Page 72 - Handbook of Thermal Analysis of Construction Materials

Section 6.0 - Properties of Cement Paste 55

is controversy over the significance of H O as an adsorbate indetermining
2

surface area. With water as an adsorbate, the surface area is estimated
2
at 200 m /g and remains constant for different w/c ratio pastes. The surface
area varies with w/c ratio when using nitrogen, methanol, isopropanol,

and cyclohexane as adsorbates. [18] With nitrogen, the values vary from 3 to
2
147 m /g.
The average characteristic of a pore structure can be represented by
the hydraulic radius which is obtained by dividing the total pore volume by
the total surface area. The pore volume of d-dried paste determined by
nitrogen, helium, or methanol, is due to capillary porosity, and the hydraulic
radius is known to vary from 30 to 107 Å for w/c ratios from 0.4 to 0.8.

6.7 Mechanical Properties

Hydrated portland cement contains several types of solid phases,
and the theoretical treatment of such a material is complex.
Many observations have led to the conclusion that the strength
development of hydrated portland cement depends on the total porosity, P.
Most data can be fitted to an exponential dependence term, e -bP , with b
values associated with different types of pores. Porosity and grain size
effects on strength become clearly separable as pores approach or become
smaller than the grain size. Uniform distributions of different types of pores
will have similar exponential strength-porosity trends, but the b values
will change. They will depend on the pore location, size, and shape. The
latter two are important only when the pore causing failure is large in
comparison with the grain size or with the specimen size. For small pores,
its location is important. Pores at grain boundaries are more critical than
pores within grains.
Correlation of porosity with mechanical property values has led to
several types of semi-empirical equations, the most common being that due
to Ryshkewitch: [19]

M = M exp (-bP)
0

where M is the mechanical strength property at porosity P, M is the value
0
at zero porosity, and b is a constant. As stated previously, b is related to the
pore shape and orientation. This equation shows good agreement with
experimental values at lower porosities. Another equation, due to Schiller,
is as follows: [20]