Badly-painted concrete is an unfortunate feature of the modern world, and is primarily caused by the use of plastic-emulsion ("latex") paints which are largely incompatible with portland cement. These two materials have highly disparate chemical characteristics, giving them the tendency to separate at the interface under changing environmental conditions, which inevitably causes the paint to blister and peel. Accordingly, if a paint could be made using portland cement itself as the binder, it should produce a seamless and durable bond with concrete, similar in principle to applying gesso over plaster, or oil paint over putty.

   While this can be achieved directly by coloring portland cement with inert mineral pigments and adding water, the resulting paint has poor initial strength and adhesion, and thickens rapidly during application. Fortunately, both of these faults can be remedied by the addition of hydrated lime. Specifically, cement and lime can be mixed in ratios from 1:1 to 1:3 to form useful binders, the strength of which is determined by the amount of cement, and the working time and adhesion determined by the amount of lime. This binder can then be mixed with roughly 10% of its weight of an inert mineral pigment, such as the oxides of iron, titanium, and chromium, or cobalt blue, which replaces ultramarine due to its resistance to alkali. This mixture can then be added to roughly its own weight of water to form a paint which remains brushable for 1-3 hours, corresponding to the weight ratio of lime to cement.

   While ideally this paint should be made with pure hydrated lime (calcium hydroxide) and white portland cement (as seen above), this is only truly necessary for the brightest colors. For most purposes ordinary gray portland cement is suitable, and certain types of masonry cement may even be used as the entire binder, as this is often simply a mixture of cement and lime. These ingredients and the desired pigment can then be mixed in dry form, by shaking the powders together in a closed container such as a jar or paint can. Additionally, if this container is marked internally with a pre-calculated fill line, water can then be added immediately before use without the need for measurement. The only caveats to the use of this paint are that the substrate should be thoroughly dampened with water before painting, and that the paint becomes significantly lighter in color when dry.

   In practice, this paint is surprisingly durable; the concrete blocks at the top of the page were painted using a 1:1 binder and 10% pigment, and after three and a half years outdoors the individual brushstrokes are still visible. It has been exposed to the direct sunlight of summer as well as the freeze-thaw cycles of winter, and shows no significant wear other than some slight abrasion and discoloration due to contact with the soil. Paint with a higher lime content will likely be somewhat less durable than this, but is still fundamentally distinct from limewash, which is made with no cement at all; cement paint produces a hard and slightly glossy coat, while limewash is fundamentally soft and chalky by nature. Overall, I am highly satisfied with the results of this paint, and plan to use it whenever a colorful and durable coating is needed for concrete or masonry structures in projects to come. In fact, the only remaining issue is the inherent complexity of portland cement itself, which I plan to address in a future article by developing either a synthesis or a substitute for this essential material.