Page 165 - Handbook of Thermal Analysis of Construction Materials

Section 2.0 - Accelerators 149

2.2 Effect of Calcium Chloride on Calcium Aluminate

Two chloroaluminate hydrates are known, viz., the low form
C A•CaCl •xH O and the high form C A• CaCl •yH O. It is generally
3 2 2 3 3 2 2
believed that the low form is the main reaction product formed under
practical conditions of hydration. [19][21][22] The low form crystallizes as
hexagonal plates and the high form as needle-shaped crystals. The basal
spacing for the low form is 8.1 Å and that for the high form is 10.15 Å. The
Differential Thermal Analysis (DTA) technique may be used to differenti-
ate between the two forms of chloroaluminate. Endotherms at about 190°C
and 350°C are caused by the monochloroaluminate, and the endotherm at
about 160°C is exhibited by the higher chloroaluminate. [20]
In the C A-gypsum-CaCl -H O system, substantial amounts of
2
3
2
chloride are immobilized within an hour whereas lower amounts of chloride
are immobilized by C S and cement after longer times. [23] The reaction
3
between C A and gypsum is accelerated by calcium chloride.
3
Monochloroaluminate is formed after gypsum is consumed in the reaction
with C A. Conversion of ettringite to monosulfoaluminate occurs only after
3
all CaCl has reacted. [3][5][21][22][24]–[27] The cubic aluminate hydrate (C AH )
2
3
6
also reacts with CaCl to form monochloroaluminate hydrate, but the
2
reaction is slower than that with C A as the starting material. [28]
3
In the hydration of C A in the presence of CaCl at temperatures of
3 2
75–100°C, calcium chloroaluminate is identified. [26][29] Using different
amounts of gypsum and CaCl , it has been found that the acceleration of the
2
reaction of gypsum with C A is accompanied by the generation of strains
3
and consequent loss in strengths. [25]
In the system C S-C A-CaCl -H O, more C AH is formed and
2
2
3
3
13
4
better strengths attained than that without CaCl . [30] The main factor may
2
be the accelerated hydration of the C S phase.
3
In the portland cement paste containing different amounts of
CaCl , the hydration of the tricalcium aluminate phase was followed as a
2
function time. [31] The results showed that tricalcium aluminate phase
reacted to form compounds such as chloroaluminates, sulfoaluminates, and
aluminate hydrates.
It appears that much less work has been done on the effect of CaCl 2
on the hydration of the ferrite phase than on other cement minerals. It is
generally assumed that the sequence of reactions in the ferrite phase is
similar to that of the tricalcium aluminate phase except that the reactions are
slower.