Concrete and air pollutants

In recent years, concrete has been clad or painted to an increasing degree. The reason for this, besides the aesthetic considerations, is to protect the concrete from aggressive media. Included as aggressive media are not just chemicals such as hydrochloric acid, sulphuric acid and certain salt solutions, but also gases, such as carbon dioxide, sulphur dioxide and nitrous oxides in aqueous solutions.

When concrete is of poorer quality, and/or the coverage is insufficient, these gases can mount serious attacks on the reinforcement and even completely destroy it under extreme conditions. Before discussing protection of concrete, it can be worthwhile to look at the chemical reactions that can occur under the influenced of CO₂, SO₂ and NO_x in combination with oxygen and water.

Composition of cement-stabilised construction material

Just as the name implies, all cement-stabilised materials contain cement as a binding agent. The cement, however, constitutes not only the glue, but also corrosive protection for the reinforcement. Cement consists primarily of calcium oxide and quartz, as well as small amounts of aluminium oxide, iron oxide, magnesium oxide, potassium oxide and sodium oxide. Common cement also contains small amounts of sulphates. Analysis data for Portland cement show approximately 63% calcium oxide (CaO), 1.2% potassium oxide (K₂O) and 0.15% sodium oxide (Na₂O).

Chemical reactions in the water-cement paste and in the hardened cement.

It is of course a well-known fact that cement hardens (sets) through reactions between the various oxides and water.

As has been already mentioned, all cement contains potassium and sodium oxide. In most publications, these are also referred to, but very little has been devoted to these oxides significance in the chemical reactions in the water-cement paste.

There are publications that address the danger of excessively high contents of alkali oxides in the cement, but then in regards to the concrete structure’s stability. These investigations mostly concern the reaction between alkali oxides and varying additives (for example, the silicon-alkali reaction in Denmark).

Experimental investigation

In a test, three core samples of concrete were used with varying strengths (K15, K35 and K55); the sample diameter was 9.5 cm. These were placed in beakers with a diameter of 10 cm and 400 cm3 of distilled carbon dioxide-free water was poured over. The volume ratio was about 1:1. All of the concrete core samples consisted of the same cement and additive.

The three beakers were covered with watch glass, but not tightly. In this way, the surface was exposed to air. The air volume between the water surface and the watch glass was about 300 cm³.

Each week, the contents of calcium, sodium and potassium were measured in the clear solution above the concrete. Additionally, the appearance and pH in the water were noted. Over an 8-week period, the contents of sodium and potassium rose in the water above the K35 and K55 samples, at the same time as the calcium content fell to nearly 0. In the water solution above sample K15, the calcium content rose somewhat more than the potassium and sodium contents. The pH value for K15 stopped at 12.5, while it rose to 13 for the K35 and K55 samples. In the K35 and K55 samples, a substantial precipitation of white, compact grained material formed; in sample K15, the precipitation was not as great. Closer examination showed that the precipitation consisted for the most part of calcium carbonate; a small amount was magnesium carbonate.

In research on the pore liquids in concrete, Heinz-Günter Smolczyk, Duisburg, has also established that with decreasing hydration time, the contents of Ca⁺², Cl^– and SO4^-2 decline, while Na⁺ and OH^– ion concentrations increase.

Reaction mechanisms

When one observes the reactions of CaO, MgO, K₂O and Na₂O with water and CO2 respectively, one discovers that with water, the hydroxides Ca(OH)₂, Mg(OH)₂, KOH and NaOH are formed. Ca(OH)₂ and Mg(OH)₂ are very difficult to dissolve in water. In the reaction with carbon dioxide, nearly insoluble products in the form of calcium carbonate (CaCO3) and magnesium carbonate (MgCO₃) are formed from the calcium and the magnesium hydroxides. Potassium and sodium hydroxide form very soluble carbonates. As can be seen in Table 1, water participates or is formed in all reactions, i.e. the reactions occur chiefly and accelerating in the liquid phase.

In Table 2, one can see that the sodium and potassium carbonates are not only very soluble, they also display a rather high pH value.

Furthermore, one sees that the potassium carbonate is about three times more soluble than sodium carbonate. In the reaction between potassium hydroxide or sodium hydroxide (strong base) and carbon acid (weak acid), potassium carbonate (sodium carbonate) and water are formed.

In the subsequent reaction between potassium carbonate (sodium carbonate) and calcium hydroxide, calcium carbonate and potassium hydroxide (sodium hydroxide) are formed.

We can now see that the carbonating process of cement (concrete) goes via the sodium and potassium hydroxides. Both NaOH and KOH have a very high pH value even in weak concentrations. One-percent solutions have a pH value of about 13.

Because of what has been said previously, it is hardly meaningful to look to the carbonating front with phenolphthalein, because the phenolphthalein’s colour shifts at pH 9. This has also been seen in comparison between phenolphthalein and complete analysis.

The carbonating front is often 3 times deeper that what the phenolphthalein shows.

Influence of sulphur oxide and nitrous oxides on concrete

The content of sulphur dioxide (SO₂) in air is usually between 0.1 and 0.2 mg/m³ air in industrial atmospheres.

The nitrous oxide in heavily trafficked environments can reach values of about 1.4 mg/m³ air. Nitrous oxides are most often denoted as NOx, which encompasses both nitrogen monoxide (NO) and nitrous oxide (NO₂).

The reaction formulas in Table 3 show that the sodium and potassium salts are very soluble in water. Magnesium sulphate is somewhat soluble; all nitrates are very soluble. None of the salts shows alkalinity, but rather the opposite, weak acid reaction.

In the action of nitrous oxide on cement in the presence of oxygen, the following reaction occurs:

If carbon dioxide is also present, the neutralisation of the concrete is substantially quickened.

