Chemical admixtures - Concrete Mix Design, Quality Control and Specification - page 79

The classification used in BS EN 934-2 is water resisting admixture,
which would be a technically correct to refer to water resisting admixtures
for hydrostatic and nonhydrostatic conditions. Water resisting admixtures
are divided into crystalline, hydrophobic, and hydrophobic pore blocking.
Crystalline admixtures were developed from surface applied products that
were designed to penetrate voids and cracks in the concrete. The suppliers
claim that the crystals, which accelerate the autogenous healing capabilities
of concrete, are able to grow to fill and block static cracks up to 0.4 mm. The
silicate reacts with calcium hydroxide (produced by the cement hydration
process) to form a calcium silicate hydrate (C-S-H) similar to that formed by
cement hydration but with a variable hydrate concentration (CSHn) (Trinder,
2000). Although much of the information available on these products is pro-
prietary, scanning electron micrographs show crystal formation in capil-
lary pores suggesting that they should be able to fill fine cracks and voids
provided the conditions are appropriate. Mitsuki et al. (1992) found crystal
growth occurring and concluded from qualitative analysis of the concrete
by an energy scattering x-ray analysis procedure that the needlelike crys-
tals were C-S-H. Anecdotal evidence suggests beneficial effects of crystalline
admixtures should be at least as good as traditional autogenous healing.
There are a number of studies that indicate improved chemical resistance
with the use of such products. For example, Trinder et al. (1999) found that
the addition of a crystalline admixture substantially improved resistance to
ammonium sulfate compared to a reference concrete. Yodmalai et al. (2009)
showed reduced surface chloride content and apparent chloride diffusion
coefficient for both the crystalline admixture and surface treatment. These
short-term studies suggest that the resultant crystals appear to be durable
and of some benefit to the concrete. Although the longer-term durability
of such crystals has not been independently established, C-S-H crystals
(the basic chemistry of the crystals and cementitious hydration) do have a
long history. The problem of evidence for longer-term durability exists with
many materials. Tests on penetrability properties of concrete (absorption,
permeability, and chloride diffusion) have tended to show limited effect
of crystalline admixtures on these parameters compared to a comparable
reference concrete of reasonable quality.
In the opinion of the authors, the primary advantage of crystalline admix-
tures is in enhancing autogenous healing of cracks and voids in concrete,
which is an important component in achieving watertightness in real struc-
ture. Although there appears to be little evidence of reduced penetrability in
higher quality concrete, there is evidence of improved chemical resistance.
The next type of water resisting admixture is the hydrophobic admixture.
Hydrophobic admixtures are believed to form a thin water-repellent layer
within the pores and voids in the concrete matrix that exhibit high contact
angles (θ) to water. Rixom and Mailvaganam (1986) suggest that concrete
containing hydrophobic admixtures can be considered to have a contact