Sulphate and Chloride Attack on Concrete – Mechanism and Prevention

Not all damage to concrete comes from overloading. Two of the most insidious forms of deterioration are sulphate attack and chloride attack — they work slowly and invisibly for years before producing visible damage. By the time you see cracks, rust stains, or spalling, the damage may already be extensive. Understanding how these attacks work and how to prevent them is essential for designing durable structures in aggressive environments.

1. Sulphate Attack — Mechanism

Sulphate attack occurs when sulphate ions from external sources (soil, groundwater, sea water, industrial effluents) react with the hydration products of cement, particularly C&sub3;A and Ca(OH)&sub2;, to form expansive crystalline compounds.

Sources of Sulphates

• Groundwater containing sodium sulphate (Na&sub2;SO&sub4;), magnesium sulphate (MgSO&sub4;), or calcium sulphate (CaSO&sub4;)
• Gypsiferous (sulphate-rich) soils — common in agricultural areas and coastal regions
• Sea water (contains ≈2.7 g/L sulphate)
• Industrial effluents and sewage (sulphate-reducing bacteria produce H&sub2;S which oxidises to H&sub2;SO&sub4;)
• Ammonium sulphate from fertilisers in agricultural land

Chemical Reactions in Sulphate Attack

Two main reactions occur:

With MgSO&sub4;, the attack is even more severe because Mg²² also attacks C-S-H gel, converting it to a non-binding magnesium silicate hydrate (M-S-H).

2. Types of Sulphate Attack

• Internal sulphate attack (DEF — Delayed Ettringite Formation): If concrete is steam-cured at high temperatures (>70°C), ettringite is not formed initially. When the concrete cools and moisture is available later, delayed ettringite forms from existing sulphate within the concrete, causing internal cracking. Common in precast concrete steam-cured at excessive temperature.
• External sulphate attack: The classic form described above — sulphates penetrate from outside through permeable concrete. Most common in foundations and underground structures.
• Thaumasite Sulphate Attack (TSA): At temperatures below 15°C, a different product — Thaumasite (CaSiO&sub3;·CaCO&sub3;·CaSO&sub4;·15H&sub2;O) — can form. It converts C-S-H gel (the strength-giving gel) itself into a soft, white, sugar-like mass. Concrete affected by TSA loses all cohesion — it can be scooped up like wet sand. Very dangerous and difficult to detect early.

3. Prevention of Sulphate Attack

• Use Sulphate Resistant Cement (SRC, IS 12330): Very low C&sub3;A content (<5%). Without C&sub3;A, the primary ettringite-forming reaction cannot occur. This is the most effective single measure.
• Use Portland Pozzolana Cement (PPC, IS 1489) or GGBS: The pozzolanic reaction consumes Ca(OH)&sub2;, removing one of the reactants needed for gypsum formation. Fly ash and GGBS also reduce permeability.
• Low W/C ratio (≤0.45): Dense, low-permeability concrete prevents sulphate-rich water from penetrating to the cement hydrates.
• Adequate cover and good compaction: Reduces ingress pathways.
• Protective coatings: For very aggressive (extreme) exposure, epoxy coatings or waterproofing membranes provide additional protection.

IS 456:2000 Clause 8.2.4 specifies cement type and concrete grade requirements based on sulphate concentration in soil or groundwater (Table 4).

4. Chloride Attack — Mechanism

Chloride attack doesn’t actually damage the concrete paste itself — it damages the steel reinforcement inside the concrete. Here’s the step-by-step mechanism:

1. Chloride ingress: Chloride ions (Cl²) penetrate the concrete either by diffusion through the pore solution (dominant in submerged structures) or by cyclic capillary absorption and drying (dominant in tidal/splash zones — the most aggressive situation).
2. Passive film disruption: Steel in the alkaline environment of concrete ( pH ≈ 12.5) is normally protected by a thin passive oxide film (Fe&sub2;O&sub3;/Fe&sub3;O&sub4;). When chloride concentration at the steel surface exceeds a threshold (typically 0.3–0.5% by weight of cement), chloride ions locally break down this passive film, exposing bare steel.
3. Pitting corrosion begins: An electrochemical corrosion cell forms — the depassivated area becomes the anode (where iron dissolves) and the passive area remains the cathode. The iron oxidises rapidly to form rust products (Fe&sub2;O&sub3;, Fe(OH)&sub2;).
4. Volume expansion: Rust products occupy 2 to 4 times the volume of the original steel. This expansion is confined within the concrete, generating tensile stresses in the cover concrete.
5. Cracking and spalling: When the tensile stress from rust expansion exceeds the tensile strength of the cover concrete, longitudinal cracks form parallel to the rebar. Eventually, the cover concrete spalls (breaks off in chunks), fully exposing the corroded steel. At this stage, the structure is in urgent need of repair.

5. Chloride Threshold and IS 456:2000 Limits

The critical chloride content (the threshold level at which corrosion initiates) is approximately 0.3–0.5% by weight of cement in the concrete, but this varies with pH, oxygen availability, and moisture content.

