Sulfate attack in concrete and mortar

Last updated December 03, 2023

Cement hydration and strength development mainly depend on two silicate phases: tricalcium silicate (C₃S) (alite), and dicalcium silicate (C₂S) (belite).^[1] 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.

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.^[3] 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.

External attack

This is the more common type 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:^[3]

ŜH + CH → CSH₂ (cement chemist notation)

C₃A + 3 CŜH₂ + 26 H → C₃A·3CŜ·H₃₂

tricalcium aluminate + gypsum + water → ettringite

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

2 C₃A + C₃A·3CŜ·H₃₂ → 3 C₃A·3CŜ·H₁₂

tricalcium aluminate + ettringite → mono-sulfate 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:^[2]

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.^[2]

Excess of gypsum in concrete

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

C₃A·3CŜ·H₁₂ + 2 CSH₂ + 16 H → C₃A·3CŜ·H₃₂

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.^[4]
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.^[2]

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.^[2]

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.^[2]

History and literature

Sulfur has long been known to contribute to damage. This is true for many materials such as metal corrosion, or concrete degradation. In King Lear, Shakespeare says:^[5]

There’s hell, there’s darkness,
there is the sulphurous pit,
Burning, scalding, stench, consumption;
fie, fie, fie!

Related Research Articles

<span class="mw-page-title-main">Cement</span> Hydraulic binder used in the composition of mortar and concrete

A cement is a binder, a chemical substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel (aggregate) together. Cement mixed with fine aggregate produces mortar for masonry, or with sand and gravel, produces concrete. Concrete is the most widely used material in existence and is behind only water as the planet's most-consumed resource.

Portland cement is the most common type of cement in general use around the world as a basic ingredient of concrete, mortar, stucco, and non-specialty grout. It was developed from other types of hydraulic lime in England in the early 19th century by Joseph Aspdin, and is usually made from limestone. It is a fine powder, produced by heating limestone and clay minerals in a kiln to form clinker, grinding the clinker, and adding 2 to 3 percent of gypsum. Several types of portland cement are available. The most common, called ordinary portland cement (OPC), is grey, but white portland cement is also available. Its name is derived from its resemblance to portland stone which was quarried on the Isle of Portland in Dorset, England. It was named by Joseph Aspdin who obtained a patent for it in 1824. His son William Aspdin is regarded as the inventor of "modern" portland cement due to his developments in the 1840s. The term portland in this context refers to a material or process, not a proper noun like a place or a person, and should not be capitalized.

Cement chemist notation (CCN) was developed to simplify the formulas cement chemists use on a daily basis. It is a shorthand way of writing the chemical formula of oxides of calcium, silicon, and various metals.

<span class="mw-page-title-main">Calcium sulfate</span> Laboratory and industrial chemical

Calcium sulfate (or calcium sulphate) is the inorganic compound with the formula CaSO₄ and related hydrates. In the form of γ-anhydrite (the anhydrous form), it is used as a desiccant. One particular hydrate is better known as plaster of Paris, and another occurs naturally as the mineral gypsum. It has many uses in industry. All forms are white solids that are poorly soluble in water. Calcium sulfate causes permanent hardness in water.

Lime is an inorganic material composed primarily of calcium oxides and hydroxides, usually calcium oxide and/or calcium hydroxide. It is also the name for calcium oxide which occurs as a product of coal-seam fires and in altered limestone xenoliths in volcanic ejecta. The International Mineralogical Association recognizes lime as a mineral with the chemical formula of CaO. The word lime originates with its earliest use as building mortar and has the sense of sticking or adhering.

Ettringite is a hydrous calcium aluminium sulfate mineral with formula: Ca₆Al₂(SO₄)₃(OH)₁₂·26H₂O. It is a colorless to yellow mineral crystallizing in the trigonal system. The prismatic crystals are typically colorless, turning white on partial dehydration. It is part of the ettringite-group which includes other sulfates such as thaumasite and bentorite.

Ye'elimite is the naturally occurring form of anhydrous calcium sulfoaluminate, Ca^₄(AlO^₂)^₆SO^₄. It gets its name from Har Ye'elim in Israel in the Hatrurim Basin west of the Dead Sea where it was first found in nature by Shulamit Gross, an Israeli mineralogist and geologist who studied the Hatrurim Formation.

Alite is an impure form of tricalcium silicate, Ca₃SiO₅, sometimes formulated as 3CaO·SiO₂, typically with 3-4% of substituent oxides. It is the major, and characteristic, phase in Portland cement. The name was given by Törnebohm in 1897 to a crystal identified in microscopic investigation of Portland cement. Hatrurite is the name of a mineral that is substituted C₃S.