Effects of corrosion on reinforcement bars

It is established that steel corrodes in the presence of sulphate ions, even in alkalic solutions. With atmospheric corrosion of steel, it is known that the presence of sulphates quickens corrosion with a factor of 103 to 104. It reacts the same with nitrates.

In testing, steel plates, which were sandblasted, primered and painted with two coats of epoxy, were submerged in solutions of sodium chloride, magnesium chloride solution with magnesium sulphate and sodium chloride, cement slurry and cement slurry with an additive of 3% sodium nitrate. After several weeks of storage, the test object in nitrate/cement slurry was so strongly corroded that the entire coating was underfilm corroded and detached; the pH value was about 12 during the entire period. Other plates demonstrated, in principle, no damages at all, with the exception of minor amounts at the cut edges.

Conclusion

Binding agents in concrete contain hydroxides of calcium, magnesium, sodium and potassium. Water solutions of sodium and potassium hydroxide show a pH of 12–13.5, even in low concentrations. The chemical process is determined by the reactivity and aggregation state (solid or in solution). The concrete’s reaction to carbon dioxide, sulphur dioxide and nitric oxides therefore occurs to a very small degree via the difficult to dissolve calcium hydroxide, but to a very large degree with the easily soluble and reactive sodium and potassium hydroxides. Carbon dioxide in air reacts first with sodium hydroxide, respectively, potassium hydroxide, first thereafter in accordance with the chemical laws, such as anion with the calcium cation. The created sodium and potassium cations react with the hydroxide anion and the easily dissolved initial hydroxides reform. In accordance with what has been previously presented and that which is also indicated in the literature (Smolczyk), it is thus incorrect to consider the pore liquids in the cement concrete as primarily saturated calcium hydroxide solution. The alkalinity in the pore liquids is determined almost exclusively by the sodium and potassium carbonate and sodium and potassium hydroxide.

After completed carbonation, i.e. when the calcium, sodium and potassium hydroxides are transferred to respective carbonates, to a much larger degree, the soluble sodium and potassium carbonates (with pH values between 12 and 13) account for the chemical reactions with sulphur and nitric oxides, respectively. Sodium and potassium carbonate are transferred to sulphates and nitrates, respectively, followed by the reaction with calcium hydroxide.

When one attempts to establish the carbonation depth with phenolphthalein solution, one is under the impression that one indicates calcium hydroxide with a colour shift to red. Actually, it is sodium and potassium carbonate, and/or sodium and potassium hydroxide that causes the colour shift, while calcium can exist as carbonate.

The phenolphthalein method does not indicate anything at all in regards to the carbonation state of concrete, and in particular, nothing of various calcium compounds.

One does often attain uncoloured zones in the phenolphthalein test, and chemically, two states can then exist.

1. The easily soluble sodium and potassium carbonates and hydroxides are washed out after complete carbonation of the calcium compounds. This case can only occur when no soluble carbonates or hydroxides, from deeper layers, can reach the surface. This can be the case if an ion concentration arises under blocked pores, for example, if a surface is infused with much water.

1. The calcium hydroxide is completely carbonated. The soluble carbonates and hydroxides (Na and K) have later reacted with sulphur and nitric oxide, respectively, and formed neutral salts.

Covering of concrete and other cement-stabilised materials

In constructions that are often subjected to normal outdoor environments, reinforcement bars are most often sufficiently protected if the covering is sufficient and if the concrete quality is good enough. If this is not the case, or if the construction is often soaked by rain in combination with air pollutants, corrosion damages can occur. Such constructions should be protected with a covering that protects against water, carbon dioxide, nitric oxides, sulphur dioxide and acid.

With today’s technology, it is primarily unsaponifiable materials based on epoxy, polyurethane or acrylate that are possible options. When it comes to visible constructions, the material must also be sufficiently weather resistant. In investigations of damages that have occurred to coverings, it is usually the case that the material has not been sufficiently saponifiable resistant. It has been shown several times that additives of saponifiable softening agent damage otherwise impervious materials.

In analysis of the content of blisters on coatings, it has been shown that they primarily contain potassium salts, and to a certain extend, sodium salts. Calcium salts almost never occur. This means that the coating material must be resistant to potassium and sodium hydroxide solution.

Table 1

Chemical sub-reactions in cement and concrete in the presence of water and carbon dioxide.

CaO + H₂O –> Ca(OH)₂^(B) + CO₂ –> CaCO₃^(A) + H₂O

(Calcium oxide reacts to water and gives calcium hydroxide. Calcium hydroxide reacting with carbon dioxide gives calcium carbonate and water.)

MgO + H₂O –> Mg(OH)₂^(B) + CO₂ –> MgCO₃^(A) + H₂O

(Magnesium oxide reacts to vatten and gives magnesium hydroxide. Magnesium hydroxide reacting with carbon dioxide gives magnesium carbonate and water.)

K₂O + H₂O –> 2 KOH^(D) + CO₂ –> K₂CO₃^(D) + H₂O

(Potassium oxide reacts to water and gives potassium hydroxide. Potassium hydroxide reacting with carbon dioxide gives potassium carbonate and water.)

Na₂O + H₂O –> 2 NaOH^(D) + CO₂ –> Na₂CO₃^(C) + H₂O

(Sodium oxide reacts to water and gives sodium hydroxide. Sodium hydroxide reacting with carbon dioxide gives sodium carbonate and water.)

Key

A. Nearly insoluble
B. Sparingly soluble
C. Partially soluble
D. Soluble

Table 2

The pH value of some compounds in cement-stabilised material and concrete, and their solubility.

Table 3

Chemical sub-reactions in cement and concrete in the presence of water (H₂O), oxygen(O₂), sulphur dioxide (SO₂) and nitrous oxide (NO_x).