Civil Engineering Reference
In-Depth Information
cement slurries with densities from 1 to 2.4 kg/lit at pumping times 2-6 hs,
and exposed to temperatures from freezing to 200°C and pressures up to 140
MPa are utilized. The types of admixtures used are many and they include
retarders (lignosulfonate-based), polyhydroxy organic acids of sugar ori-
gin, carboxymethyl/hydroxyethyl cellulose, etc. Accelerators are utilized
for lowering thickening times and increasing early strengths when cement
surface casing and conductor strings below 40°C. Accelerators are: calcium
chloride, sodium chloride, sodium silicate, sea water and calcium sulfate
hemihydrate, sodium aluminate, etc.
Strength retrogression is countered by the addition of active silica
sand or flour, natural pozzolans, or fly ash. Fluid loss controllers are used
to maintain constant w/s ratio. Polymers such as polyacrylamide or
polyethylene amines are also used. Many other types of admixtures are used
for various purposes such as gas migration controllers, weighting
agents, extenders, dispersants, thixotropic agents, defoamers, foamers,
coloring agents, etc. Details of these admixtures are described in a chapter
of Ref. 91.
7.13
Antifreezing Admixtures
Antifreezing admixtures are capable of depressing the freezing
point of water in concrete considerably. Antifreezing admixtures have been
used in Russia for concreting at temperatures of -30°C. These admixtures
allow earlier stripping and use of forms, earlier completion of construction
projects and occupancy. Many admixtures such as Na nitrite, Na chloride,
weak electrolytes, and organic compounds (high molecular weight alcohols
and carbamide) have been advocated. In Table 11, strength gain in concrete
cured at -5°C and containing antifreezing admixtures is given.
[40]
The
binary admixture at lower dosages imparts higher strengths than when used
individually. Some novel formulations have been proposed that contain
superplasticizers. Much more information has to be obtained before these
admixtures can be used in practice. Antifreezing admixtures are expensive,
especially as they are used in high dosages. Compatibility with other
admixtures and long term durability of concrete has yet to be established for
each mixture and curing regime.
