chemistry

Sulfate attack in concrete and mortar

Sulfate attack in concrete and mortar is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Sulfate attack in concrete and mortar rather than just read about it. In short: Sulfate attack typically happens to ground floor slabs in contact with soils containing a source of sulfates. Sulfates dissolved by ground moisture migrate into the concrete of the slab where they react with different mineral phases of the hardened cement paste.

Key takeaways

Sulfate attack in concrete and mortar belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Sulfate attack in concrete and mortar to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Sulfate attack in concrete and mortar from memory before moving on to harder problems.

Reference excerpt

Sulfate attack typically happens to ground floor slabs in contact with soils containing a source of sulfates. Sulfates dissolved by ground moisture migrate into the concrete of the slab where they react with different mineral phases of the hardened cement paste. The attack arises from soils containing SO2−4 ions, such as MgSO4 or Na2SO4 soluble and hygroscopic salts. The tricalcium aluminate (C3A) hydrates first interact with sulfate ions to form ettringite (AFt). Ettringite crystallizes into small acicular needles slowly growing in the concrete pores. Once the pores are completely filled, ettringite can develop a high crystallization pressure inside the pores, exerting a considerable tensile stress in the concrete matrix causing the formation of cracks. Ultimately, Ca2+ ions in equilibrium with portlandite (Ca(OH)2) and C-S-H and dissolved in the concrete interstitial water can also react with SO2−4 ions to precipitate CaSO4·2H2O (gypsum). A fraction of SO2−4 ions can also be trapped, or sorbed, into the layered structure of C-S-H. These successive reactions lead to the precipitation of expansive mineral phases inside the concrete porosity responsible for the concrete degradation, cracks and ultimately the failure of the structure.

Background: the key mineral phases Cement hydration and strength development mainly depend on two silicate phases: tricalcium silicate (C3S) (alite), and dicalcium silicate (C2S) (belite). Upon hydration, the main reaction products are calcium silicate hydrates (C-S-H) and calcium hydroxide Ca(OH)2, written as CH in the cement chemist notation. C-S-H is the phase playing the role of the glue in the cement hardened paste and responsible of its cohesion. Cement also contains two aluminate phases: C3A and C4AF, respectively the tricalcium aluminate and the tetracalcium aluminoferrite. C3A hydration products are AFm, calcium aluminoferrite monosulfate, and ettringite, a calcium aluminoferrite trisulfate (AFt). C4AF hydrates as hydrogarnet and ferrous ettringite.

External attack This is the more common type of sulfate attack, and typically occurs where groundwater containing dissolved sulfate are in contact with concrete. Sulfate ions diffusing into concrete react with portlandite (CH) to form gypsum:

When the concentration of sulfate ions decreases, ettringite breaks down into monosulfate aluminates (AFm):

When it reacts with concrete, it causes the slab to expand, lifting, distorting and cracking as well as exerting a pressure onto the surrounding walls which can cause movements significantly weakening the structure. Some infill materials frequently encountered in building fondations and causing sulfate attack are the following:

Red Ash (shale) Black ash Slag Grey fly ash Other industrial materials and building rubble can also cause problems. These materials were used extensively in the North West of England as they were widely available and waste products from industries such as coal mines, steelworks, foundries and power stations.

Excess of gypsum in concrete If gypsum is present in excess in concrete, it reacts with the monosulfate aluminates to form ettringite:

A fairly well-defined reaction front can often be observed in thin sections; ahead of the front the concrete is normal, or near normal. Behind the reaction front, the composition and the microstructure of concrete are modified. These changes may vary in type or severity but commonly include:

Extensive cracking Expansion Loss of bond between the cement paste and aggregate Alteration of hardened cement paste composition, with monosulfate aluminates phase converting to ettringite and, in later stages, gypsum formation. The necessary additional calcium is provided by the calcium hydroxide and calcium silicate hydrate in the cement paste The effect of these changes is an overall loss of concrete strength. The above effects are typical of attack by solutions of sodium sulfate or potassium sulfate. Solutions containing magnesium sulfate are generally more aggressive, for the same concentration. This is because magnesium also takes part in the reactions, replacing calcium in the solid phases with the formation of brucite (magnesium hydroxide) and magnesium silicate hydrates. The displaced calcium precipitates mainly as gypsum.

Sources of sulfates Oxidation of pyrite in clay formations in contact with concrete – this produces sulfuric acid which reacts with concrete. Bacterial activity in sewers – anaerobic sulfate reduction at work in the organic-rich sludges accumulated under water in the conduits produces hydrogen sulfide gas (H2S). After its released in the air of the galleries, H2S is further oxidized into sulfuric acid by atmospheric oxygen. In masonry, sulfates produced by the oxidation of pyrite in clay materials can be present in bricks. They are gradually released over a long period of time, causing sulfate attack of mortar, especially where moisture movement concentrates the sulfates. Seawater: sulfate is the second anion present in seawater after chloride.

Identification Sulfate attacks are identified through a remedial survey but they can often be overlooked when undertaking a damp survey as they can be considered as a structural rather than a dampness issue but moisture is required to promote the reaction. A first visual and leveling inspection of the structure and the underlying terrain is a first step to recognize a sulfate issue. To characterize the type and depth of the infill, exploration holes are needed. If water is present in the subfloor of the structure, a structural engineer may need to be instructed, subject to the level of damage or movement to the walls.

Remedial action The remedial action depends on the severity of the attack and on the risk related to its evolution. If repairs are required because of the extent of damages, often, the affected slab must be demolished and removed, the spoil should not be used as hardcore under the replacement slab.

See also Concrete degradation Pitting corrosion (effect of sulfur and sulfides)

References

Worked examples

Example 1 — a first encounter with Sulfate attack in concrete and mortar

Start with the simplest possible case. Write down what Sulfate attack in concrete and mortar claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Sulfate attack in concrete and mortar before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Sulfate attack in concrete and mortar ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Sulfate attack in concrete and mortar

In research: Sulfate attack in concrete and mortar appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Sulfate attack in concrete and mortar in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Sulfate attack in concrete and mortar is common in secondary-school and first-year university syllabi. It links to neighbouring topics Chemistry of construction methods, Concrete, Materials degradation, so understanding it makes those chapters shorter.
In everyday life: Look for Sulfate attack in concrete and mortar outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Sulfate attack in concrete and mortar in 20 minutes

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Frequently asked questions

What is Sulfate attack in concrete and mortar in simple terms?

Why does Sulfate attack in concrete and mortar matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Sulfate attack in concrete and mortar?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Sulfate attack in concrete and mortar.