strength-gain characteristics under subzero curing conditions are much better than for port-
land cements. CAC has been used successfully in the past for cold weather construction,
and in fact manufacturers of CAC provide data sheets promoting its use at temperatures
below 3C (27F), and claim successful results at temperatures as low as 40C (38F).
It may be noted that the heat liberation rate of freshly hydrated CAC can be as high as 9 cal/
g per hour, which is about three times as high as the rate for high-early-strength portland
cements. Although CAC does not set faster than portland cement, it does develop strength
and evolve heat at a much faster rate after the initial set has commenced. After 24 hours, the
strength of CAC concrete can reach 90% of its ultimate strength, and its rapid strength gain
is accompanied by rapid evolution of heat. For this reason, CAC, after initial setting, can
tolerate much colder ambient conditions than its portland cement counterpart. However,
CAC concrete must be protected from freezing, in much the same way as portland cement
concrete, until setting and some strength development has occurred.
CAC hydration can also be accelerated like portland cement by the addition of small
quantities (~ 0.5 wt% of the water) of various compounds. Among many candidates, LiCl
has been reported to produce the greatest effect. In terms of decreasing effectiveness, the
cations have been found to follow the order Li << Na < none < K < Ca < Mg < Sr < NH4,
while the anions follow the order OH << none << Cl < NO3 < Br < acetate. Hydroxy
compounds, such as sodium and calcium hydroxide, generally accelerate the CAC hydra-
tion, whereas both magnesium and barium hydroxides have been found to act as retarders.
Citric acid is the most common retarder for the system, although glycols, glycerine, sugars,
casein, and chloride salts such as NaCl, KCl, CaCl2, and MgCl2 can also work as retarders.
It has been found also that up to 20 wt% portland cement addition to CAC accelerates its
set reaction, and at the same time, small quantities of CAC act as set accelerators for the
portland cement. Various proprietary formulations based on the mixture of portland ce-
ment and CAC have been marketed, and preliminary research work conducted on these
versa, is variable and depends strongly on the type and source of the CAC. Therefore, prior
trial runs should be done before any combination is used for field applications. It was also
found that when fondu cement was accelerated with lithium carbonate, it performed well in
terms of strength development on small-size specimens; however, on larger-size speci-
mens, the strength development was very poor at room temperature, probably because of
the large amount of heat generation, which might have altered the nature and morphology
of the crystalline calcium aluminate hydrates.
Currently, the major use of CAC is as refractory cements where chemical bonds develop
at high temperature, obliterating the effect of any conversion reactions. However, for use
under cold weather conditions, CAC seems to possess many of the required properties,
although the deleterious strength retrogression during service in warmer climates must be
taken into consideration before the concrete is designed for a particular job.
Calcium-sulfate (gypsum) cement
Crystallization of gypsum needles from a hydrated gypsum cement is the cause of set-
ting and hardening that has been exploited by the wallboard industry. However, gypsum is
not stable in water and therefore gypsum cement is nonhydraulic. Because of this poor
moisture resistance, its use has been mainly reserved for indoor applications.
Although unmodified gypsum cements have restricted use, the material CaSO4 has been
used as additives in many cement blends and formulations for both portland cements and
CAC to produce cement formulations for low-temperature applications.
Magnesium phosphate cement
If fast-setting and cold-weather characteristics are required simultaneously, then the most
cost-effective alternative are the phosphate cements based on MgOammonium phosphate
water chemistry. These cements are not compatible with regular portland cement. Labora-
tory tests have been performed to show that these cements can be formulated to set up