Page 363 - Handbook of Thermal Analysis of Construction Materials

340 Chapter 8 - Supplementary Cementing Materials

physico-chemical and durability characteristics. Some work has also been
carried out to investigate the influence of polymers on the hydration of
C S. [92] In Fig. 34, DTA curves of neat C S pastes (a) are compared with the
3
3
paste mixed with polyvinyl sulfonic acid (PESA)(b). [92] The sample ESA(c)
was prepared by using vinyl sulfonic acid monomer with an initiator
potassium persulfate and sodium bisulfite. In the figure, the period of curing
is shown in the circles. The endotherm in the range 100–300°C (not shown)
is mainly attributable to the loss of water from the C-S-H phase, and that at
450–530°C, to the decomposition of calcium hydroxide. The endotherm for
the composite material is weaker than the corresponding peak for the neat
paste, and it also appears at a lower temperature. The decrease in the
endotherm is taken as evidence that there is a reaction between the sulfonate
groups of the polymer and the calcium ions or calcium hydroxide released
during the hydration of the silicate. It is also possible that the sulfonate
groups, being hygroscopic, absorb water thus diminishing water available
for the hydration of the silicate. Similar conclusions were drawn from the
TG results.
Cook, et al., and others have examined several polymer-cement
systems by DTA. [6][93] The polymers were mixed with monomers and
polymerized either by irradiation or thermal catalysis. The mixes were
hydrated for 14 days before examination by DTA (Fig. 35). The intensity
of the calcium hydroxide peak varies, depending on the polymer. Polysty-
rene does not have much effect, but acrylonitrile, methyl methacrylate
(MMA), vinyl acetate, and polyester styrene reduce the intensity of the
Ca(OH) peak, suggesting the occurrence of an interaction between these
2
polymers and calcium hydroxide. Caution should be exercised in the
interpretation of the thermal curves in the polymer-cement systems. The
decrease in intensities could be due to the dilution effect, masking by the
exothermal peaks, and interactions during heating.
Polymer-impregnated concrete is obtained by impregnating con-
crete with a monomer and then polymerizing it by various means. Such
concrete, in addition to exhibiting a three-fold increase in compressive
strength, manifests better durability than normal concrete under different
exposure conditions. Several investigators have reported that an interaction
occurs between the polymer and Ca(OH) of the cement paste.
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Ramachandran and Sereda studied the DTA of polymethyl methacrylate-
impregnated cement pastes obtained at w/c ratios of 0.3 and 0.7. [94]
Thermograms of cement and that impregnated with polymethyl methacry-
late are shown in Fig. 36. In cement paste A, the endothermal effects at
215–225°C and 490–505°C are due to the dehydration of water from the