Rethinking Water-to-Cement Ratios in Pool Plaster
Jon Temple is out to stir things up in the pool-plaster industry. One of his most provocative and potentially impactful crusades is all about water-to-cement ratios in mix design and its implications in plaster appearance and durability. It’s an issue he says that needlessly keeps plasterers in the dark.
By Jon Temple
In my many years as a plasterer, my goal has always been the same: reduce warranty calls. Every callback costs money, damages credibility, and points to something that went wrong in the process. And yet, as an industry, we continue to overlook some very basic technical realities about plaster mix design—particularly when it comes to the water-to-cement ratios.
I’ve spent more than 20 years informally studying this subject, running experiments in small test pools, under different seasonal and environmental conditions—from humid Florida to the arid Middle East.
What I’ve learned, through trial, observation, and data, is that our industry’s assumptions about water-to-cement ratios are often based on incomplete or inaccurate information. That gap in understanding is one of the root causes of persistent problems like check cracking, pigment inconsistency, and premature degradation.
No Standard, No Clarity
Unlike concrete, plaster is not considered a structural material under ACI 318 standards. That’s why there is no defined or recommended water-to-cement ratio for pool plaster. Absent that standard, our industry is essentially the Wild West.
We all know, or at least should know, the practical reasons why the water content varies—different aggregates, moisture levels, workability, pumpability, weather conditions—but the lack of a standard or even a framework means every mix is its own experiment. Too much water? You get check cracking. Not enough? The mix becomes unworkable and brittle.
But the deeper issue is that the ratio itself is built on a false assumption—that the bag of cement you’re using actually contains as much cement as the label says. It doesn’t.
The problem boils down to one primary factor, the unknown percentages of additives in bags of cement.
When you buy a 92.6-pound bag of white Portland cement, you might assume it contains 92.6 pounds of actual cementitious material. In reality, the true cement content can be 10–15% less due to additives, extenders, and performance modifiers.
In my discussions with manufacturers, I’ve confirmed this variation firsthand. That means if you base your water content, pigment dose, accelerators, or pozzolans on a nominal 92 pounds, but the bag actually contains 82 pounds of true cement, you’re automatically introducing too much water and other admixtures.
That seemingly small discrepancy compounds quickly. Extra water means more capillary pathways during curing. More capillaries mean higher porosity, faster chemical intrusion, and ultimately, shorter plaster life.
Think of it this way: if cement is the glue, water is the solvent. Add too much solvent and the glue loses its bonding power.
Check Cracking and Porosity
Check cracking is one of the most common—and misunderstood—symptoms of poor water-to-cement control. It’s often dismissed as merely aesthetic, but that’s a dangerous misconception.
Every check crack is a portal for water intrusion. Once water gets in, it starts dissolving the calcium compounds within the plaster, weakening the matrix from within. It’s the beginning of a slow, chemical unraveling that leads to etching, discoloration, and failure.
Too much water doesn’t just make plaster more prone to cracking; it fundamentally changes its chemistry. As the water evaporates, it leaves behind voids—capillaries—through which water and aggressive pool chemistry can later travel. That porosity is the Achilles’ heel of any plaster finish.
A proper water-to-cement ratio, verified against true cement weight, closes those pathways. It creates a denser, stronger, less permeable surface that resists chemical intrusion and remains cleaner over time.
Here’s how I often explain it to crews: Imagine you pour one shot of vodka into a glass and drink it straight—you feel it. Now pour that same shot into a gallon of water. You’ll barely taste it.
That’s what happens when you add too much water to cement. You’re diluting the strength, spreading out the particles so far apart that they can’t bond properly. Another analogy is if you hit something with an open hand, your fingers are splayed out and there’s a limited amount of force. But if you make a fist, with your fingers curled together, the impact is much greater.
To much water spreads out the particles meaning there’s less strength. If you could visualize it under a microscope, a proper mix would look like the tightly clenched fist—dense, compact, powerful. A mix with too much water looks like an open hand—spread out, weak, and full of gaps.
From Discs to Pools
My experiments have included sample discs and test pools, each with controlled variations in water content. By adjusting the water between 12 and 15 gallons per 277 pounds of true cement, I observed consistent, measurable differences.
A single gallon of water can change the color, density, and texture of a finish. In side-by-side comparisons, the disc with just one gallon more water was significantly lighter, less saturated in color, and measurably softer under surface abrasion tests.
These results hold true across aggregate types—marble dust, quartz, polished, or pebble. They hold true across climates, too. When I plastered in Dubai using corrected ratios, I saw no check cracking, despite extreme heat and rapid hydration conditions.
Sequencing in the mix process is another overlooked variable. Water-to-cement ratio isn’t just about quantity—it’s also about timing and sequencing.
In many mixes, we see polymers, pozzolans, pigments, and accelerators all being dumped in together. The problem is, each of these materials interacts differently with cement depending on when they’re introduced.
If polymers go in before the cement has begun hydration, they can block pigment from bonding to the cement particles, causing mottling or color fade. Add accelerators too early, and they tie up available calcium ions, resulting in residual precipitates that mimic calcium hydroxide scaling.
Even mix time plays a role. Under-mixing can leave pockets of weak material; over-mixing can entrain too much air. The correct sequence ensures that the cement hydrates first, the pigment bonds to that hydration structure, and the polymers and pozzolans reinforce the matrix rather than disrupt it.
Why It Matters
These aren’t academic distinctions. They’re the difference between a pool that lasts 20 years and one that needs refinishing in five.
By understanding and controlling the true water-to-cement ratio—starting with the actual cement weight, not the number on the bag—you eliminate a huge source of variability. You reduce check cracking, color inconsistency, and porosity-related failures.
When porosity is reduced, so is biofouling—algae has less to grab onto, and surfaces stay cleaner longer. That means fewer service calls, fewer acids washes, and longer intervals between remodels.
And, I’ve found that using the correct amount of water, along with proper sequencing, mix times and compression troweling (a subject we’ll look at in a future article), you dispel the concept that plaster can’t be left dry. I have a test pool we’ve left empty for months and there is zero check cracking.
That’s all part of why if I could give one piece of advice to every plasterer and builder in the industry, it’s this: Know your true cement content before you start. Without that number, you’re guessing. And guessing almost always means adding too much water.
When you dial in the ratio correctly, everything else—additives, troweling, curing—starts to work the way it should. The surface gets denser, colors get richer, and most importantly, warranty calls disappear.
That’s the future of plastering. Not more additives or gimmicks. Just better fundamentals.
Jon Temple is the owner of Tempool Inc. in Jacksonville, Florida. A master applicator and industry innovator, he has devoted decades to improving the science and craftsmanship of pool plastering.