Page 135 - Handbook of Thermal Analysis of Construction Materials

118 Chapter 3 - Formation and Hydration

Cements have been suggested as materials that could be used to
immobilize heavy metals produced by various industries. Cadmium and its
compounds are highly toxic and can effectively be retained in concrete
provided the pH does not fall below 7. The main mechanism of Cd
stabilization is related to its precipitation as cadmium hydroxide and
2+
2+
physical entrapment. The possibility of Cd substituting Ca by solid
diffusion or dissolution mechanism, forming a precipitate of Ca•Cd(OH) 4
has been proposed by Goni, et al., [69] based on TG/DTG studies.
The potentiality of the emanation thermal analysis for the investi-
gation of the hydration of cement and cement compounds has been
discussed by Balek. [67] This technique is based on the measurement of
radioactive gases released from the hydrating phase. The amount of gas
released depends on the physico-chemical processes taking place in the
solid. A quantitative estimation of the rate of hydration of cement and C S
3
at early stages has been obtained.

8.0 CaO-SiO -Al O -H O AND RELATED
2
2
3
2
SYSTEMS
The binary, ternary, and quaternary systems containing oxides of
relevance to cement science have been studied at ambient temperatures or
under autoclaving conditions. Various hydrated products, crystalline as
well as poorly crystallized, are formed.
The thermal analysis techniques were found to be of great value
[1]
by Kalousek and co-workers for investigating these systems. In the
CaO-SiO -SiO system studied at normal temperatures, the formation of
2
2
C-S-H products with different C/S ratios gave peaks at different tempera-
[1]
tures. In Fig. 34, reported by Kalousek, the C-S-H compounds with C/S
ratios of 1.0 and 1.33 show distinct exothermal peaks beyond 800°C. The
peak denotes a crystallization effect. The peak is shifted as the C/S ratio
increases. Even after subjecting the samples to extraction by acetoacetic
ester the curves (dotted) are not modified. This suggests that lime in these
compounds is strongly bound. Generally, in the system containing lime-
silica-water, with age the peak due to lime disappears and an exothermal
peak caused by the crystalline effect at higher temperatures increases in
intensity.