Sulfate attack on concrete has been reported from many other
parts of the world. As early as 1936 the concrete construction
manual published by the U. S. Bureau of Reclamation warned that
concentrations of soluble sulfates greater than 0.1 percent in soil
may have a deleterious effect onconcrete, and more than 0.5 percent
soluble sulfate in soil may have a serious effect.
Most soils contain some sulfate in the form of gypsum (typically
0.01 to 0.05 percent expressed as SO4); this amount is harmless to
concrete. Higher concentrations of sulfate in ground waters are
generally due to the presence of magnesium and alkali sulfates.
Decay of organic matter in marshes, shallow lakes, mining pits,
and sewer pipes often leads to the formation of H2S, which can be
transformed into sulfuric acid by bacterial action.Thus, it is not
uncommon to find deleterious concentrations of sulfate in natural
and industrial waters.
When concrete cracks, its permeability increases and the
aggressive water penetrates more easily into the interior, thus
accelerating the process of deterioration. Sometimes, the expansion
of concrete causes serious structural problems.
Sulfate attack can also take the form of a progressive loss of
strength and loss of mass due to loss ofcohesiveness in the cement
hydration products.
Deterioration of hardened Portland cement paste by gypsum
formation goes through a process that, atfirst leads to reduction
of stiffness and strength, then by expansion and cracking, and
eventually the transformation of concrete into a mushy or
non-cohesive mass.
Na2SO4 +Ca(OH) 2 +2H2O CaSO4.2H2O + 2NaOHthe formation of sodium
hydroxide as a byproduct of the reaction ensures the continuation
of high alkalinity in the system, which is essential for the
stability of the cementitious material C-S-H.
MgSO4 +Ca(OH) 2 +2H2O CaSO4.2H2O + Mg(OH) 23 MgSO4 + 3CaO .2SiO2
.3H2O + 8 H2O -> 3CaSO4.2H2O + 3 Mg(OH) 2 + 2SiO2.H2OFormation
of relatively insoluble magnesium hydroxide. In the absence of
hydroxyl ions in the solution C-S-H is no longer stable and is also
attacked by the sulfate solution. The magnesium sulfate attack is,
therefore, more severe on concrete. Amount and nature of the
sulfate present, level of the water table and its seasonal
variation, flow of groundwater and soil porosity, form of
construction, quality of concrete.
The quality of concrete, specifically a low permeability, is the
best protection against sulfate attack. Adequate concrete
thickness, high cement content, low water/cement ratio and proper
compaction and curing of fresh concrete are among the important
factors that contribute to low permeability.
ACI 318
Negligible attack: When the sulfate content is under 0.1 percent
in soil, or under 150 ppm (mg/liter) in water, there shall be no
restriction on the cement type and water/cement ratio.
Moderate attack: When the sulfate content is 0.1 to 0.2 percent
in soil, or 150 to 1500 ppm in water, ASTM Type II Portland cement
or Portland pozzolan or Portland slag cement shall be used, with
less than an 0.5 water/cement ratio for normal-weight concrete.
Severe attack: When the sulfate content is 0.2 to 2.00 percent
in soil, or 1500 to 10,000 ppm in water, ASTM Type V Portland
cement, with less than an 0.45 water/cement ratio, shall be
used.
Very severe attack: When the sulfate content is over 2 percent
in soil, or over 10,000 ppm in water, ASTM Type V cement plus a
pozzolanic admixture shall be used, with less than an 0.45
water/cement ratio.
