Cement
A cement is a binder, a chemical substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel (aggregate) together. Cement mixed with fine aggregate produces mortar for masonry, or with sand and gravel, produces concrete. Concrete is the most widely used material in existence and is behind only water as the planet's most-consumed resource.[2]
For other uses, see Cement (disambiguation).
Cements used in construction are usually inorganic, often lime or calcium silicate based, which can be characterized as hydraulic or the less common non-hydraulic, depending on the ability of the cement to set in the presence of water (see hydraulic and non-hydraulic lime plaster).
Hydraulic cements (e.g., Portland cement) set and become adhesive through a chemical reaction between the dry ingredients and water. The chemical reaction results in mineral hydrates that are not very water-soluble. This allows setting in wet conditions or under water and further protects the hardened material from chemical attack. The chemical process for hydraulic cement was found by ancient Romans who used volcanic ash (pozzolana) with added lime (calcium oxide).
Non-hydraulic cement (less common) does not set in wet conditions or under water. Rather, it sets as it dries and reacts with carbon dioxide in the air. It is resistant to attack by chemicals after setting.
The word "cement" can be traced back to the Ancient 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. In modern times, organic polymers are sometimes used as cements in concrete.
World production of cement is about 4.4 billion tonnes per year (2021, estimation),[3][4] of which about half is made in China, followed by India and Vietnam.[3][5]
The cement production process is responsible for nearly 8% (2018) of global CO2 emissions,[4] which includes heating raw materials in a cement kiln by fuel combustion and resulting release of CO2 stored in the calcium carbonate (calcination process). Its hydrated products, such as concrete, gradually reabsorb substantial amounts of atmospheric CO2 (carbonation process) compensating near 30% of initial CO2 emissions, as estimations suggest.[6]
$_$_$DEEZ_NUTS#0__titleDEEZ_NUTS$_$_$
$_$_$DEEZ_NUTS#0__descriptionDEEZ_NUTS$_$_$
Reduced-footprint cement[edit]
Growing environmental concerns and the increasing cost of fossil fuels have resulted, in many countries, in a sharp reduction of the resources needed to produce cement, as well as effluents (dust and exhaust gases).[86] Reduced-footprint cement is a cementitious material that meets or exceeds the functional performance capabilities of Portland cement. Various techniques are under development. One is geopolymer cement, which incorporates recycled materials, thereby reducing consumption of raw materials, water, and energy. Another approach is to reduce or eliminate the production and release of damaging pollutants and greenhouse gasses, particularly CO2.[87] Recycling old cement in electric arc furnaces is another approach.[88] Also, a team at the University of Edinburgh has developed the 'DUPE' process based on the microbial activity of Sporosarcina pasteurii, a bacterium precipitating calcium carbonate, which, when mixed with sand and urine, can produce mortar blocks with a compressive strength 70% of that of concrete.[89] An overview of climate-friendly methods for cement production can be found here.[90]
$_$_$DEEZ_NUTS#6__descriptionDEEZ_NUTS$_$_$
Setting, hardening 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 and harden. The interlocking of the hydrates gives cement its strength. Contrary to popular belief, hydraulic cement does not set by drying out — proper curing requires maintaining the appropriate moisture content necessary for the hydration reactions during the setting and the hardening processes. If hydraulic cements dry out during the curing phase, the resulting product can be insufficiently hydrated and significantly weakened. A minimum temperature of 5 °C is recommended, and no more than 30 °C.[51] The concrete at young age must be protected against water evaporation due to direct insolation, elevated temperature, low relative humidity and wind.
The interfacial transition zone (ITZ) is a region of the cement paste around the aggregate particles in concrete. In the zone, a gradual transition in the microstructural features occurs.[52] This zone can be up to 35 micrometer wide.[53]: 351 Other studies have shown that the width can be up to 50 micrometer. The average content of unreacted clinker phase decreases and porosity decreases towards the aggregate surface. Similarly, the content of ettringite increases in ITZ. [53]: 352
$_$_$DEEZ_NUTS#2__titleDEEZ_NUTS$_$_$
$_$_$DEEZ_NUTS#2__descriptionDEEZ_NUTS$_$_$
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 (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.[54] Reducing agents such as ferrous sulfate (FeSO4) are often added to cement to convert the carcinogenic hexavalent chromate (CrO42−) into trivalent chromium (Cr3+), a less toxic chemical species. Cement users need also to wear appropriate gloves and protective clothing.[55]
$_$_$DEEZ_NUTS#5__titleDEEZ_NUTS$_$_$
$_$_$DEEZ_NUTS#5__descriptionDEEZ_NUTS$_$_$
$_$_$DEEZ_NUTS#6__titleDEEZ_NUTS$_$_$
$_$_$DEEZ_NUTS#8__titleDEEZ_NUTS$_$_$
$_$_$DEEZ_NUTS#8__subtextDEEZ_NUTS$_$_$
$_$_$DEEZ_NUTS#1__titleDEEZ_NUTS$_$_$
$_$_$DEEZ_NUTS#1__subtextDEEZ_NUTS$_$_$
$_$_$DEEZ_NUTS#4__titleDEEZ_NUTS$_$_$
$_$_$DEEZ_NUTS#4__subtextDEEZ_NUTS$_$_$
$_$_$DEEZ_NUTS#3__titleDEEZ_NUTS$_$_$
$_$_$DEEZ_NUTS#3__subtextDEEZ_NUTS$_$_$
$_$_$DEEZ_NUTS#7__titleDEEZ_NUTS$_$_$
$_$_$DEEZ_NUTS#7__subtextDEEZ_NUTS$_$_$