Geoscience Reference
In-Depth Information
most likely cause of internal sulfate attack is the use of
sulfate- or sulfide-contaminated aggregates. These may
release sulfate ions within the concrete that may react
with the cement matrix to form ettringite and/or gypsum
as described on p. 102. Although less common, internal
sulfate attack can result from cement or mixing water
with high sulfate contents. In thin section, the
appearance of concrete suffering from internal sulfate
attack includes the occurence of a network of cement
matrix cracks, that along with air voids, are filled with
secondary deposits of ettringite or gypsum. Unlike
external sulfate attack, evidence of deterioration will not
be confined to the surfaces.
Internal sulfate attack is responsible for 'the mundic
problem' in southwest England. Here, the expansive
cracking of concrete blocks and mass concrete has been
caused by the use of mining waste aggregate containing
pyrite (FeS
2
). The mechanisms of deterioration involve
oxidation of the pyrite (Bromley & Sibbick, 1999).
Consequently the equity value and resale potential of
many, mainly pre-1950s domestic properties have been
adversely affected by uncertainties about the nature of the
concrete used in their original construction. A
petrography-based sampling and testing procedure
(described in Stimson, 1997) was therefore adopted to
determine if buildings contained concrete with potentially
expansive aggregate (and the likely risk of future
deterioration). The procedure involves taking
representative core samples (50 mm diameter) from the
concrete elements of the building and examining them in
hand specimen using a low-power microscope (and in thin
section if uncertainties arise). This screens for evidence of
deterioration and is used to classify the aggregate
lithologies present into either Group 1 (inert) or Group 2
(potentially deleterious). A detailed atlas of concrete
aggregates used in southwest England has been collated by
Bromley (2002). Figure
208
shows concrete blocks made
with china clay waste, which is a commonly seen Group
1 aggregate. Figure
209
shows concrete made with
potentially deleterious crushed metasedimentary mining
waste aggregate, which is classified as Group 2. Providing
that the concrete appears sound and contains less than
30% of Group 2 aggregate then the property is likely to be
considered safe for mortgage purposes by lenders. Since its
introduction, the petrographic assessment procedure has
enabled approximately 80% of previously blighted houses
in southwest England to be saleable again.
Delayed ettringite formation (DEF) is a rare but
potentially severe form of internal sulfate attack of concrete
(Quillin, 2001). A certain amount of ettringite (hydrous
calcium sulfoaluminate) forms as part of the normal
cement hydration products. However, if the concrete is
subjected to elevated temperatures during curing (>65°C)
this ettringite formation may be 'delayed' and if the
hardened concrete is subsequently exposed to moisture, the
ettringite may re-form causing expansion and cracking,
months or years after construction. Concrete may be
exposed to high temperatures during curing as a result of
208
209
208
Concrete block from southwest England
containing Group 1/1 china clay waste aggregate,
consisting chiefly of quartz (grey), with minor feldspar
(grey), muscovite mica (brightly coloured), and
tourmaline (orange). The uncarbonated cement matrix
appears black; XPT, ×35.
209
Concrete block from southwest England containing
Group 2/1 crushed phyllite ('killas') aggregate. The
aggregate particles are coated by iron oxide (dark red)
and the cement matrix is cracked (yellow) suggesting
deterioration due to pyrite oxidation. The carbonated
cement matrix appears brown; XPT, ×35.
