Page 74 - Handbook of Thermal Analysis of Construction Materials

56 Chapter 2 - Introduction to Portland Cement Concrete

M = D ln P CR
P

where D is a constant and P is the porosity at zero strength. It shows good
CR
agreement at high porosities.
Feldman and Beaudoin [16] correlated strength and modulus of
elasticity for several systems over a wide range of porosities. The systems
included pastes hydrated at room temperature, autoclaved cement paste
with and without additions of fly ash, and those obtained by other workers.
Porosity was obtained by measurement of solid volume by a helium
pycnometric technique and apparent volume, through the application of
Archimedes’ principle.
Several attempts have been made to relate the strength of cement
paste to the clinker composition. A series of equations was proposed by
Blaine, et al., in 1968 [21] to predict strength against a number of clinker
compositions, ignition loss, insoluble fraction, and air and alkali contents.
Other investigators have also proposed equations expressing the relation-
ship between the clinker composition and the 28 day strength. [22]
The data on the effect of clinker composition on strength are rather
conflicting, although it is recognized the multiple regression equations
reflect reasonably well the relationship for narrow ranges of cement
composition. It is recognized that other effects such as the texture, presence
of minor components, particle size distribution, and amount of gypsum will
have a significant influence on the potential strength of cement.

7.0 PERMEABILITY OF CEMENT PASTE

The rate of movement of water through concrete under a pressure
gradient, termed permeability, has an important bearing upon the durability
of concrete. The measure of the rate of fluid flow is sometimes regarded as
a direct measure of durability.
It is known that the permeability of hardened cement paste is
mainly dependent on the pore volume. However, pore volume, at different
water/cement ratios and degrees of hydration, does not uniquely define the
pore system and, thus, is not uniquely related to the permeability.