g
2w2h QIQIS:12269–1987 (53 grade OPC), IS:12230–1988 (Sulphate resisting cement), IS:1489-1976 (Portland
Puzzolana cement), IS:455–1976 (Portland slag cement). All types of Portland cements are
interchangeable for mix design, and the most commonly used ones are OPC, PPC, PSC, and
SRC.
After water is added to the cement hydration occurs and continues as long as the relative
humidity in the pores is above 85 per cent and sufficient water is available for the chemical
reactions. On an average, 1 g of cement requires 0.253 g of water for complete hydration. As
hydration proceeds, the ingress of water by diffusion through the deposit of hydration products
around the original cement grain becomes more and more difficult, and the rate of hydration
continuously decreases. In mature paste, the particles of calcium-silicate hydrates form an
interlocking network which is a ‘gel’ having a specific surface of about 200 m^2 /g. This gel is
poorly crystalline, almost amorphous, and appears as randomly oriented layers of thin sheets
or buckled ribbon. The gel is the heart of the concrete and is a porous mass. The interstitial
spaces in the gel are called ‘gel pores’. The strength giving properties and phenomena, such as
creep and shrinkage are due to the porous structure of the gel, and the strength is due to the
bond afforded by the enormous surface area.
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The size of the aggregate, particle shape, colour, surface texture, density (heavyweight or
lightweight), impurities, all of which have an influence on the durability of concrete, should
conform to IS: 383–1970.
During the process of hydration the products of hydration completely surround and bind
together the aggregate particles in a solid hardened mass. Aggregates constitute nearly 70–75
per cent of the total volume of concrete. The strength of concrete is governed by the weakest
element, be it the cement paste, the aggregate or the interface of the aggregate-cement paste.
Strong aggregates are also more sound and durable in aggressive environments. The strength
at the aggregate mortar interface is perhaps more critical, hence the shape, size and texture of
the coarse aggregate is important. The aggregate should be clean, hard, strong, and durable,
free from chemicals or coatings of clay or other fine material that can affect the bond with the
cement paste.
Very sharp and rough aggregate particles or flat and elongated particles require more fine
material to produce a workable concrete. Accordingly the water requirement and there from
the cement content increases. Excellent concrete is made by using crushed stone but the
particles should be roughly cubical or spherical in shape. Natural rounded aggregates having
a smooth surface are better from the point of view of workability, but their bond with mortar
may be weaker and are likely to produce concrete of lower flexural strength.
The maximum size of aggregate governs the strength and workability of the concrete. For a
lean mix, a larger maximum size of aggregate gives better results, because for a given volume
of aggregate, the total surface area is less. Depending on the maximum size of aggregate, the
cement content for a specific strength is altered because at the same water-cement ratio different
ranges of strength are possible for different sizes of aggregate. However, for a mix of high
compressive strength a smaller maximum size of aggregate is preferable, and it is just not
economically possible to make concretes of 28-days compressive strengths exceeding 40 N/
mm^2 using 40-mm aggregate.
A statement of fact valid for all mix design is: The smaller the maximum size of aggregate the
greater the proportion of fine aggregate needed for concretes of identical cement contents and workability.
Also, the lower the cement content of the mix, and/or the more angular the coarse aggregate,
the greater is the proportion of the fine aggregate required.