Materials for engineering, 3rd Edition - (Malestrom)

Glasses and ceramics 147
temperatures >1720 K. The process is conducted in a nitrogen atmosphere
and the product contains little porosity, although there is a proportion of
residual silicon in the microstructure, which limits the high temperature
strength of the material. Strengths superior to RBSN may be achieved, although
considerable scatter in strength is found.
Sintered silicon carbide (SSC) is prepared from fine powders, using boron
and carbon as sintering aids. Properties are marginally better than for RSSC,
although the growth of coarse (>50 µm) grains during sintering is a strengthlimiting
factor.
4.4 Cement and concrete
Concrete is a composite material made up of a matrix of cement into which
is embedded particles of sand and aggregate: the cement provides a means of
binding together low cost sand and gravel into a product of engineering
value. The mix must contain enough cement to coat all the aggregate particles
and to fill the spaces between them. The aggregate should consist of a range
of gravel particle sizes, so that the smaller particles help to fill the spaces
between the larger particles, thus reducing the amount of cement required,
Fig. 4.8.
Portland cement (OPC) is the most important of the binders used in
concrete. It was patented in the 1820s and so named because its colour
resembles that of the natural limestone found at Portland in Dorset. It is
made by firing a suitable mixture of clay and chalk and then grinding the
product to a fine powder. It consists of a mixture of particles of ionically
bonded solids; its constitution (in weight %) is as follows, with a contracted
symbol shown in brackets to represent some of the compounds:
4.8 Graded aggregate in a matrix of cement.