Page 75 - Handbook of Thermal Analysis of Construction Materials

58 Chapter 2 - Introduction to Portland Cement Concrete

explains its behavior. There are two main models. In the Powers-Brunauer
model, the cement paste is considered a poorly crystalline gel and layered.
2
The gel has a specific surface area of 180 m /g with a minimum porosity of
28%. The gel pores are assumed to be accessible only to water molecules
because the entrance to these pores is less than 0.4 nm in diameter. Any
space not filled with cement gel is called capillary space. The mechanical
properties of the gel are described using this model. The particles are held
together mainly by van der Waal’s forces. [24] Swelling on exposure to water
is explained by the individual particles separating, due to layers of water
molecules existing between them. Creep is the result of water being
squeezed out from between the particles during the application of stress
This model recognizes the existence of some chemical bonds between the
particles and the existence of layers.
In the Feldman-Sereda model, [16][24] the gel is considered as a
poorly crystallized layered silicate and the role of water is much more
complex (Fig. 7) than is recognized by the Powers-Brunauer model. Water
does not re-enter the interlayer after d-drying. Water, in contact with the d-
dried gel, acts in several ways:
a) It interacts with the free surface, forming hydrogen bonds.
b) It is also physically adsorbed on the surface.
c) It also enters the collapsed layered structure of the mate-
rial even at humidities below 10% RH.
d) It fills large pores by capillary condensation at higher
humidities.
According to this model creep is a manifestation of aging, i.e., the material
moves towards a lower total energy by aggregation of sheets due to the
formation of more layers. Surface area is reduced by this process. Aggre-
gation is accelerated by stress and facilitated by the presence of interlayer
water.

10.0 MATHEMATICAL MODELS

Recently, there has been a significant interest in the development
of computer-based models for the microstructure, hydration, and structural
development in cement-based materials. Factors such as composition and
shape of cement particles, w/c ratio, and curing conditions have been
considered for obtaining mass and volume fraction of phases in cement