A cement is a binder, a substance that sets and hardens and can bind other materials together. The word "cement" can be traced back to the Roman term opus caementicium, used to describe masonry resembling modern concrete that was made from crushed rock with burnt lime as binder. The volcanic ash and pulverized brick supplements that were added to the burnt lime, to obtain a hydraulic binder, were later referred to as cementum, cimentum, cäment, and cement.

Cements used in construction can be characterized as being either hydraulic or non-hydraulic,

depending upon the ability of the cement to set in the presence of water (see hydraulic and non-

hydraulic lime plaster).

Non-hydraulic cement will not set in wet conditions or underwater; rather, it sets as it dries and

reacts with carbon dioxide in the air. It can be attacked by some aggressive chemicals after setting.

Hydraulic cements (e.g., Portland cement) set and become adhesive due to a chemical reaction

between the dry ingredients and water. The chemical reaction results in mineral hydrates that are

not very water-soluble and so are quite durable in water and safe from chemical attack. This allows

setting in wet condition or underwater and further protects the hardened material from chemical

attack. The chemical process for hydraulic cement found by ancient Romans used volcanic ash

(activated aluminium silicates[citation needed]) with lime (calcium oxide).

The most important uses of cement are as a component in the production of mortar in masonry, and of concrete, a combination of cement and an aggregate to form a strong building material.

Chemistry[edit]

Non-hydraulic cement, such as slaked lime (calcium hydroxide mixed with water), hardens

by carbonation in the presence of carbon dioxide which is naturally present in the air. First calcium

oxide is produced by lime calcination at temperatures above 825 °C (1,517 °F) for about 10 hours

at atmospheric pressure:

CaCO3 → CaO + CO2

The calcium oxide is then spent (slaked) mixing it with water to make slaked lime:

CaO + H2O → Ca(OH)2

Once the excess water is completely evaporated (this process is technically called setting),

the carbonation starts:

Ca(OH)2 + CO2 → CaCO3 + H2O

This reaction takes a significant amount of time because the partial pressure of carbon

dioxide in the air is low. The carbonation reaction requires the dry cement to be exposed

to air, for this reason the slaked lime is a non-hydraulic cement and cannot be used

under water. This whole process is called the lime cycle.

Conversely, hydraulic cement hardens by hydration when water is added. Hydraulic

cements (such as Portland cement) are made of a mixture of silicates and oxides, the

four main components being:

Belite (2CaO·SiO2);

Alite (3CaO·SiO2);

Tricalcium aluminate (3CaO·Al2O3) (historically, and still occasionally, called 'celite');

Brownmillerite (4CaO·Al2O3·Fe2O3).

The silicates are responsible of the mechanical properties of the

cement, the tricalcium aluminate and the brownmillerite are essential to

allow the formation of the liquid phase during the kiln sintering (firing).

The chemistry of the above listed reactions is not completely clear and

is still the object of research.[

Safety issues[edit]

Bags of cement routinely have health and safety warnings printed on them because not only is

cement highly alkaline, but the setting process is exothermic. As a result, wet cement is

strongly caustic (water pH = 13.5) and can easily cause severe skin burns if not promptly washed off

with water. Similarly, dry cement powder in contact with mucous membranes can cause severe eye

or respiratory irritation. Some trace elements, such as chromium, from impurities naturally present in

the raw materials used to produce cement may cause allergic dermatitis.[1] Reducing agents such as

ferrous sulfate (FeSO4) are often added to cement to convert the carcinogenic

hexavalent chromate (CrO42-) in trivalent chromium (Cr3+), a less toxic chemical species. Cement

users need also to wear appropriate gloves and protective clothing

Setting and curing[edit]

Cement starts to set when mixed with water which causes a series of hydration chemical reactions.

The constituents slowly hydrate and the mineral hydrates solidify; the interlocking of the hydrates

gives cement its strength. Contrary to popular perceptions, hydraulic cements do not set by drying

out; proper curing requires maintaining the appropriate moisture content during the curing process. If

hydraulic cements dry out during curing, the resulting product can be significantly weakened.

