Diverse Applications of Surface and Colloid Chemistry in Science and Industry - Surface and Colloid Chemistry: Principles and Applications - page 220

to form a high-density surface layer that carbonates in contact with air to produce a
low-permeability surface of calcite. The use of this procedure leads to the protection
of the Ca(OH) 2 layer (such as seen in the Roman structure Hadrian's Wall). Lime
mortars were still used in domestic construction until relatively recently. Partial
dehydration of natural gypsum (200°C), and calcination of calcite (850°C).
It is likely that, during the early Roman period, silica and alumina, as well as
volcanic earths, were used as cements. Portland cement is made from finely ground
limestone and finely divided clay to give a burned product containing 70% CaO,
20% SiO 2 , 4% Fe 2 O 3 , and 4% Al 2 O 3 , plus smaller amounts of minor oxides (e.g.,
Na 2 O, K 2 O, MgO, etc.).
The major raw materials used for modern cement making are
Limestones, argillaceous shales
Chalks, schists
Shells, clays
Calcerous muds, other iron-bearing aluminosilicates
The essential step is the efficient grinding and blending of raw materials. The final
properties of cement strongly depend on its mineral composition so that raw com-
position and firing conditions are adjusted, depending on the type of cement to be
produced. The microstructure of the steel fiber-cement paste interface was studied
by scanning electron microscopy (SEM). The interfacial zone surrounding the fiber
was found to be substantially different from the bulk paste further away from the
fiber surface. The interfacial zone consisted of
1. A thin (1 or 2 m thick) duplex film in actual contact with the reinforcement.
2. Outside this, a zone of perhaps 10 to 30 m thickness, which, in reasonably
well-hydrated systems, is largely occupied by relatively massive calcium
hydroxide crystals, with occasional interruptions of more porous regions.
3. Outside this, a highly porous layer parallel to the interface. The interaction
of cracks initiated in the matrix with this interfacial zone was observed.
In the cement industry, the term hydration is used to describe a range of reactions
between cement and water to produce a hardened product. A cement clinker par-
ticle is a multiphase solid having massive calcium silicate grains (50-100 μm) in
a matrix of interstitial aluminate and ferrite. This is described as analogous to a
distorted clay sequence, which traps regions of porosity-pore size distribution from
nanometer to micrometer.
The most important characteristic of cement is its pore structure and aqueous
phase; hence, the microstructure of the hardened cement paste via the pore system.
It is highly alkaline (pH > 13) due to rapid and almost quantitative dissolution of
Na and K salts from the cement clinker. The porosity of the paste comprises inter-
connected and isolated pores, the pore sizes of which are important to the strength
and dimensional stability of cement products. Different types of cement are used
to meet different performance criteria. Properties can be estimated from composi-
tions and fineness (i.e., particle size and size distribution). In the past, additives