Page 71 - Handbook of Thermal Analysis of Construction Materials

54 Chapter 2 - Introduction to Portland Cement Concrete

6.4 Density

The density value quoted in the literature for a given material is
accepted without question because it depends simply on mass and volume
at a given temperature. An accurate assessment of density, however, is one
of the most important factors in determining porosity, assessing durability
and strength, and estimating lattice constants for the C-S-H phase in
hydrated portland cement.
Traditionally, density of hydrated portland cement was measured
in the d-dried state by pycnometric methods, using a saturated solution of
calcium hydroxide as a fluid. Since the d-dried hydrated portland cement
rehydrates on exposure to water, this method is of questionable value. More
realistic values can be obtained by proper conditioning of the sample and
using fluids that do not affect the structure of the paste.
Drying to 11% RH and measuring with a saturated solution of
Ca(OH) gives an uncorrected value of 2.38 g/cc, as compared to a corrected
2
value of 2.35 g/cc and 2.34 g/cc using helium. At the d-dried state, a higher
value obtained by the Ca(OH) solution technique is due to the penetration
2
of water into the interlayer positions of the layered structure of the
crystallite.

6.5 Pore Structure

Pore structure influences the strength development and durability
of concrete. Pore structure is modified in the presence of admixtures.
Porosity and pore size distribution are usually determined using mercury
porosimetry and nitrogen or water adsorption isotherms. Total porosity is
obtained by using organic fluids or water as a medium. But water cannot be
used as it may interact with the body. Mercury porosimetry involves forcing
mercury into the vacated pores of a body by the application of pressure. The
technique measures a range of pore diameters down to about 3 nm.
Beaudoin [17] measured total porosity by Hg porosimetry using pressures up
to 408 MPa and concluded that the porosimetry and He pycnometry
methods could be used interchangeably to determine porosity of cement
paste formed at a w/c ratio equal to or greater than 0.40.

6.6 Surface Area and Hydraulic Radius

Surface area is the area available to gases or liquids by way of pores
and the external area. Hydrated portland cement is very complex and there