Sulfate attack in concrete and mortar

Sulfate attack in cement, mortar and concrete.

Cement is primarily composed of two minerals: tri-calcium silicate (C3S), and di-calcium silicate (C2S).[1] Upon hydration, the main reaction products are calcium silicate gels (C-S-H) and calcium hydroxide Ca(OH)2, written as CH in cement chemistry notation. Moisture is necessary for this reaction which can cause serious structural damage to both walls and wall slabs in buildings.

Sulfate attacks typically happen to ground floor slabs. This issue generally affects properties from the 1950s and 1960s but can affect earlier structures where a concrete floor slab has been installed.[2] They occur when the infill material beneath the slab contains sulphates and these are taken up into solution by ground moisture which then migrates into the concrete which forms the floor slab.

The attacks can come from MgSO4 salts, NaSO4 salts, and other salts containing SO3- ions. The interaction of Ca2+ ions with SO4 present in the solution will produce CaSO4 or gypsum. The effect of gypsum on C-S-H gel, which is the principal component of hardened cement, is a debatable topic.[3] Other components present in cement, such as tri-calcium aluminate, also interact with sulfate ions. This reaction is properly established in literature.

They can be 'external' or 'internal'.

External attackEdit

This is the more common type and typically occurs where water containing dissolved sulfate penetrates the concrete. Sulfate ions that penetrate the concrete react with CH to form gypsum[3]

ŜH+CH→CSH2 (Cement chemist notation)

C3A + 3CŜH2 + 26H→C3A.3CŜ.H32

tricalcium aluminate + gypsumettringite

when concentration of sulfate ions decrease the ettringite breaks down into monosulfates

2C3A + C3A.3CŜ.H32 → 3C3A.3CŜ.H12

When it reacts with concrete, it causes the slab to expand, lifting, distorting and cracking as well as applying pressure to surrounding walls which can mean structural movement, significantly weakening the concrete.

The main infill materials to a solid floor which result in a sulphate attack:[2]

-       Red Ash (Shale)

-       Black Ash

-       Slag

-       Grey Fly Ash

-       Other industrial materials and building rubble could be used and present a potential problem

These materials originated were used extensively in the North West of England as they were widely available and virtually free from sources such as Coal Mines, Steelworks, Foundries and Power Stations.[2]

Internal attackEdit

This form occurs when the source of sulfates is excess gypsum . The gypsum present in the concrete it reacts with monosulfates to form ettringite

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

A fairly well-defined reaction front can often be seen in polished sections; ahead of the front the concrete is normal, or near normal. Behind the reaction front, the composition and microstructure of the concrete will have changed. 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 paste composition, with monosulfate 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.

Other sources of sulfate which can cause sulfate attack includeEdit

  • Seawater
  • Oxidation of sulfide minerals in clay adjacent to the concrete - this can produce sulfuric acid which reacts with the concrete
  • Bacterial action in sewers - anaerobic bacterial produce sulfur dioxide which dissolves in water and then oxidizes to form sulfuric acid
  • In masonry, sulfates present in bricks and can be gradually released over a long period of time, causing sulfate attack of mortar, especially where sulfates are concentrated due to moisture movement.[4]

How it is identifiedEdit

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 what causes the reaction.[2]

Initially a visual and levelling inspection to the property will be sufficient to recognise there is a sulfate issue. To establish the type and depth of infill, trial holes will need to be used.

If water is present in the subfloor structure, a structural engineer may need to be instructed, subject to the level of damage or movement to the walls.[2]

What remedial action can be taken?Edit

The action depends on how severe the attack is and how high the future risk is. Sometimes inspections are related to mortgages therefore it may also depend on the degree of assurance required for a lender.

If repair is required for the level of damage, the slab must be broken out and removed, the spoil should not be used as hardcore under the replacement slab.[2]

See alsoEdit


  1. ^ Lea, F.M.; Hewlett, P.C. (1998). Lea's chemistry of cement and concrete (4th ed.). London: Arnold. ISBN 0340565896. OCLC 38879581.
  2. ^ a b c d e f Dawson, Adrian. "Certified Surveyors". Olympic Construction. Retrieved 2019-10-07.
  3. ^ a b 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.
  4. ^ "Sulfate attack in concrete". Retrieved 2015-03-03.