3.2. Chemical Deterioration

Sulfate Attack: Sulfate ions react with hydrated cement products forming expansive compounds (ettringite, gypsum), causing cracking.

Sulfate attack occurs when sulfate ions from external sources come into contact with concrete. These ions react with hydration products of cement, leading to the formation of new compounds like ettringite and gypsum. These compounds expand within the concrete, which can create internal pressure and ultimately lead to cracks in the material, compromising its integrity over time.

Imagine a sponge that expands when it absorbs water. If you were to keep adding more water to the sponge, it would eventually break or tear due to the pressure. Similarly, when sulfate ions mix with the compounds in concrete, they cause it to expand, resulting in cracks and damage.

Chloride Attack: Chloride ions penetrate concrete and cause corrosion of embedded steel reinforcement.

Chloride attack mainly refers to the invasion of chloride ions into concrete, which are often found in de-icing salts or seawater. Once these ions penetrate, they can lead to the corrosion of the steel reinforcement bars (rebar) embedded within the concrete. This corrosion is harmful because it produces rust, which expands and can create cracks and spalling in the concrete, severely weakening the structure.

Think of the steel in reinforced concrete like the frame of a bicycle covered by layers of paint. If water (especially salty water) gets to the frame and starts to corrode it, the paint (concrete) can't protect the frame anymore, and the bike could eventually fall apart due to the weakened structure. This is similar to how chloride ions corrode rebar in concrete.

Acid Attack: Reaction with acidic environments degrades the cement matrix.

An acid attack happens when acidic substances, such as sulfuric acid or organic acids, come into contact with concrete. The acid reacts with the components of the cement matrix, leading to the breakdown of the material. This degradation weakens the concrete, reduces its durability, and can cause the structure to fail if the exposure is significant and prolonged.

Consider how vinegar can dissolve certain food items. If you leave a piece of calcium carbonate (like chalk) in vinegar, it will slowly dissolve away. In a similar way, when concrete is subjected to acidic conditions, its constituents can dissolve and weaken the structure over time.

Carbonation: Carbon dioxide reacts with calcium hydroxide in concrete, reducing pH and leading to corrosion of reinforcement.

Carbonation occurs when carbon dioxide in the air penetrates concrete and reacts with calcium hydroxide, a component of cement. This reaction forms calcium carbonate, which lowers the pH of the concrete. A lower pH can lead to the corrosion of the embedded steel rebar because the protective alkaline environment is diminished, making the steel susceptible to rusting.

Think of a battery that works best in a specific pH environment. Once that environment changes, the battery starts to corrode and fails. Similarly, when the pH of concrete drops due to carbonation, the protective qualities are lost, leading to the corrosion of the rebar, much like how that poorly balanced environment can cause battery failure.

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