ASTM C 150 defines portland cement as "hydraulic cement (cement
that not only hardens by reacting with water but also forms a
water-resistant product) produced by pulverizing clinkers
consisting essentially of hydraulic calcium silicates, usually
containing one or more of the forms of calcium sulfate as an inter
ground addition." Clinkers are nodules (diameters, 0.2-1.0 inch
[5-25 mm]) of a sintered material that is produced when a raw
mixture of predetermined composition is heated to high temperature.
The low cost and widespread availability of the limestone, shales,
and other naturally occurring materials make portland cement one of
the lowest-cost materials widely used over the last century
throughout the world. Concrete becomes one of the most versatile
construction materials available in the world.The manufacture and
composition of portland cements, hydration processes, and chemical
and physical properties have been repeatedly studied and
researched, with innumerable reports and papers written on all
aspects of these properties.
The composition of portland cements is what distinguishes one type
of cement from another. ASTM C 150 and AASHTO M 85 present the
standard chemical requirements for each type. The phase
compositions in portland cement are denoted by ASTM as tricalcium
silicate (C3S), dicalcium silicate (C2S), tricalcium aluminate
(C3A), and tetracalcium aluminoferrite (C4AF). However, it should
be noted that these compositions would occur at a phase equilibrium
of all components in the mix and do not reflect effects of burn
temperatures, quenching, oxygen availability, and other real-world
kiln conditions. The actual components are often complex chemical
crystalline and amorphous structures, denoted by cement chemists as
"elite" (C3S), "belite" (C2S), and various forms of aluminates. The
behavior of each type of cement depends on the content of these
components. Characterization of these compounds, their hydration,
and their influence on the behavior of cements are presented in
full detail in many texts. Some of the most complete references
dealing with the chemistry of cement include those written by Bogue
(1955), Taylor (1964), and Lea (1970). Different analytical
techniques such as x-ray diffraction and analytical electron
microscopy are used by researchers in order to understand fully the
reaction of cement with water (hydration process) and to improve
its properties.
