Concrete

Summary

Cement is usually a fine, abrasive, grey powder made by melting a mixture of chalk and clay in a rotating kiln to produce a clinker which is ground to a powder. When mixed with water the surface of the cement grains go into solution and crystals slowly grow on the grains. It is called grout and can be used to fill small cavities. If also mixed with sand it is called mortar and is used to hold bricks apart. If also mixed with stones (or aggregate) it is called concrete and is used in many structures either unreinforced or reinforced with steel bars to overcome the relative weakness of concrete in tension.

Concrete is a mixture of water, cement, aggregate (sand, stone) and possibly admixtures to alter properties.

Unlike homogeneous materials, like steel, the tensile strength of concrete is only about 10% of its compressive strength. Steel reinforcement is used to take tensile forces.

The cement is a complex mixture of chemicals which, when water is added, go into solution at various rates. Crystals grow out of the solution forming largely calcium silicate hydrate and occupy a greater volume than the solids they came from. As the crystals interlock they increase the strength and impermeability of the concrete. The pores should be kept full of water (called curing) while the crystals are growing.

A typical portland cement contains (in industry shorthand for the various oxides):

C=CaO (67%), S=SiO₂ (21%), A=Al₂O₃ (5%), F=Fe₂O₃, (3%), M=MgO (1%) & 3% others.

These form the four main constituents:

C3S representing tricalcium silicate (3CaO.SiO₂) or Alite,

C2S representing dicalcium silicate (2CaO.SiO₂) or Belite,

C3A representing tricalcium aluminate (3CaO.Al₂O₃) or the 'aluminate phase',

C4AF representing tetracalcium aluminoferrite (4CaO.Al₂O3.Fe₂O₃) or the ‘ferrite’ phase.

The main oxide, when hydrated, becomes calcium hydroxide:

CaO + H₂O = Ca(OH)₂.

When exposed to air this very slowly carbonates to become calcium carbonate:

Ca(OH)₂ + CO₂ = CaCO₃ + H₂O

Cement is typically made from limestone or chalk which is calcium carbonate (CaCO₃) and clay or shale to provide silicates. These raw materials are extracted from a quarry crushed to a very fine powder and then blended in the correct proportions. This blended raw material is called the 'raw feed' or 'kiln feed' and is heated in a rotary kiln where it reaches a temperature of about 1400°C to 1500°C. In its simplest form, the rotary kiln is a brick lined tube up to 200 metres long and perhaps 6 metres in diameter, with a long flame at one end. The raw feed enters the kiln at the cool end and gradually passes down to the hot end, then falls out of the kiln and cools down.

The material formed in the kiln is described as 'clinker' and is typically composed of rounded nodules between 1mm and 25mm across.

After cooling and recovery of some heat, the clinker may be stored temporarily in a clinker store, or it may pass directly to the cement mill.

The cement mill grinds the clinker to a fine powder. A small amount of gypsum - a form of calcium sulfate (CaSO₄) - is normally ground up with the clinker. The gypsum controls the setting properties of the cement when water is added.

Other cements:

- calcium aluminate cement; also known as high alumina cement,

- calcium sulfoaluminate cement,

- hydraulic lime and

- extended cements – mixtures of Portland Cement with other reactive material which take part in the hydration process, such as fly ash or ground slag; also known as blended cements or composite cements or combination cements.

The high alkalinity of cement (pH 12) prevents rusting of the embedded steel (provided the cover is dense enough to prevent, for long enough, carbonation by CO₂ reaching the steel).

Mortar is concrete without the stone. However, for brick or block-work it will usually also contain lime or entrained air to plasticize it and weaken it to ensure that movement cracks occur in mortar and not in the bricks or blocks.

Grout is a mixture of water and cement used to fill cracks. It may be mixed with some sand to extend it for filling larger cavities.

More water is always needed, to make a mixture sufficiently workable or mouldable, than is needed for the chemical reaction. A theoretical water / cement ratio of only 0.16 (by weight) is required to completely hydrate all the cement, but many times this may be required to make the mix workable.

It is desirable to prevent water evaporating from the concrete (curing) while it hardens to maximise strength gain (It does not "dry" to harden!) and to reduce 'drying shrinkage cracking'. The less water used the greater the strength and durability, provided compaction is achieved. Various admixtures can be added to reduce the water required.

After placing concrete should be compacted to remove most trapped air. Normally about 2% remains after compaction but 5% of potential strength is lost for every 1% of air (or water) left in the concrete. However, about 5% air, consisting of 0.2 mm bubbles, may be purposefully entrained to provide frost protection of the hardened concrete. The bubbles provide pressure relief chambers as ice forms and expands in saturated pores. The presence of both water and air reduce strength and durability. Good mix design minimises this loss.

After compacting a slab some water will bleed to the surface. This can evaporate or be vacuumed out. Further compaction is then achieved by floating and trowelling the surface. Trowelling gives a harder, more durable and smoother surface, provided that further evaporation is prevented as soon as trowelling is finished.

The aggregate is a filler in the cement paste. It reduces drying shrinkage but does not increase strength. It can be crushed rock, sand, gravel or any material which is suitably strong and durable. It should be stronger than the cement paste whose strength is dependant on the ratio of water to cement. The maximum size is normally 20 mm, but I have used as much as 150 mm in dam construction and as little as 0.6 mm in a cup winning concrete canoe.

Doubling the maximum aggregate size decreases the cement requirement by about 17%, for the same strength and durability (W/C = 0.55):

Max. aggregate size (mm) 0.15 0.3 0.6 1.2 2.4 5 10 20 40 80 150

Cement content (kg/m³) 1150 960 800 660 550 440 370 310 260 210 180

Examples of use Grout Canoe Mortar Normal Dams

The relative density of aggregates is about: flint 2.54, limestone 2.65, granite 2.65, quartzite 2.7, basalt 2.9. Cement is heavier at 3.14 and water is 1. From this the density of a concrete can be calculated: A cubic metre of fresh wet concrete may contain 2% air (20 litres), 179 kg of water (/1=179 litres), 350 kg of cement (/3.14=111 litres) [ W/C ratio of 0.51 by weight], 583 kg of sand (/2.65=220 litres) and 1250 kg of stone or coarse aggregate (/2.65=470 litres), giving a total density of (179 +350+583+1250) kg / (20 +179+111+220+470) litres = 2362 / 1000 = a relative density of 2.362 or a density of 2362 kg/m³.