Understanding defects, testing and inspection - Concrete Repair - page 18

Figure 1.12 Concrete with severe shrinkage cracking due to shrinkable aggregates.
Figure 1.13 Freeze thaw damage to a concrete slab - note the loose aggregate exposed
as the cement matrix erodes (photo courtesy of the Concrete Society).
Sulfate attack
Concrete buried in soils or groundwater containing high levels of sulfate
salts, particularly in the form of sodium, potassium or magnesium salts, may
be subjected to sulfate attack under damp conditions. An expansive reaction
occurs between the sulfates and the C3A phase of the hydrated cement to
form calcium sulfoaluminate (ettringite) with consequent disruption to the
matrix. As a guide, levels of sulfate above about 4% of cement (expressed
as SO 3 ) may indicate the possibility of sulfate attack, provided sufficient
moisture is present. Sulfate attack requires prolonged exposure to damp
conditions. However, there has been recent concern with another form of
sulfate attack, as follows