Page 133 - Handbook of Thermal Analysis of Construction Materials

116 Chapter 3 - Formation and Hydration

A semi-isothermal method of derivative thermogravimetry has
been suggested for better resolution of thermal effects and for quantitative
analysis of calcium hydroxide in cement pastes. [61] The formation of several
compounds during the hydration of cement results in interfering effects.
Butler and Morgan [62] dehydrated cement at 200°C in N , followed by
2
carbonation and subsequent thermal decomposition of CaCO . Only cal-
3
cium hydroxide was carbonated. Although it was not possible to determine
C-S-H accurately, it was possible to determine by DSC other products such
as sulfoaluminate, hexagonal aluminate hydrates, cubic aluminate hydrate,
and calcium hydroxide.
A cement exposed to moisture and carbon dioxide during storage
may partially hydrate and carbonate. The kinetics of hydration and strength
development of cement may be affected. DTA/TG techniques have been
applied to identify the hydrated products in such a cement. Thiesen and
Johansen [74] investigated the effect of prehydration of cement on compres-
sive strength. The degree of adsorbed water was determined by TG and the
compounds (mainly calcium aluminate-based compounds) were identified
by DTA. DTA showed the formation of ettringite when the cement was
exposed to a low temperature and a humid atmosphere. In the TG curve,
weight loss occurred in four stages. From these losses the amount of
combined water with clinker minerals was calculated. In commercial
clinkers the combined water varied from 0.15 to 0.30%. Each cement has
a critical value above which strength may deteriorate. Strengths, in
certain severe cases were reduced by 30%.
It is important to acquire information on the hydration processes
that occur when cement is subjected to low temperatures. Even at tempera-
tures below zero cement hydrates slowly, as water does not freeze due to the
presence of soluble materials in the pores. The products that form at low
temperatures are calcium aluminate hydrates at earlier periods and after
several months, the products of hydration of tricalcium silicate. A weak
endothermal peak at about 330°C could be due to the presence of silica gel
and this is preceded by the appearance of calcium hydroxide and C-S-H
gel. [5]
Supersulfated cement has a lower heat of hydration and shows
better resistance to sulfate attack than normal portland cement. It has lower
CH contents and most of aluminum is bound as ettringite. This cement may
contain 80–85% slag, 10–15% anhydrite, and 5% activator. The main
hydration products are C-S-H and ettringite. A comparison of the conduc-
tion calorimetric curves of the supersulfated cement with that of normal