What is Pool Plaster?
Understanding spot etching first requires a closer inspection of pool plaster itself. What is pool plaster made of? In this installment, we will explain the fundamentals of plaster and how they interact with water, which is obviously important for anything that is going to line a pool.
Making pool plaster, or any cement, requires combining five base parts. Each plays its own function in the overall process, and each comes with positive and negative contributions to the final product. Pool plasters will have slightly different concentrations of each chemical to optimize resistance against water, but the fundamentals are universal. This part might feel a little technical, but bear with us. It’s not as difficult as it sounds.
This is commonly referred to as Alite. It’s one of the two primary minerals in all plasters and cements, and it is responsible for the strength and durability of cement while it cures. Often called the “early” strength, Alite is what lets the plaster hold its form while it dries. When it mixes with water, it produces a fair amount of calcium hydroxide, which we’ll talk more about in a moment.
This is commonly called Belite in the industry, and it is the most important part of a plaster mix. Belite mixes with water to release calcium hydroxide. The result is a needle-like structure that is very malleable until it dries. Once it sets, the needle structure condenses, hardening the bonds and essentially creating the final form of concrete. Belite is very resistant to the effects of water, and it is what supplies the strength of a cement or plaster after it sets.
Another chemical with a long name, this part of the mixture is mostly necessary to make the pre-mix form of cement. It is essential for the heating and drying process that happens long before the plaster ever comes near your pool. When it is mixed with water so it can be poured and set, the aluminate releases a large amount of calcium hydroxide. The aluminate also contributes to some of the less desirable traits of plaster, so it is usually a minimal part of the mix and is also tempered by other chemicals.
Tetra Calcium Aluminoferrite
Speaking of those other chemicals, this is the main one. The aluminoferrite is really just in the mix to prevent the aluminate from reacting badly. It minimizes how much damage water can do to the plaster over time and helps the entire mix to stay stable and consistent, which is key to having a smooth, reliable plaster coat.
We keep mentioning this chemical, so maybe it’s time to explain. Calcium hydroxide is not intentionally mixed into plaster. Instead, it is the byproduct of the reaction that happens when water is added to the dry mix. The Belite and Alite are the main contributors to this. Essentially, when they are mixed with water for the pouring stage, a small chemical reaction occurs and calcium hydroxide is produced. This is inescapable, and plasters account for it, but it is also what creates the challenge of spot etching in the first place. The main concern is that calcium hydroxide is very soluble in water, so over time, some of the pool water will inevitably leach some of the calcium hydroxide out of the plaster. The next segment talks more about the specific interactions that take place during the etching process.
Pool Plastering with the Pros: Spot Etching #1 – What is Pool Plaster?