![]() The addition of calcium chloride to a plaster mix is also known to cause graying of cement, in addition to making the plaster less durable over time. A very low water-to-cement ratio almost always leaves a darker finish color. Late hard troweling can cause a darkening of the color of white cement because it decreases the water-to-cement ratio of the plaster surface. This graying discoloration is difficult to remove, and is often termed as a “hydration problem” or “entrapped moisture.” This type of dark discoloration should not be confused with metal or calcium scale staining. ![]() Usually, this type of staining can be easily and safely removed from the surface by acid washing, sanding, using sequestering or chelating chemicals, or other stain removal products.įor this article, we are discussing a blotchy or streaky, and very smooth gray discoloration that sometimes develops during the first few months after plastering and being filled with water. (Concrete Slab Surface Defects: Causes, Prevention, Repair Portland Cement Association 2001).Īlthough some discolorations are the result of metals precipitating and depositing various color residues onto the plaster surface, these are referred to as surface staining. It is also called “pinto concrete” in the cement industry. Trowel “burn” is another cause of extreme dark discolorations of white plaster. Gray (also spelled “grey”) or darkened blotchy or streaky discoloration of pool plaster (cement flatwork) is known to be caused by several improper plastering practices, including the addition of calcium chloride to the mix (even less than 2%), late hard or dry troweling, and sometimes a cement material issue can lead to this problem. ![]()
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