Page 196 - Handbook of Thermal Analysis of Construction Materials

180 Chapter 4 - Introduction to Concrete Admixtures

protective layer may be severely affected under salt exposure conditions.
One way to improve the resistance of concrete is to enhance its imperme-
ability by incorporation of silica fume, fly ash, slag, or polymer modified
mortar. Some chemicals known as corrosion inhibitors have been advo-
cated to counter corrosion. They are Ca/Na nitrites, Na benzoate, Na/K
chromates, Na salts of silicates and phosphates, stannous chloride, hydra-
zine hydrates, etc. Extensive work has been carried out on the effect of
nitrite-based compounds, which have shown promise as efficient corrosion
inhibitors.

7.10 Alkali-Aggregate Expansion Reducing Admixtures

The alkali-aggregate expansion involves chemical interaction be-
tween the alkali hydroxides derived from the cement and reactive compo-
nents of aggregates. In the alkali-silica reaction, hydrolysis of reactive silica
and alkali forms alkali-silica gel. The alkali-silicate reaction is the result of
aggregate containing greywackes. The alkali-carbonate reaction is differ-
ent from others as the affected concrete does not contain any significant
amounts of silica gel. The effective methods of counteracting this expan-
sion are: use of silica fume, fly ash, slag, air-entrainment, and chemical
admixtures. Of the chemicals studied lithium salts, Cu sulfate, Al
powder, and some proteins were found to be effective in decreasing the
alkali-aggregate expansion reaction. [90]

7.11 Polymer-Modified Mortars/Concrete

These admixtures are used to increase the bond strength in repair
applications, to decrease shrinkage, increase tensile strength, etc. The
polymers used include latexes, redispersible polymer powders, water-
soluble polymers, liquid resins, and monomers. In practical applications,
styrene butadiene rubber, polyacrylic ester and polyvinylidene chloride-
vinyl chloride, methylcellulose, etc., have been used.

7.12 Admixtures for Oil Well Cements

Oil well cement needs to have the properties such as low permeabil-
ity, satisfactory bond with the rock or bore holes. Depending on conditions,