Water is near to being a universal solvent. Even in its present state it can cause the dissolution of all but the most stable of materials.

Water is near to being a universal solvent and it can cause the dissolution of all but the most stable of materials.

The water and whatever else is present in the water will almost always have some effect on the container.

The composition of the container will be the determining factor in the nature and extent of the effects. To determine the effects of swimming pool water, and the chemicals used in the water, on white cement it is of importance that we know the composition and properties of the various substituents which are present in the cement.

Water and whatever else is present in the water will almost always have some effect on its container.

The composition of the container, in this case the marcite shell of the swimming pool, will be the determining factor in the nature and extent of these effects. To determine the effects of swimming pool water and the chemicals used in the water on white cement it is of importance to know the composition and properties of the various substituents which are present in portland cement.

The primary raw material for the manufacture of Portland cement is calcium carbonate or limestone. The limestone is mixed with clay containing silica, aluminum and ferric oxides in the proper proportions and then burned in a rotary kiln at a temperature of about 1450ºC (2640ºF). A hard "clinker" is formed which is cooled and then ground in a mill. During the grinding about three percent of gypsum is added to help control the setting time of the finished cement.

The primary compounds entering into the composition of finished Portland cemet are: Tricalcium silicate, 3 CaO•SiO₂ (54%); Dicalcium silicate, 2 CaO•SiO₂ (17%); Tricalcium aluminate, 3 CaO•Al₂o₃ (11%) and Tetracalcium aluminoferrite, 4 CaO•Al₂o₃•fe₂ O₃ (9%). White Portland cement is richer in alumina but free of ferric and magnesium oxides.

The setting of cement is essentially a hydration reaction to form tricalcium disilicate hydrate, 3CaO•2SiO₂•3h₂O. The hydrate forms a colloidal "mineral glue" which is called "Tobermite", because it is practically identical with a rare, naturally occuring mineral of that name. The reactions which occur are given below.

(2) (3CaO•SiO₂) + 6 H₂O -> 3CaO•2SiO₂•3H₂O + 3 Ca(OH)₂

(2) (2CaO•SiO₂) + 4H₂O -> 3CaO•2SiO₂•3H₂O + Ca(OH)₂

In the first reaction about forty percent by weight of calcium hydroxide, Ca(OH)₂, or as it is sometimes called calcium hydrate, CaO•H₂O, is formed. In the second reaction eighteen percent by weight of calcium hydroxide is formed. The hydration of the tricalcium aluminate is as follows:

(3) 3CaO•Al₂o₃ + 6H₂O -> 3CaO•Al₂o₃•6H₂ O

As soon as the cement is mixed with water, a rapid reaction starts. In a few minutes, the gauging water becomes saturated with calcium hydroxide. The di_ and tricalcium silicate is hydrated to a gel, releasing calcium hydroxide which slowly crysallizes from the solution. A microscopic examination of the hardened cement paste shows small amounts of unreacted clinker minerals and crystals of calcium hydroxide. This latter material is a basic or alkaline salt and is of considerable significance in connection with the resistance of cement to attack by aggressive reagents some of which will be discussed later.

The primary raw material for the manufacture of portland cement is calcium carbonate or limestone and clay containing silica, aluminum and ferric oxides in the proper proportions. The mixture is calcined in a rotary kiln at a temperature of about 1450ºC to produce the required product.

The primary compounds entering into the composition of finished portland cemet are: tricalcium silicate, 3CaO•SiO₂ (54%); dicalcium silicate, 2CaO•SiO₂ (17%); tricalcium aluminate, 3CaO•Al₂o₄ (11%) and tetracalcium aluminoferrite, 4CaO•Al₂o₃•fe₂ O₃ (9%). White portland cement is richer in alumina and essentially free of ferric and magnesium oxides.

The setting of cement is essentially a hydration reaction to form calcium silicate hydrates. The hydrate forms a colloidal "mineral glue" which is called "tobermite", because it is practically identical with a rare, naturally occuring mineral of that name. The reactions which occur are given below.

3CaO•SiO₂ + (X+1)H₂O -> Ca₂SiO₄•XH₂O + Ca(OH)₂ + Heat(500J/g) (1)

2CaO•SiO₂ + XH₂O -> Ca₂SiO₄•XH₂O + Heat(260J/g) (2)

In reaction (1) about forty percent by weight of calcium hydroxide, Ca(OH)₂, or as it is sometimes called calcium hydrate, CaO•H₂O, is formed. The hydration of the tricalcium aluminate is as follows:

3CaO•Al₂O₃ + 6H₂O -> Ca₃Al₂(OH)₁₂ + Heat(865J/g) (3)

As soon as the cement is mixed with water, a rapid reaction starts. In a few minutes, the gauging water becomes saturated with calcium hydroxide. The di_ and tricalcium silicate is hydrated to a gel, releasing calcium hydroxide which slowly crysallizes from the solution. reactions (1) and (3) produce rapid setting but low strength in the cement. Reaction (2) is a slower setting reaction but produces higher strength. Microscopic examination of the hardened cement paste shows small amounts of unreacted calcined minerals and crystals of calcium hydroxide. The latter is a basic or alkaline material and is of considerable significance in connection with the resistance of cement to attack by aggressive agents as is discussed below.

Adding water which has a high pH (basic) to a newly plastered pool which as we have seen has an alkaline surface can further raise the pH of the water to a sufficient level to convert the bicarbonate ion (HCO₃_) present in the water to carbonate ions (CO₃=). This ion then combines with soluble magnesium and calcium ions in the water to precipitate as insoluble magnesium and calcium carbonate.

HCO₃^_ + OH^_ = CO₃⁼ + H₂O

Ca⁺⁺ + CO₃⁼ -> CaCO₃

This is sometimes referred to as "Plaster Dust", although none of it comes from the cement itself. With water as low as 25 ppm total hardness as much as four pounds of fine chalk dust can be precipitated from the water of a 20,000 gallon pool.

Adding water which has a high pH (basic) to a newly plastered pool which has an alkaline surface can further raise the pH of the water to a sufficient level to convert the bicarbonate ion (HCO₃_) present in the water to carbonate ions (CO₃=). This ion then combines with soluble magnesium and calcium ions in the water to precipitate insoluble magnesium and calcium carbonates.

HCO₃^_ + OH^_ = CO₃⁼ + H₂O (6)

Ca⁺⁺ + CO₃⁼ -> CaCO₃ (7)

This is sometimes referred to as "plaster dust", although none of it comes from the cement itself. With water as low as 25 ppm total hardness as much as four pounds of fine chalk dust can be precipitated from the water of a 20,000 gallon pool.

It involves a chemical attack on the cement walls of the swimming pool resulting in the selective solvation or leaching out of the cement surface specific minerals.

...can result in the chemical attack on the walls of the swimming pool resulting in the selective solvation or leaching out of mineral constituents from the marcite surface.