Effect of Acid Attack on Cement Mortar and Concrete

Effect of Acid Attack on Cement Mortar and Concrete
1 Introduction
Acid attack represents one of the most destructive forms of chemical degradation affecting cement mortar and concrete structures. It occurs when acidic solutions—whether from industrial effluents, acid rain, sewer systems, agricultural waste, or natural acidic waters—come into contact with cementitious materials. The highly alkaline nature of hydrated cement paste, with a pH typically above 12.5, makes it inherently susceptible to neutralization and dissolution when exposed to low-pH environments .
The consequences of acid attack are severe and multifaceted. Deterioration manifests as surface softening, mass loss, increased porosity, reduced mechanical strength, and ultimately structural failure. The economic implications are substantial, as acid attack limits the service life of concrete structures in wastewater treatment plants, sewer pipes, agricultural facilities, industrial buildings, and infrastructure exposed to acidic precipitation . Understanding the mechanisms, influencing factors, and consequences of acid attack is essential for designing durable concrete structures and developing effective mitigation strategies.
2 Mechanisms of Acid Attack
2.1 Fundamental Chemical Processes
The fundamental mechanism of acid attack involves the reaction of hydronium ions (H₃O⁺) from the acidic solution with the alkaline constituents of the cementitious matrix. The process can be described as a three-step sequence: (i) transport of H₃O⁺ ions to the reaction zone, (ii) chemical reaction with cement hydration products, and (iii) removal of reaction products from the reaction zone .
The primary targets of acid attack are the hydration products of Portland cement: calcium hydroxide (portlandite, CH) and calcium silicate hydrate (C-S-H). The reaction neutralizes the alkaline pore solution and causes decalcification and dissolution of these phases. A highly porous corroded layer develops on the concrete surface, consisting essentially of hydrated silicates and silica gel .
2.2 Phase Dissolution and pH Thresholds
From a thermodynamic perspective, each solid phase in the cement matrix has a corresponding pH value below which it becomes unstable and dissolves . The sequence of phase dissolution follows a hierarchy based on stability:
At pH values below approximately 12.5, calcium hydroxide (portlandite) begins to dissolve. This is the most soluble and least acid-resistant component of the hardened cement paste . The congruent dissolution behavior of Ca(OH)₂ means that this initial stage is reaction-controlled .
As the pH continues to decrease, the stability of other hydrates diminishes. AFm and AFt phases (calcium aluminate hydrates) dissolve at lower pH values, followed by the decalcification of C-S-H. The decomposition of C-S-H occurs through incongruent dissolution, leaving behind a silica-rich gel layer at the interface between the intact matrix and the aggressive solution .
At very low pH values (< 4), even the aluminum hydroxide and iron hydroxide phases become soluble, and the silica gel itself begins to dissolve .
2.3 Formation of Corrosion Products
The reaction products formed during acid attack significantly influence the degradation process. The type and solubility of calcium salts formed determine whether they precipitate within the matrix or dissolve and leach away .
Gypsum formation. In sulfuric acid attack, calcium ions react with sulfate to form gypsum (CaSO₄•2H₂O). Gypsum has limited solubility and precipitates within the corroded layer. While gypsum precipitation can initially reduce the rate of acid penetration by blocking pores, its expansive nature can also cause internal cracking and spalling . Under acidic conditions at pH ≤ 2.0, gypsum formation is strongly promoted .
Soluble salts. In attack by nitric acid, hydrochloric acid, and organic acids such as acetic acid, the calcium salts formed are highly soluble. Calcium nitrate (Ca(NO₃)₂), calcium chloride (CaCl₂), and calcium acetate (Ca(CH₃COO)₂) dissolve readily in the aggressive solution and are removed from the reaction zone. This dissolution prevents the formation of a protective barrier and accelerates degradation, as fresh acid can continuously reach the reaction front .
3 Influence of Acid Type
The severity and nature of acid attack vary significantly depending on the type of acid, its concentration, and the solubility of the resulting calcium salt.
3.1 Mineral Acids
Mineral acids—including sulfuric acid (H₂SO₄), hydrochloric acid (HCl), and nitric acid (HNO₃)—are generally strong acids that dissociate completely in solution. Their aggressive effect on hardened cement paste follows the order HNO₃ > HCl ≫ H₂SO₄, with sulfuric acid being the least aggressive among the three due to the low solubility of gypsum .
Sulfuric acid. Sulfuric acid attack combines dissolution and expansion effects. The reaction of H₂SO₄ with portlandite and C-S-H produces gypsum and ettringite, both of which form with significant volume increases, leading to internal pressure, cracking, and structural failure . In sewage systems, biogenic sulfuric acid produced by sulfur-oxidizing bacteria has been identified as a major cause of concrete corrosion, with corrosion rates reaching several millimeters per year .

    Date: 2026-09-18 00:00:00.000000