Belite is an industrial mineral important in Portland cement manufacture. Its main constituent is dicalcium silicate, Ca₂SiO₄, sometimes formulated as 2 CaO · SiO₂ (C₂S in cement chemist notation).

<span class="mw-page-title-main">Cement clinker</span> Main component of Portland cement

Cement clinker is a solid material produced in the manufacture of portland cement as an intermediary product. Clinker occurs as lumps or nodules, usually 3 millimetres (0.12 in) to 25 millimetres (0.98 in) in diameter. It is produced by sintering limestone and aluminosilicate materials such as clay during the cement kiln stage.

Tricalcium aluminate Ca₃Al₂O₆, often formulated as 3CaO·Al₂O₃ to highlight the proportions of the oxides from which it is made, is the most basic of the calcium aluminates. It does not occur in nature, but is an important mineral phase in Portland cement.

<span class="mw-page-title-main">Calcium aluminoferrite</span> One of the four main mineral phases of the Portland cement clinker

Calcium aluminoferrite is a dark brown crystalline phase commonly found in cements. In the cement industry it is termed tetra-calcium aluminoferrite or ferrite. In cement chemist notation (CCN), it is abbreviated as C^₄AF meaning 4CaO·Al^₂O^₃·Fe^₂O^₃ in the oxide notation. It also exists in nature as the rare mineral brownmillerite.

An AFm phase is an "alumina, ferric oxide, monosubstituted" phase, or aluminate ferrite monosubstituted, or Al₂O₃, Fe₂O₃ mono, in cement chemist notation (CCN). AFm phases are important hydration products in the hydration of Portland cements and hydraulic cements.

Thaumasite is a calcium silicate mineral, containing Si atoms in unusual octahedral configuration, with chemical formula Ca₃Si(OH)₆(CO₃)(SO₄)·12H₂O, also sometimes more simply written as CaSiO₃·CaCO₃·CaSO₄·15H₂O.

The alkali–silica reaction (ASR), also commonly known as concrete cancer, is a deleterious swelling reaction that occurs over time in concrete between the highly alkaline cement paste and the reactive amorphous silica found in many common aggregates, given sufficient moisture.

Calcium silicate hydrates are the main products of the hydration of Portland cement and are primarily responsible for the strength of cement-based materials. They are the main binding phase in most concrete. Only well defined and rare natural crystalline minerals can be abbreviated as CSH while extremely variable and poorly ordered phases without well defined stoichiometry, as it is commonly observed in hardened cement paste (HCP), are denoted C-S-H.

Friedel's salt is an anion exchanger mineral belonging to the family of the layered double hydroxides (LDHs). It has affinity for anions as chloride and iodide and is capable of retaining them to a certain extent in its crystallographical structure.

Concrete degradation may have many different causes. Concrete is mostly damaged by the corrosion of reinforcement bars due to the carbonatation of hardened cement paste or chloride attack under wet conditions. Chemical damages are caused by the formation of expansive products produced by various chemical reactions, by aggressive chemical species present in groundwater and seawater, or by microorganisms. Other damaging processes can also involve calcium leaching by water infiltration and different physical phenomena initiating cracks formation and propagation. All these detrimental processes and damaging agents adversely affects the concrete mechanical strength and its durability.

The pozzolanic activity is a measure for the degree of reaction over time or the reaction rate between a pozzolan and Ca²⁺ or calcium hydroxide (Ca(OH)₂) in the presence of water. The rate of the pozzolanic reaction is dependent on the intrinsic characteristics of the pozzolan such as the specific surface area, the chemical composition and the active phase content.

AFt Phases refer to the calcium Aluminate Ferrite trisubstituted, or calcium aluminate trisubstituted, phases present in hydrated cement paste (HCP) in concrete.

References

↑ Lea, F.M.; Hewlett, P.C. (1998). Lea's chemistry of cement and concrete (4th ed.). London: Arnold. ISBN 0340565896. OCLC 38879581.
1 2 3 4 5 6 Dawson, Adrian. "Certified Surveyors". Olympic Construction. Retrieved 2019-10-07.
1 2 Tian, Bing; Cohen, Menashi D (January 2000). "Does gypsum formation during sulfate attack on concrete lead to expansion?". Cement and Concrete Research. 30 (1): 117–123. doi:10.1016/S0008-8846(99)00211-2.
↑ "Sulfate attack in concrete". Understanding-cement.com. Retrieved 2015-03-03.
↑ Neville, Adam (2004-08-01). "The confused world of sulfate attack on concrete". Cement and Concrete Research. 34 (8): 1275–1296. doi:10.1016/j.cemconres.2004.04.004. ISSN 0008-8846 . Retrieved 2022-02-22.