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Sulfate attack in concrete and mortar
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<p>Cement hydration and strength development mainly depend on two silicate phases: <a href="/facts/Alite/IkG9wHey">tricalcium silicate</a> (C3S) (<a href="/facts/Alite/IkG9wHey">alite</a>), and <a href="/facts/Belite/UMfP7Abp">dicalcium silicate</a> (C2S) (<a href="/facts/Belite/UMfP7Abp">belite</a>). Upon <a href="/facts/Hydration_reaction/T1Vb2u1d">hydration</a>, the main reaction products are <a href="/facts/Calcium_silicate_hydrate/rYewAGuv">calcium silicate hydrates</a> (C-S-H) and <a href="/facts/Calcium_hydroxide/Kz2KzRd8">calcium hydroxide</a> Ca(OH)2, written as CH in the <a href="/facts/Cement_chemist_notation/VgxwK98o">cement chemist notation</a>. 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 <a href="/facts/Ettringite/yN9up1Ur">ettringite</a>, a calcium aluminoferrite trisulfate (AFt). C4AF hydrates as <a href="/facts/Hydrogrossular/vffjhrgL">hydrogarnet</a> and ferrous ettringite. </p><p>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. </p><p>The attack arises from soils containing SO2−4 ions, such as MgSO4 or Na2SO4 soluble and hygroscopic salts. The <a href="/facts/Tricalcium_aluminate/IfHadBXN">tricalcium aluminate</a> (C3A) hydrates first interact with sulfate ions to form <a href="/facts/Ettringite/yN9up1Ur">ettringite</a> (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 <a href="/facts/Stress_(mechanics)/ErAXDbbd">tensile stress</a> in the concrete matrix causing the formation of cracks. Ultimately, Ca2+ ions in equilibrium with <a href="/facts/Portlandite/vAo4Y3St">portlandite</a> (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 (<a href="/facts/Gypsum/MItYU70n">gypsum</a>). 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. </p>