C-S-H is the phase playing the role of the glue in the cement hardened paste and responsible of its cohesion.
[2] Sulfates dissolved by ground moisture migrate into the concrete of the slab where they react with different mineral phases of the hardened cement paste.
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.
Sulfate ions diffusing into concrete react with portlandite (CH) to form gypsum:[3] 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:[2] 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] A first visual and leveling inspection of the structure and the underlying terrain is a first step to recognize a sulfate issue.