Deterioration of Concrete by Cation-Exchange Reactions

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Concrete exposed to aggressive chemical solutions will result in some deleterious reactions which is carried out by cation-exchange. The reaction occurs between the aggressive solution and the components of portland cement in the concrete. These deleterious cation exchange reactions can result in the deterioration of concrete.

Types of Cation-Exchange Reactions

Different Cation-Exchange reactions are categorised based on the harmful products formed. They are:

1. Formation of Calcium Salts
2. Formation of Soluble Calcium Salts
3. Formation of Magnesium Salts

1. Formation of Calcium Salts

Insoluble salts of calcium are found when anions present in aggressive water react with cement paste. The damage to the concrete due to these reaction products is not caused if the calcium salt formed are either expansive or washed out by running water seepage or vehicular traffic. In most cases the reaction between calcium hydroxide in concrete and acids like tannic hydrofluoric, phosphoric, tartaric, oxalic and humic acids will give products of calcium salts which are categorised as insoluble and non-expansive. Humic acids is mainly found in decaying organic matters. When concrete is exposed to such materials humic acid is responsible for chemical deterioration.

2. Formation of Soluble Calcium Salts

In industries, the environment mostly contains acidic solution which have anions. These anions can form soluble calcium salts. Most of the effluents present in the chemical industries consist of nitric acids sulfuric acid and hydrochloric acid. Most of the food products will have lactic, formic, and acetic acid. Soft drinks consist carbonic acid and natural water with a high concentration of carbon dioxide. Between the acidic solution and constituent material of portland cement paste, there take place cationic exchange reactions. This will give rise to soluble source of calcium. Some of these salts are calcium acetate, calcium bicarbonate and calcium chloride. These soluble salts are removed by leaching. Cation – exchange reactions are common in environments like fertiliser and agriculture industries. These cation – exchange reactions transform the cement paste components into soluble products. The main solutions that are involved in such reactions in these industries are ammonium chloride and ammonium sulphate.

2NH₄Cl + Cl (OH)_{2 ->} CaCl₂ + 2NH_4­ OH     Eq (1)   

The products of above reaction are soluble, this makes the attack more severe. If the solution was magnesium chloride the products would have been calcium chloride and magnesium hydroxide which are insoluble that won’t increase the permeability and porosity of the whole system, thus making the attach less severe. Below mentioned is the cationic exchange reaction between carbonic acid and calcium hydroxide.

Ca (OH)₂ + H₂CO₃ -> CaCO­₃ + 2H₂O         Eq (2)

CaCO­₃ + CO₂ + H₂O -> Ca (HCO₃)₂            Eq (3)

The precipitation of insoluble calcium carbonate during Eq (2) mentioned above will stop, given that there is no carbon dioxide in water. As per Eq (3) the calcium carbonate is transformed to soluble bicarbonate. The free carbon dioxide will facilitate the hydrolysis of calcium hydroxide. This reaction is reversible so some amount of free carbon dioxide is required to maintain the equilibrium of the reaction. If this free carbon dioxide is more than or less than what required for balancing, will result in aggressive action on cement paste. The hardness of the water will affect the balancing of the carbon dioxide. Note: Acidity of naturally occurring water is mainly due to the presence of dissolved carbon dioxide. This is major in ground water, sea water and mineral water. As a general rule the concentration of carbon dioxide is negligible for ground water and sea water when the ph is greater than or equal to 8. The ph less than 7 will show harmful concentration of carbon dioxide.

3. Formation of Magnesium Salts

Some of the industrial effluents, ground water and sea water will have magnesium chlorides or magnesium sulphates or magnesium bicarbonates. The reaction takes place between the magnesium solution and calcium hydroxide present in portland cement. This will result in salts of calcium. The reaction of magnesium solutions is found to be very aggressive as it results in sulphate attack on alumina bearing hydrates present in cement. Note: An important characteristics of magnesium ion attack is that the extended reaction will give calcium silicate hydrate (C-S-H) which is an essential and principle constituent of concrete. If this contact of C-S-H with the magnesium is sustained for long period the product will lose calcium ion which can be replace by magnesium. Hence the final product will be magnesium silicate hydrate which have adverse effect on cement characteristics.