First, the best cement/aggregate ratio is about 1 part cement to 1.5 – 1.75 parts aggregate (marble sand). This produces a higher quality pool plaster product than either richer (more cement) or leaner (more sand) mixes. Plaster that is too rich tends to shrink and crack, and plaster that is too lean is unworkable and isn’t durable. The cement and aggregate should also be selected with care – it needs to be of good quality, consistent, and of an appropriate grade for use in pool plaster. (This also applies to any admixtures.)
http://poolhelp.com/handouts/oB_Ten%20S ... andout.pdf
