Page 419 - Handbook of Thermal Analysis of Construction Materials

396 Chapter 9 - Non-Portland Cement Binders and Concrete

material increases its volume on rehydration and hardening. Heating of
gypsum or hemihydrate to about 185–200°C yields soluble anhydrite. At
400–500°C, gypsum is dehydrated to an insoluble anhydrite of the compo-
sition CaSO .
4
The exact mechanism by which gypsum addition to portland
cement controls setting behavior is not known although much work has
been reported in this field. [81] It is clear, however, that interactions between
C A and gypsum are integral to the stiffening and hardening processes.
3
Feldman and Ramachandran investigated the hydration character
of C A-CaSO •2H O systems containing up to 20% CaSO •2H O. [82] They
2
2
4
4
3
made extensive use of thermal analysis and coupled the hydration study
with length change measurements using compacted solid bodies with an
equivalent water/solid ratio of 0.1. It was concluded that the formation of
ettringite had no direct effect upon reaction rate, but that the reactivity of
2-
C A was reduced by the sorption of SO ions on active sites of its surface.
4
3
A second mechanism retarding the normal hydration of C A is the reduced
3
rate of conversion of the hexagonal hydroaluminates to the cubic hexahy-
2-
drate probably by sorption of SO ions. The retardation of the hydration
4
of C A at a particular temperature is affected by a balance between the
3
2-
following: the concentration of SO ions on and in proximity to the surface
4
2-
of the C A; the rate of reaction of SO ions with the hexagonal
4
3
hydroaluminates; and the thickness of the hexagonal hydroaluminate layer
around the C A grain. The large volume of sulfoaluminate product may
4
contribute to a general decrease in porosity and retard the reaction of C A
3
in this way. The disruptive expansions that occur when the sulfoaluminate
is formed suggest that it cannot form an impermeable layer.
The engineering properties of cement minerals including gypsum
have been determined using the compact technique. [83] The use of compacts
(porous bodies formed by pressure compaction of powdered material) as
structural models of cement systems has been extensively studied. [84] The
co-linearity of mechanical property—porosity relationships for in-situ
hydrated and compacted cement paste powders—provides strong evidence
for the absence of “chemical” bonds between particulates as it is unlikely
that these would not be broken (if present) during compaction. The fact that
the values for modulus of elasticity of compacts fit so closely to that of the
paste supports the idea that the system has none or very few chemical bonds.
This concept of the nature of inter-particle bonds in cement paste allows for
the possibility of one particle breaking its bond with one neighboring
particle and remaking a similar bond with another neighbor. The result
would be permanent deformation, but no net loss of strength when the