Concrete Composition and its Acid Reactions
Acid attack is the consequences while concrete reacts with acid. By the way, the concrete is an alkaline base structure which support the development of a metropolitan city. In addition, the concrete is a reaction substance from several ingredients, and no doubt, the cement is the major raw material.
Even though the cement is the main ingredient for a concrete but these two products have difference composition. Hence the chemical resistance ability is difference too.
Cement： Main composition is Calcium Oxide.
Concrete: Main composition is C-H-S and Calcium Hydroxide.
Cement main composition
Cement is a mixture within calcium oxide and silica oxide. Below are main ingredients for a cement, Ca2SiO4 or 2Cao.SiO2
- Lime or Calcium Oxide, CaO.
- Silica Oxide, SiO.
Concrete is a mixture and reaction product from cement, sand, water and gravel. The mixture form Calcium-Silicate-Hydrate as well as the Calcium Hydroxide. Below is one of the concrete mixing ratio.
Concrete is a mixing product as below ratio
- Cement: 10%
- Water: 20%
- Sand: 30%
- Gravel: 40%
While cement mix with the mixture and water, it forms Calcium Silicate Hydrate (3CaO · 2SiO2 · 3H2O) and Calcium Hydroxide (Ca(OH)2). The Calcium Hydroxide is a the composition react between Calcium Oxide and water. While a concrete contacts with an acid solution, the initial reaction is between the acid and Calcium Hydroxide.
Concrete Chemical Reaction with Acid
So far we know the concrete is product of Calcium Silicate Hydrate + Calcium Hydroxide. While the concrete gets contact with an acid solution, the acid solution will react with calcium hydroxide. The product from this reaction is calcium salt and water. Let us see the general chemical formula while the concrete reacts with acid.
Acid X + Ca(OH)2 ==> Ca.X + Water
Concrete + Sulfuric acid
While sulfuric acid pour on a concrete surface, the sulfuric acid immediate react with Calcium Hydroxide and form Calcium Sulfate (CaSO4). The precipitate calcium sulfate is a gypsum by-product which not soluble in water. By the way, the calcium sulfate induces an additional tensile strength which cause the concrete spalling and cracking.
in conclusion, calcium sulfate is poor solubility in water and white in colour.
This is the Sulfuric acid corrosion mechanism.
Concrete + Phosphoric acid
Base on earlier general concrete reaction with acid X guideline, while the concrete get in touch with phosphoric acid, the reacting salt is Calcium phosphate (Ca3(PO4)2). Calcium phosphate is white in colour and insoluble in water.
The Calcium phosphate soluble in dilute nitric acid and hydrochloric acid. It also mild soluble in CO2 containing solution.
Calcium phosphate could be found in human bone, since it will mild soluble in CO2 solution, hence the soft drink (consist of CO2) is actually dissolve the calcium phosphate.
Concrete + Hydrochloric Acid
Hydrochloric acid (or muriatic) is dissolving the concrete too. Again, base on the earlier reaction formulation, the Hydrocholoric acid will react with the Calcium Hydroxide. The product we get from this reaction is the Calcium Chloride and water.
The Calcium Chloride (CaCl2) is a white colour crystalline salt which very soluble in water.
In this case, if you want to remove a concrete splash on a surface, just pour some diluted hydrochloric acid on the surface and the concrete splash will remove.
Concrete + Nitric Acid
The reaction between concrete and nitric acid is significant. While Nitric acid in touch with the concrete, it will form Calcium Nitrate salt. This white colour salt is highly soluble in water.
Calcium nitrate mostly is used in agriculture industry as a fertilizer and also decease control. Since calcium nitrate is soluble in water, so it is more easy for the plant to intake. By applying Calcium Nitrate, the plant will get 2 nutrients, the Calcium and Nitrogen.
Above all, here is the summary for concrete reacts with acid.
|Concrete + Sulfuric Acid||Calcium Sulfate Salt||Poor Solubility|
|Concrete + Phosphoric Acid||Calcium Phosphate Salt||Insoluble|
|Concrete + Hydrochloric Acid||Calcium Chloride Salt||High Solubility|
|Concrete + Nitric Acid||Calcium Nitrate Salt||High Solubility|
Acid Reaction with Calcium Silicate Hydrate
The calcium hydroxide in the concrete composition will react with acid prior to Calcium Silicate Hydrate. Once the calcium hydroxide exhausted, then the acid will start react with the Calcium Silicate Hydrate.
The Calcium Silicate Hydrate mostly written as C-S-H, an abbreviation for these 3 composition. Unfortunately, there is without a firm formulation as it could change under difference mixing recipe. While the Calcium Silicate Hydrate gets in touch with acid, the calcium ion will be leached out and replace by the hydrogen ion. The leaching out of calcium ion will deteriorate the entire concrete structure.
Calcium Salt is the product while concrete reacts with an acid solution. Some of the Calcium salt are water soluble but some are not.
In conclusion, an alkaline base concrete structure is rigid in property unfortunately while it exposes to acidity condition, the concrete starts react with the acid solution. The reaction is a kind of acid attack which will deteriorate the strength and structure.