Civil Engineering Reference
In-Depth Information
The influence of lignosulfonate on cements containing different
amounts of SO
3
has been studied by conduction calorimetry.
[20]
The
hydration of the C
3
S component was retarded. In the presence of CLS, the
rate of heat evolution in the first few minutes is sustained longer suggesting
the immediate retarding effect of gypsum on C
3
A has not occurred, but an
acceleration of C
3
A hydration has resulted. In the first twenty-four hours,
for Type I cement containing 1.65% SO
3
the total heat was reduced by 26%
compared to that in the control sample. For the same cement with 2.15%
SO
3
, the corresponding reduction was only 14%. It was found that CLS
increased the optimum gypsum content. The cement low in C
3
A was more
strongly retarded and gave lower strengths.
The hydration of cement undergoes a significant modification in
the presence of a mixture of lignosulfonate and alkali carbonate. The heat
evolution peaks occur separated by two induction peaks. The possibility of
a highly anionic complex between lignosulfonate and carbonate ions has
been proposed to explain its more effective dispersing effect than lignosul-
fonate. The first induction period is due to the competitive interaction of one
or more of the admixture with C
3
A.
[21]
3.0
SUGAR-FREE LIGNOSULFONATE
Lignosulfonate is a complex molecule of high molecular weight. It
is derived from substituted phenyl propane containing hydroxy, methoxy,
phenolic, and sulfonate groups. The commercial lignosulfonates, not being
pure, contain varying amounts of sugars such as mannose, glucose,
xylose, galactose, arabinose, and fructose. Sugars are known to be good
retarders of setting of cement, hence, the major part of the retarding action
of a commercial lignosulfonate is believed to be due to the sugars
contained in it.
[8]
Conduction calorimetric curves of C
3
S hydrated up to 55 hours
show that the exothermic peak at about 7-8 hours found in the reference C
3
S
is retarded or completely annulled at dosages of about 0.3% sugar-free
Na-lignosulfonate.
[8]
Work of Monosi, et al.,
[13]
also has shown the
effectiveness of sugar-free lignosulfonate on C
3
S hydration. In the DTG of
C
3
S hydrated with or without 0.3% sugar-free lignosulfonate, the endother-
mal peak at about 500°C for Ca(OH)
2
decomposition is absent in the sample
containing sugar-free lignosulfonate (Fig. 10).
[13][22]
