Cement

[edit] Introduction

Cement is a substance used for binding and hardening other materials. Water and cement set and harden through a chemical reaction known as 'hydration'. The process of hardening is described as 'curing', which requires particular conditions of temperature and humidity.

Cement can be mixed with a fine aggregate and water to produce mortar, used in masonry construction as a bedding and adhesive to bind and fill the gaps between adjacent blocks of brick, concrete or stone.

It can be also be mixed with water, aggregates (such as gravel, sand or rock), and sometimes admixtures, to form concrete, and can be used to make renders, screeds and so on. The ratio of water and cement will determine the overall strength and quality of the mix.

The exact properties of the cement paste are very important:

It must be fluid enough for some time after mixing to allow the mix to be formed into its final shape.
It must then set and gain strength so that it binds the aggregates together to produce a strong material.

[edit] Types of cement

[edit] Portland cement

Almost all concrete is made with Portland cement. It is also the principal cement used in most masonry mortars and renders. It is manufactured by heating together limestone (or chalk) and clay (or shale) in large rotary kilns. The chemistry of Portland cement largely consists of calcium silicate which reacts with water to form a strong, durable cement paste.

For more information, see Portland cement.

[edit] Low heat blast-furnace Portland cement

This is a special blended cement with low heat of hydration characteristics for mass concreting. The advantage of this kind of cement over ordinary Portland cement is that, although it has a slower strength gain, it has a higher ultimate strength, as well as better workability.

[edit] Rapid hardening cement

This hardens faster than Portland cement, as it includes more silicates, however, the final strength is only slightly higher. The one-day strength of this cement is equal to the three-day strength of Portland cement with the same water-cement ratio. It is mainly used where formwork has to be removed for reuse.

[edit] Sulphate resisting cement

Sulphates exist in rain and sea water and can be harmful to building materials. Sulphate resisting cement is a type of modified Portland cement that can be used in conditions where concrete is exposed to the risk of deterioration due to sulphate attack.

[edit] High alumina cement

High alumina cement (HAC, sometimes known as calcium aluminate cement (CAC) or aluminous cement) is composed of calcium aluminates rather than calcium silicates. It is manufactured from limestone or chalk and bauxite.

Historically, HAC was used in marine applications where it was considered to be resistant to chemical attack, as well as in structural concrete such as pre-cast beams. However, HAC is no longer used in structural concrete in the UK as it prone to a crystalline re-arrangement (or 'conversion'), which can result in reduced strength and vulnerability to chemical attack when exposed to water for long periods.

For more information, see High alumina cement.

[edit] Ferro-cement

Ferro-cement is a composite material made up of mortar and light wire steel mesh. It is a highly versatile form of reinforced concrete.

For more information, see Ferro-cement.

[edit] Others

[edit] Storage of cement

Cement is hydroscopic which means it absorbs water. If it is not protected in air-tight silos or bags on-site, it can hydrate. Water initiates the hydration process which can lead to a 20% reduction in strength over 3 months, and a 50% reduction in strength over 2 years.

It is important therefore to ensure hydration does not occur; for example, paper bags of cement incorporate a plastic layer and to inhibit moisture uptake.

[edit] Aggregates

Aggregates are mixed with cement and water to provide bulk and to modify the physical and chemical properties of the mix. There are several desirable properties of aggregates:

Clean: Free from any organic substances such as dust or other fine materials.
Hard/strong: Most cement in construction is used to resist compressive forces.
Durable: Must be able to resist forces over a long time period.
Capable of good adhesion: The cement paste needs to physically bind to other aggregates and grip effectively.
Good shape: Smooth and round aggregates provide good workability, whereas angular and rough aggregates tend to give better compressive strength.

The two most common sources for aggregates are rivers (smooth) or crushed rock from quarries (rough), both make very good concrete.

Fine: < 5 mm (sand or crushed rock)
Coarse: > 5 mm (naturally occurring gravel or crushed rock)

[edit] Grading of aggregates

Grading is the distribution of sizes of particles in aggregates, usually expressed in terms of cumulative percentages larger or smaller than each other. It is important in terms of preventing voids.

For grading, a sample of aggregate is placed in a container of sieves of successively reducing sizes. The container is vibrated for a specified period and the aggregate retained on each sieve is measured. This provides a profile of the distribution of different particle sizes in the aggregate.

[edit] Setting and hardening

For some time, the mix remains plastic and may be moulded into the desired shape. After a period (dependent upon temperature, humidity, etc.), the mixture loses its plasticity and begins to harden and gain strength. The gain in strength is rapid in early stages, but it will continue to gain strength for some months provided there is still water to combine with cement.

NB Durability Guaranteed - Pulhamite Rockwork - Its conservation and repair, published by Historic England in 2008, defines cement as: ‘A combination of argillaceous and calcareous (limestone) compounds, burned at a controlled temperature and pulverised to produce a powder that hardens by chemical reaction with water. Cements typically contain 25 to 40 per cent silica and alumina and will set very rapidly (15 minutes to one hour) under water.’