Effect of Sulphate Attack on Cement-Based Concrete

Effect of Sulphate Attack on Cement-Based Concrete
1 Introduction
Sulphate attack is one of the most significant chemical deterioration mechanisms affecting the durability of cement-based concrete in aggressive environments. It is a complex process involving the reaction of sulphate ions from external sources—such as soils, groundwater, seawater, and industrial effluents—with the hydrated constituents of cement paste. The resulting chemical and microstructural changes can lead to expansion, cracking, spalling, softening, and ultimately loss of strength and serviceability.
Sulphate attack is classified as a "very severe" threat to concrete durability according to building codes such as ACI 318 . The deterioration processes involve multiple interacting mechanisms that depend on the concentration and type of sulphate, the associated cations, the pH of the environment, the composition of the cementitious binder, and the transport properties of the concrete. This report investigates the mechanisms, influencing factors, and consequences of sulphate attack on cement-based concrete, drawing on recent experimental studies and established technical knowledge.
2 Mechanisms of Sulphate Attack
2.1 Chemical Reactions and Products
External sulphate attack (ESA) occurs when sulphate ions from an external source penetrate the concrete's pore network and react with hydration products. The primary reactions involve calcium hydroxide (portlandite, CH) and calcium aluminate hydrates .
The principal reaction products are gypsum (CaSO₄•2H₂O) and ettringite (3CaO•Al₂O₃•3CaSO₄•32H₂O). Gypsum forms through the reaction of sulphate ions with calcium hydroxide:
Na₂SO₄ + Ca(OH)₂ + 2H₂O → CaSO₄•2H₂O + 2NaOH
Ettringite forms when sulphate ions react with monosulphoaluminate hydrate (AFm) and calcium aluminate hydrate in the presence of calcium hydroxide:
C₃A•CS̄•H₁₈ + 2CH + 2S̄ + 12H → C₃A•3CS̄•H₃₂
Both products form with significant volume expansion compared to the original hydration products. The solid volume increase can reach up to 2.2 times that of the initial reacting phases, generating internal pressure within the confined pore structure . This expansion initially fills pores and may even increase mechanical strength temporarily, but continued crystal growth ultimately causes internal stresses that lead to cracking and deterioration.
2.2 The Role of pH and Solution Composition
The pH of the external solution and the pore solution plays a critical role in determining the course of sulphate attack. Research using thermodynamic-microstructural modeling has demonstrated that a reduction in pH alone—independent of sulphate concentration—can cause rapid and significant transformation of monosulphate to ettringite . This finding highlights that the chemical evolution during sulphate attack is not solely driven by sulphate ingress but is also influenced by changes in the alkaline environment of the pore solution.
Ettringite stability is pH-dependent, being unstable below approximately pH 10.5, where it dissolves to form gypsum and aluminium sulphate . The pH of the surrounding solution, along with sulphate concentration and the associated cation type, determines the extent and kinetics of degradation .
2.3 Gypsum Formation and Softening
While ettringite formation is primarily associated with expansion and cracking, gypsum formation is more closely linked to softening and strength loss. Research comparing C₃A-rich and C₃A-poor cement pastes exposed to low-concentration sodium sulphate solution demonstrated that these two degradation mechanisms can be differentiated . The C₃A-rich paste promoted ettringite formation, while the C₃A-poor paste promoted gypsum formation. The study confirmed that gypsum formation contributes significantly to the loss of mechanical properties.
The reaction of sulphate ions with calcium hydroxide to form gypsum results in the consumption of portlandite and the release of hydroxyl ions, which can raise the pH locally. However, the formation of gypsum also causes decalcification of the cementitious matrix and softening of the paste .
3 Influence of Associated Cations
The cation associated with the sulphate anion significantly influences the severity and nature of deterioration. Sodium sulphate (Na₂SO₄) and magnesium sulphate (MgSO₄) represent the two most common forms in natural environments, and their effects differ substantially.
3.1 Sodium Sulphate Attack
Sodium sulphate attack primarily proceeds through the formation of gypsum and ettringite as described above. The sodium ions remain in solution as sodium hydroxide, which is highly soluble and does not contribute to the formation of expansive solid products. The attack is characterized by expansion and cracking, with deterioration typically manifesting as spalling and surface scaling .
Concrete specimens exposed to sodium chloride plus sodium sulphate solutions for four years did not show significant deterioration, indicating that sodium sulphate alone at moderate concentrations may not cause rapid degradation when chlorides are present .