For this reason, the low-temperature properties of these phases or blended cements have
not been thoroughly studied. Moreover, because of limited use, large cement producers do
not usually market these special cements. On the other hand, companies who sell these
materials are very secretive about releasing any data on the cements except strength.
High iron cements
Just like the previous two modified portland cements, the high iron cements derive their
rapid setting and hardening characteristics from the formation of large amounts of ettringite
during the early hydration period. However, unlike the VHE cement, both sulfoaluminate
(C4A3S) and C4AF phases provide the aluminate ions. This cement is still under develop-
ment and may have considerable appeal to the construction industry again, perhaps for
Pyrament was the trade name given to a blended hydraulic cement product manufac-
tured by Pyrament, Inc., of Houston, Texas. This cement was the result of a new invention
by the company and was marketed as cement compositions curable at low temperatures. It
was claimed that by employing the formulation of the invention, contrary to using existing
cements, cures could be effected at temperatures well below the freezing point of water
and, in fact, cure could be accomplished at temperatures as low as 16F (3.2F).
This cement is composed of portland cement, slag, pozzolans including metakaolin, and
admixtures including potassium carbonate and water-reducing agents. Use of potassium
carbonate with the metakaolin is required to ensure continuing cure of the cement at tem-
peratures below the freezing point of water.
Calcium aluminate cement
Compared to portland cement, calcium aluminate cement (CAC) possesses many unique
properties, such as high early strength, hardening even under low-temperature conditions,
and superior durability to sulfate attack. However, CAC is not recommended for structural
use because the hardened cement can experience gradual strength retrogression. Therefore,
in most countries, CAC is now used mainly for making castable refractory linings for high-
temperature furnaces. However, since the cement has some desirable characteristics, fur-
ther examination of its properties has been advocated.
CAC is the product obtained by pulverizing calcium aluminate cement clinker manufac-
tured by partially or completely fusing calcareous materials, such as limestone or chalk,
and an alumina source, such as bauxite, to convert them to hydraulic calcium aluminates.
This is the reason why in France and Germany the cement is called ciment fondu and
tonerdeschmelz zement, respectively. Thus, unlike portland and modified portland cements,
in which di- and tricalcium silicates are the principle cementing compounds, in CAC the
monocalcium aluminate (CA) is the principal cementing compound, with C12A7, CA2,
C2AS, β-C2S, and Fss (iron solid solution) as minor compounds. Typically, the chemical
analysis of ordinary CAC corresponds to approximately 40 wt% Al2O3, and some cements
contain even higher alumina content (50 to 80 wt%). For these high alumina contents, CAC
is also referred to as high-alumina cement (HAC).
The bauxite ore used to produce CAC contains a considerable amount of iron as an
impurity, which accounts for the 10 to 17 percent iron (expressed as Fe2O3) usually present
in ordinary CAC. This is why, unlike portland cement clinker, the CAC clinker containing
high iron is in the form of completely fused melts that are made in specially designed
furnaces. On the other hand, cements meant for high-temperature applications containing
very low iron and silica are made by sintering in rotary kilns.
Although CAC materials have setting times comparable to ordinary portland cement,
the rate of strength gain at early ages is quite high, mainly due to the high reactivity of CA.
Within 24 hours of hydration, the strength of normally cured CAC concrete can attain
values equal to or exceeding the seven-day strength of ordinary portland cement. Also, the