The petrographic examination of concrete and concrete repairs - Concrete Repair - page 63

Sulphate attack
Sulphate attack is most commonly encountered in environments where
concrete is buried in ground containing sulphates. There are several forms of
sulphate attack and establishing the type of sulphate attack is of importance
in assessing the most appropriate repair materials. The thaumasite form of
sulphate attack (TSA) has been documented in the case of some motorway
structures on Liassic clay in Gloucestershire (Eden, 2003b) and equally
Figure 2.6 Thin section photomicrograph illustrating the thaumasite form of sulphate
attack within a concrete repair material at the contact with a concrete substrate.
The thaumasite occurs either side of a horizontal crack running from left to right
and appears bright in the image. A limestone coarse aggregate particle occurs at
the top of the field of view and the bottom of the field of view is occupied by the
Portland cement based matrix in the repair material. The development of sulphate
attack within the repair in this sample is related to the ingress of moisture containing
sulphate ions along the interface between the repair and substrate.
Figure 2.7 Thin section view showing the development of sulphate attack within a
repair mortar. Note the abundant cracks infilled with ettringite crystals that appear
mottled white to grey. The cracks occupy much of the field of view and are surrounded
by porous paste that contains limestone dust. A limestone aggregate particle occurs
on the right side of the field of view. The sulphate attack is related to the continued
ingress of moisture containing sulphate ions along the repair/substrate interface.