Page 70 - Handbook of Thermal Analysis of Construction Materials

52 Chapter 2 - Introduction to Portland Cement Concrete

6.2 Microstructure

Many of the properties of the cement paste are determined by its
chemical nature and microstructure. Microstructure constitutes the nature
of the solid body and that of the non-solid portion, viz., porous structure.
Microstructural features depend on many factors, such as the physical and
chemical nature of the cement, type and the amount of admixture added to
it, temperature and period of hydration, and the initial w/c ratio. The solid
phase study includes examination of the morphology (shape and size),
bonding of the surfaces, surface area, and density. Porosity, pore shape, and
pore size distribution analysis is necessary for investigating the non-solid
phase. Many of the properties are interdependent, and no one property can
adequately explain the physico-mechanical characteristics of cement paste.
A study of the morphology of the cement paste involves observa-
tion of the form and size of the individual particles, particularly through
high resolution electron-microscopes. The most powerful techniques that
have been used for this purpose are: Transmission Electron Microscopy
(TEM), Scanning Electron Microscopy (SEM), the High Voltage Trans-
mission Electron Microscope using environmental cells, the Scanning
Transmission Electron Microscope (STEM) of ion-beam thinned sections,
and the High Resolution SEM using STEM instruments in reflection mode.
The morphology of C-S-H gel particles has been divided into four
types and described by Diamond. [15]
• Type I C-S-H, forming elongated or fibrous particles,
occurs at early ages. It is also described as spines, acicu-
lar, aciculae, prismatic, rod-shaped, rolled sheet, or by
other descriptions. These are a few micrometers long.
• Type II C-S-H is described as a reticular or honey-combed
structure and forms in conjunction with Type I. It does not
normally occur in a C S or C S paste, unless it is formed
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in the presence of admixtures.
• In hardened cement pastes, the microstructure can be
nondescript and consist of equant or flattened particles
(under 1000 Å in largest dimension); such a morphologi-
cal feature is described as Type III.
• Type IV, a late hydration product, is compact and has a
dimpled appearance, and is believed to form in spaces
originally occupied by cement grains. This feature is also
found in C S pastes.
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