Page 418 - Handbook of Thermal Analysis of Construction Materials

Section 9.0 - Jet Set-Based Cement Sysyems 395

9.2 Durability

The sulfate resistance of Reg Set cement concrete to magnesium
and sodium sulfate solutions is generally poor. However, it appears that Reg
Set containing larger amounts of fluoroaluminate promotes better resis-
tance to sulfate attack. [76] A 20% replacement of Reg Set cement by fly ash
provides a substantial improvement in sulfate resistance; this is, at least
partly, due to the consumption of CH by fly ash.
The curing of Reg Set cement concrete at elevated temperatures (up
to 38°C) appears to have no adverse effects; it even results in increased
strengths. Concrete made with Reg Set cement having greater than normal
amounts of C A •CaF and anhydrite appears to be more frost resistant,
11 7 2
giving comparable frost resistance to an air-entrained portland cement
concrete.
Concrete with a high early strength development is desirable for
cold weather concreting. Reg Set cement concrete can be used for this
application. Strength determinations for Reg Set concrete cured for one
hour at 20°C and subsequently at –10°C for 28 days give similar strengths
to those for Reg Set concrete cured continuously at 20°C for the same
period. [80]

9.3 Gypsum

Calcium sulfate in the form of gypsum (CaSO •2H O) is added in
4 2
small quantities to the other constituents of cement during its manufacture
to control setting. The calcium sulfate can also be the hemihydrate
(CaSO •1.2H O); anhydrite (anhydrous calcium sulfate, CaSO ) or a
4
4
2
combination. The use of blends of gypsum (dehydrated to the more soluble
hemihydrate) and natural anhydrite can be effective in controlling stiffen-
ing or false-setting problems.
Pure gypsum (CaSO •2H O) contains 79.1% of calcium sulfate
4
2
and 20.9% of water or 32.5–32.6% calcium oxide, 46.5–46.6% of sulfur
trioxide, plus the water. Gypsum crystallizes in the monoclinic system.
Well-formed crystals are colorless, transparent, and flattened parallel to the
plane of symmetry. Commercial gypsum is massive consisting of an
aggregate of crystals which have interfered with the growth of one another.
Gypsum when heated at temperatures between 128° and about
163°C loses approximately 75% of its hydrate water and hemihydrate
forms. This is mixed with some gypsum and “soluble anhydrite.” This