IS 456:2000 Clause 8.2.5 specifies maximum chloride content in concrete:

Note: Calcium chloride (CaCl&sub2;) as an accelerator is STRICTLY PROHIBITED in reinforced and prestressed concrete per IS 456:2000 (due to chloride-induced corrosion risk).

6. Prevention of Chloride-induced Corrosion

• Low W/C ratio (≤0.40 for marine structures): Reduces concrete permeability and slows chloride diffusion rate.
• Adequate cover (IS 456 Table 16): More distance for chlorides to travel before reaching steel — provides a “time buffer” of years or decades.
• Use of GGBS (IS 16714): Most effective SCM for chloride resistance — GGBS dramatically reduces chloride diffusion coefficient and binds chlorides in the paste.
• Corrosion inhibitors: Calcium nitrite (Ca(NO&sub2;)&sub2;) is an anodic inhibitor — reinforces the passive film on steel.
• Epoxy-coated or stainless steel reinforcement: For very aggressive marine environments.
• Cathodic protection: For existing corroding structures — impressed current or sacrificial anode systems.
• Never use sea water for mixing RCC — chloride limit exceeded immediately.

7. Sulphate vs Chloride Attack — Key Comparison

8. Diagram — Sulphate & Chloride Attack Comparison

9. Exam Tips (RTMNU)

• ✅ Sulphate attack: expansive ettringite from C&sub3;A + sulphate → cracks from inside — write the reaction equation.
• ✅ Prevention for sulphate: SRC (IS 12330) with C&sub3;A <5% — always state the IS code.
• ✅ Chloride attack: Cl² breaks passive film on steel → rust expands 2–4× → cracking and spalling — explain step by step.
• ✅ IS 456 chloride limits: 0.3% (RCC), 0.1% (PSC), 0.6% (PCC) — these exact percentages are asked regularly.
• ✅ CaCl&sub2; as admixture = PROHIBITED in RCC — always state this.
• ✅ Compare both attacks in a table format for 10-mark questions — examiners love clear comparisons.
• ✅ Thaumasite Sulphate Attack (TSA): at <15°C, attacks C-S-H itself — mention for bonus marks.

10. Key Takeaways

• Sulphate attack: SO&sub4;²² reacts with C&sub3;A → expansive ettringite → cracking and disintegration. Prevented by SRC (C&sub3;A <5%).
• Chloride attack: Cl² depassivates steel → corrosion → rust expansion → cracking and spalling of cover concrete.
• IS 456 max chloride: 0.30% (RCC), 0.10% (PSC), 0.60% (PCC) — by weight of cement.
• CaCl&sub2; is strictly prohibited as admixture in RCC and PSC (IS 456:2000).
• Best SCM for sulphate: SRC or PPC. Best SCM for chloride: GGBS.
• Both attacks are controlled by: low W/C ratio, adequate cover, good compaction, correct cement type.

11. FAQs

Q1. What is sulphate attack and how does it damage concrete?

Sulphate attack occurs when sulphate ions (from soil, groundwater, or sea water) penetrate concrete and react chemically with cement hydration products — particularly C&sub3;A (tricalcium aluminate) and Ca(OH)&sub2;. The primary reaction forms ettringite, an expansive crystalline compound that expands as it grows, generating internal pressures that crack and disintegrate the concrete from within. Gypsum formation also weakens the paste.

Q2. What cement prevents sulphate attack?

Sulphate Resistant Cement (SRC, IS 12330) is specifically formulated with very low C&sub3;A content (<5%). Without C&sub3;A, the main ettringite-forming reaction cannot occur. Portland Pozzolana Cement (PPC) and GGBS blended cements are also effective because they consume Ca(OH)&sub2; (pozzolanic reaction) and reduce permeability.

Q3. How does chloride attack damage reinforced concrete?

Chloride ions diffuse through permeable concrete and accumulate at the steel surface. When chloride concentration exceeds the critical threshold (~0.3% by cement weight), they locally destroy the passive protective oxide film on the steel. An electrochemical corrosion cell forms, the steel corrodes rapidly, and the rust products occupy 2–4 times the original steel volume, generating expansive forces that crack and spall the concrete cover.

Q4. What is the maximum chloride content allowed in RCC per IS 456?

IS 456:2000 Clause 8.2.5 specifies: Reinforced Concrete = 0.30% by weight of cement. Prestressed Concrete = 0.10% by weight of cement. Plain Cement Concrete (no reinforcement) = 0.60%. These limits apply to the total chloride in the concrete from all sources (mixing water, aggregates, admixtures).

Q5. What is Thaumasite Sulphate Attack?

Thaumasite Sulphate Attack (TSA) is a particularly severe and damaging form of sulphate attack that occurs at low temperatures (below 15°C). Unlike classical sulphate attack, TSA attacks the C-S-H gel itself (the main strength-giving component of concrete), converting it into a soft, white, sugar-like mass with no structural strength. Concrete affected by TSA literally dissolves into a pulp. It requires the simultaneous presence of sulphates, carbonates, and silicates at low temperatures.

🔗 Related: Durability of Concrete – IS 456 Requirements

🔗 Related: Permeability of Concrete – Causes and Control

📚 Reference: IS 456:2000 Clause 8.2.4, 8.2.5 – Chemical Attack on Concrete, BIS