Environmental impacts[edit]

Cement manufacture causes environmental impacts at all stages of the process. These include

emissions of airborne pollution in the form of dust, gases, noise and vibration when operating

machinery and during blasting in quarries, and damage to countryside from quarrying. Equipment to

reduce dust emissions during quarrying and manufacture of cement is widely used, and equipment

to trap and separate exhaust gases are coming into increased use. Environmental protection also

includes the re-integration of quarries into the countryside after they have been closed down by

returning them to nature or re-cultivating them.

Green cement[edit]

Green cement is a cementitious material that meets or exceeds the functional performance

capabilities of ordinary Portland cement by incorporating and optimizing recycled materials, thereby

reducing consumption of natural raw materials, water, and energy, resulting in a more sustainable

construction material.

The manufacturing process for green cement succeeds in reducing, and even eliminating, the

production and release of damaging pollutants and greenhouse gasses, particularly CO2.

Growing environmental concerns and increasing cost of fuels of fossil origin have resulted in many

countries in sharp reduction of the resources needed to produce cement and effluents (dust and

exhaust gases).[55]

Peter Trimble, a design student at the University of Edinburgh has proposed 'DUPE' based

on Sporosarcina pasteurii, a bacterium with binding qualities which, when mixed

withsand and urine produces a concrete said to be 70% as strong as conventional material

ENVIRONMENTAL IMPLICATIONSMany of the aspects of the cement making process are potentially environmentally damaging,although these risks can be minimised. The areas of potential concern are listed below.

Dust emissionsThe manufacture of cement generates large quantities of dust. These must be prevented (bothon environmental and economic grounds) from escaping to the atmosphere. The two areaswhere dust has the potential to escape are via air streams that have been used to carry cement(e.g. the mills or kiln) and directly from equipment used to transport cement (e.g. the variousconveyor belts). Thus to prevent dust emissions all transport equipment is enclosed, and theair both from these enclosures and from the kiln and mills is treated in an electrostaticprecipitator to remove its load of dust. Here dust-laden air passes between an electrode

carrying 50 000 volts and an earthed collection plate. The electrostatic discharge between theelectrode and the plate forces the dust onto the plates, from which it is removed.The current emission limit from the main stack at Golden Bay is 250 mg m-3 and at Milburnis 150 mg m-3, while in Europe emission limits of down to 50 mg m-3 are becoming common.This poses a significant challenge to the manufacturing operation both in capital cost toIX-Materials-B-Cement-8reduce emissions and monitoring of emissions to ensure compliance with existing resourceconsents.

CO2

Cement manufacture is an energy intensive process. One of the most significant challengesfacing the industry into the 21st century is a requirement to reduce CO2 emissions. CO2 isproduced during the calcination phase of the manufacturing process and also as a result ofburning fossil fuels. Opportunity to reduce emissions through increased energy efficiency isonly possible on the latter of the CO2 emissions. Quarry and plant water runoffRunoff of storm water and treatment of waste water from quarries is a problem for almost allquarry operations. Usually this is trapped in wetland areas where the water is treated in acontrolled manner. Within the factory runoff can be contaminated by oils and lubricants, butthe runoff is monitored and training programmes are reguarly undertaken to ensure this doesnot happen. Chrome bricksKiln bricks used to be made of hexavalent chrome, which is a carcinogen and causesdermititus in some people. Since the problems associated with its use were identified bothMilburn and Golden Bay (along with almost all cement manufacturers internationally)replaced these bricks with environmentally-sound magnesium-spinel bricks.

THE ROLE OF THE LABORATORYThe laboratory forms an integral part of the control systems on site with testing from rawmaterials to finished product. The laboratory operates a 24 hour facility in line with acontinuous manufacturing facility responsible for the following aspects:

• Testing raw materials prior to blasting in the quarry and assisting with developmentof quarrying strategies forms the first step in the process.

• Analysing rock samples from the raw mill at regular intervals during the day andnight and fine tuning the process to ensure chemical control is maintained.

• Analysing clinker at the end of the cooler (before grinding) to ensure that themanufactured process meets specification.

• Checking that cement mills are undertaking grinding correctly and that customersreceive the right product.

• Checkings despatched materials for quality and compliance with NZ Standardsrequirements. Certificates of conformance are issued to customers based on theseanalyses.

• Product development.Testing work within the laboratory ranges from simple air permeability measurements to hightechnology X-ray fluoresence analysis (see air pollution article). Qualifications within thelaboratory relect this wide range of testing and skills requirement with university qualified

staff working alongside others with minimal formal qualifications.