Properties of concrete - Concrete Mix Design, Quality Control and Specification - page 91

aggregates, superfine CaCO 3 , or dissolved carbonate in the water. One of
the greatest threats of thaumasite attack is friction piles where a surface
thaumasite attack would effectively eliminate the friction between the pile
and the ground. BRE Special Digest 1 provides a full discussion of the
problem and recommendations. GGBS is considered particularly helpful
for thaumasite resistance (Neville, 2011). Surface carbonation has been
found to improve resistance to thaumasite.
5.1.6 Physical salt attack
Physical salt attack is the deterioration of concrete (and masonry) due to the
accumulation of salt due to evaporation. This can be a problem, particu-
larly where there are high concentrations of sodium sulfate. The problem
can also be called salt weathering, salt damp, salt crystallisation or physical
salt distress. Physical salt attack is considered to be caused by the cycling of
sodium sulfate between its anhydrous and hydrated forms, which results in
an expansion of over three times. Other salts may also be involved. Other
researchers have suggested that it is caused by the supersaturated salt solu-
tions. The key parameter involved in the accumulation of the salts appears
to be sorptivity.
5.1.7 Chemical attack
Portland cement based concrete is not resistant to acid, although concrete
with a low penetrability will be attacked less rapidly. Supplementary
cementitious materials tend to improve chemical resistance. Supersulfated
cement, which contains 80% to 95% GGBS, has been found to be resistant
to acidic conditions down to a pH of 3.5. Unfortunately supersulfated
cement is no longer readily available. Surface carbonation has been found
to improve acid resistance.
High alumina or calcium aluminate cements have been found to be
resistant to acids and a range of other chemicals. Provided they are used in
accordance with the manufacture's recommendations to limit conversion
issues, they are a suitable material and are making a resurgence in sewer
linings. Interestingly, some geopolymer concrete is also resistant to acid
attack.
Proven high performance water resisting admixtures have been found
to greatly increase resistance of Portland cement concrete to a range of
chemicals.
Distilled or soft water will also attack Portland cement based concrete
by leaching the calcium hydroxide and then decalcification of the calcium
silicate hydrate (C-S-H). Fertilisers such as ammonium nitrate also tend to
leach Portland cement concrete and cause progressive deterioration.