Sulfate Attack Resistance Test of Cement Mortar

The sulfate attack resistance test of cement mortar is a durability evaluation that measures how cementitious binders withstand sulfate-rich environments such as seawater, saline groundwater, and industrial soils that can cause expansion, cracking, and strength loss in concrete. Conducted in accordance with GB/T 749, ASTM C1012, and ASTM C452, the test determines the corrosion resistance coefficient and expansion behavior of mortar specimens. This article explains the test principles, sample requirements, testing procedures, acceptance criteria, and typical engineering applications for cement qualification and durability design.

Technical Overview

Sulfate attack is one of the most common chemical deterioration mechanisms affecting concrete structures. Sulfate ions (SO₄²⁻) present in seawater, saline soils, groundwater, and industrial effluents penetrate hardened cement paste and react with calcium hydroxide and hydrated aluminate phases to form ettringite and gypsum. These reaction products occupy a larger volume than the original compounds, generating internal expansion stresses that lead to cracking, spalling, softening, and progressive loss of strength and stiffness.

The sulfate attack resistance test of cement mortar quantifies this deterioration under controlled laboratory conditions. Two complementary evaluation philosophies are used worldwide: the strength-retention approach, in which specimens immersed in a sulfate solution are compared with reference specimens cured in water (as specified in GB/T 749), and the expansion-based approach, in which length change of mortar bars is monitored during sulfate exposure (as specified in ASTM C1012 and ASTM C452). The results are used to classify cements, verify conformity with durability requirements, and support service-life design of structures exposed to aggressive sulfate environments.

• Corrosion resistance coefficient method: GB/T 749-2008, with acceptance criteria in GB 748-2005 for sulfate-resisting portland cement
• Length-change exposure method: ASTM C1012, referenced by ASTM C595 and ASTM C1157 for moderate (MS) and high (HS) sulfate-resistance classifications
• Potential sulfate expansion method: ASTM C452, referenced by ASTM C150 for optional sulfate expansion limits
• Specimen preparation and strength testing: GB/T 17671, ISO 679, EN 196-1, ASTM C109, and ASTM C778 standard sand requirements

Testing Samples

Standard Mortar Prisms for Strength-Based Evaluation

For the strength-ratio method, mortar is prepared with cement, standard graded sand, and water at the mix proportions specified by the governing standard, typically a cement-to-sand ratio of 1:3 with a water-to-cement ratio of 0.50 for ISO-method prisms (40 mm × 40 mm × 160 mm) or small prisms of approximately 10 mm × 10 mm × 60 mm for rapid corrosion testing. Specimens are cast in steel or plastic molds, covered, and demolded after approximately 24 hours of initial curing.

Mortar Bars for Length-Change Evaluation

Expansion tests use mortar bars of approximately 25 mm × 25 mm × 285 mm cast with embedded stainless-steel gauge studs, following the apparatus requirements of ASTM C490. For ASTM C452 potential expansion testing, the mortar is prepared with calcium sulfate (gypsum) blended into the cement at a calculated dosage so that the total SO₃ content is adjusted to approximately 7.0% by mass of cement, deliberately creating a sulfate-rich internal chemistry to reveal the intrinsic susceptibility of the cement.

Companion and Reference Specimens

Each test program requires control specimens of identical composition cured in water or lime-saturated water, companion cubes used to verify curing maturity (for example, ASTM C1012 requires mortar cubes to reach a compressive strength of 20.0 MPa before immersion begins), and replicate specimens at each test age to ensure statistical validity. A typical program includes at least three specimens per condition per age, plus spares for contingency.

Testing Items

Depending on the selected standard and project specification, the following parameters are measured and reported:

• Flexural strength of mortar prisms after sulfate exposure and after water curing
• Corrosion resistance coefficient K, defined as the ratio of flexural strength in sulfate solution to flexural strength in water at the same age
• Length change (expansion rate) of mortar bars at specified exposure ages
• Mass change of specimens during exposure
• Visual deterioration assessment, including cracking, edge spalling, surface softening, and efflorescence
• Optional compressive strength retention and microstructural analysis for ettringite and gypsum identification
• Monitoring of solution pH and sulfate concentration throughout the exposure period

Testing Methods

Strength-Ratio Immersion Method (GB/T 749)

Mortar prisms are demolded and cured in water at 20 ± 1 °C to the specified initial age. The test group is then immersed in a sodium sulfate solution of the concentration defined by the standard or the project specification (commonly in the range of 3% to 5% Na₂SO₄ by mass), while the control group remains in water under identical temperature conditions. The solution volume to specimen volume ratio is maintained at a level sufficient to preserve the aggressiveness of the exposure, and the solution is renewed at prescribed intervals. At the specified age, commonly 28 days of exposure, both groups are tested for flexural strength. The corrosion resistance coefficient is calculated as K = f(solution) / f(water). Under GB 748-2005, sulfate-resisting portland cement is required to achieve a K value of not less than 0.85.

