Flexible, fiber-reinforced, waterproofing quartz cement mortar, modified with acrylic, water-repellent polymers and special additives. It is applied to the waterproofing of surfaces made of concrete, plaster, bricks, cement blocks, cement boards, porous concrete, etc. It ensures watertightness in fountains, swimming pools, inverted roofs, bathrooms, showers that are to be covered with tiles or pressed cement mortar and in general any mineral surface exposed to partial or continuous moisture, subject to vibrations or contractions and which exhibits or is expected to exhibit capillary cracks.