Length-Change Method in Sulfate Solution (ASTM C1012)

Mortar bars proportioned per ASTM C109 (cement to standard sand at 1:2.75, water-to-cement ratio of 0.485) are cured in lime-saturated water until companion cubes confirm a compressive strength of 20.0 MPa. Initial length readings are taken, and the bars are fully immersed in a sodium sulfate solution containing 50 g/L Na₂SO₄, with the solution volume at least four times the volume of the immersed specimens. Length change is measured at prescribed intervals, typically at 1, 2, 3, 4, 8, 13, and 15 weeks, and then at 6 months, 12 months, and 18 months. Commonly specified acceptance limits include a maximum expansion of 0.10% at 6 months for moderate sulfate resistance and 0.05% at 6 months with 0.10% at 12 months for high sulfate resistance.

Potential Sulfate Expansion Method (ASTM C452)

This short-duration method evaluates the inherent sulfate susceptibility of portland cement chemistry. Gypsum-adjusted mortar bars are stored in water at 23.0 ± 2.0 °C, and expansion resulting from internal ettringite formation is measured at 14 days, with an optional 8-week reading. Where specified by purchasing documents, expansion is limited to 0.040% at 14 days. The method is particularly effective for comparing cements with different tricalcium aluminate (C₃A) contents.

Accelerated Wet-Dry Cycling

For severe service simulations, laboratories may perform cyclic immersion and drying regimes in sulfate solution in accordance with project specifications or durability test frameworks analogous to GB/T 50082 for concrete. Cycling concentrates sulfate crystals in pores during the drying phase and intensifies attack, providing an accelerated comparative ranking of binder systems when long-term immersion data are not yet available.

Applications

Test results are used across the cement and construction industries for multiple purposes:

• Conformity assessment and certification of sulfate-resisting cements against GB 748, ASTM C150 Types II and V, ASTM C595 MS and HS designations, and ASTM C1157 MS and HS performance classifications
• Durability design and material selection for marine structures, harbors, tunnels, foundations in saline or sulfate-bearing soils, wastewater treatment plants, and chemical industry facilities
• Research and development of blended cements, supplementary cementitious materials, and admixtures that improve sulfate durability
• Production quality control at cement plants, including verification of clinker mineralogy and gypsum control
• Forensic investigation of deteriorated structures to confirm sulfate attack as the deterioration mechanism
• Independent verification by accredited third-party laboratories for tender documentation, import-export conformity, and specification compliance

FAQ

What is the corrosion resistance coefficient K?

K is the ratio of the flexural strength of mortar specimens stored in sulfate solution to the flexural strength of identical specimens cured in water for the same period. A K value of 1.0 indicates no measurable strength loss due to sulfate exposure, while values below 1.0 indicate progressive attack. A minimum K of 0.85 is commonly required for sulfate-resisting cement.

Which solution and concentration are used in the test?

Sodium sulfate solution is the standard exposure medium: ASTM C1012 specifies 50 g/L Na₂SO₄, while GB/T 749 and project specifications commonly use concentrations in the 3% to 5% range. Magnesium sulfate solutions may be used for specific service environments because magnesium attack is generally more aggressive.

How long does the test take?

The strength-ratio method typically requires about 28 days of sulfate immersion plus initial curing. ASTM C452 potential expansion testing is completed in 14 days. ASTM C1012 length-change testing is a long-term evaluation that can extend to 12 or 18 months, although 6-month data are often used for classification.

What sample quantities are required?

A typical program requires at least three prisms or bars per exposure condition per test age, plus water-cured controls and companion cubes. Laboratories generally recommend submitting sufficient cement (typically 10 to 25 kg), standard sand, and specifying the required standards and ages in advance to avoid delays.

How does ASTM C452 differ from ASTM C1012?

ASTM C452 measures expansion caused by sulfates already blended into the mortar as gypsum, with bars stored in plain water, and is a rapid indicator of cement chemistry susceptibility. ASTM C1012 measures expansion caused by sulfate ions penetrating from an external solution, making it better suited to evaluating blended and hydraulic cements under realistic exposure conditions.

Can the test predict field performance directly?

The test provides a controlled comparative ranking of sulfate resistance rather than an exact field service-life prediction. Actual deterioration in service also depends on sulfate concentration, temperature, wet-dry cycling, permeability of the concrete, and quality of construction, all of which should be considered in durability design by qualified engineers and accredited third-party laboratories.