Cement - Wikipedia
Cement - Wikipedia
Cement - Wikipedia
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. In modern times, organic polymers are sometimes used as
cements in concrete.
Chemistry
Cement materials can be classified into two distinct categories: non-hydraulic cements and hydraulic cements
according to their respective setting and hardening mechanisms. Hydraulic cements setting and hardening
involve hydration reactions and therefore require water, while non-hydraulic cements only react with a gas and
can directly set under air.
Non-hydraulic cement
Non-hydraulic cement, such as slaked lime (calcium oxide mixed with water), hardens by carbonation in
contact with carbon dioxide, which is present in the air (~ 412 vol. ppm ≃ 0.04 vol. %). First calcium oxide (lime)
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is produced from calcium carbonate (limestone or chalk) by
calcination at temperatures above 825 °C (1,517 °F) for about 10 hours
at atmospheric pressure:
The calcium oxide is then spent (slaked) mixing it with water to make
slaked lime (calcium hydroxide):
This reaction takes time, because the partial pressure of carbon dioxide in the air is low (~ 0.4 millibar). The
carbonation reaction requires that the dry cement be exposed to air, so the slaked lime is a non-hydraulic cement
and cannot be used under water. This process is called the lime cycle.
Hydraulic cement
Conversely, hydraulic cement hardens by hydration of the clinker
minerals when water is added. Hydraulic cements (such as Portland
cement) are made of a mixture of silicates and oxides, the four main
mineral phases of the clinker, abbreviated in the cement chemist
notation, being:
History
Perhaps the earliest known occurrence of cement is from twelve million years ago. A deposit of cement was
formed after an occurrence of oil shale located adjacent to a bed of limestone burned due to natural causes. These
ancient deposits were investigated in the 1960s and 1970s.
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Cement, chemically speaking, is a product that includes lime as the primary curing ingredient, but is far from the
first material used for cementation. The Babylonians and Assyrians used bitumen to bind together burnt brick or
alabaster slabs. In Egypt stone blocks were cemented together with a mortar made of sand and roughly burnt
gypsum (CaSO4 · 2H2O), which often contained calcium carbonate (CaCO3).
There is... a kind of powder which from natural causes produces astonishing results. It is found in
the neighborhood of Baiae and in the country belonging to the towns round about Mt. Vesuvius.
This substance when mixed with lime and rubble not only lends strength to buildings of other
kinds, but even when piers of it are constructed in the sea, they set hard under water.
The Greeks used volcanic tuff from the island of Thera as their pozzolan and the Romans used crushed volcanic
ash (activated aluminium silicates) with lime. This mixture could set under water, increasing its resistance. The
material was called pozzolana from the town of Pozzuoli, west of Naples where volcanic ash was extracted. In
the absence of pozzolanic ash, the Romans used powdered brick or pottery as a substitute and they may have used
crushed tiles for this purpose before discovering natural sources near Rome. The huge dome of the Pantheon in
Rome and the massive Baths of Caracalla are examples of ancient structures made from these concretes, many of
which still stand. The vast system of Roman aqueducts also made extensive use of hydraulic cement. Roman
concrete was rarely used on the outside of buildings. The normal technique was to use brick facing mate-
rial as the formwork for an infill of mortar mixed with an aggregate of broken pieces of stone, brick, pot-
sherds, recycled chunks of concrete, or other building rubble.
Middle Ages
Any preservation of this knowledge in literature from the Middle Ages is unknown, but medieval masons and
some military engineers actively used hydraulic cement in structures such as canals, fortresses, harbors, and
shipbuilding facilities.[14][15] A mixture of lime mortar and aggregate with brick or stone facing material was used
in the Eastern Roman Empire as well as in the West into the Gothic period. The German Rhineland continued to
use hydraulic mortar throughout the Middle Ages, having local pozzolana deposits called trass.
16th century
Tabby is a building material made from oyster-shell lime, sand, and whole oyster shells to form a concrete. The
Spanish introduced it to the Americas in the sixteenth century.
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18th century
The technical knowledge for making hydraulic cement was formalized by French and British engineers in the 18th
century.
John Smeaton made an important contribution to the development of cements while planning the construction of
the third Eddystone Lighthouse (1755–59) in the English Channel now known as Smeaton's Tower. He needed a
hydraulic mortar that would set and develop some strength in the twelve-hour period between successive high
tides. He performed experiments with combinations of different limestones and additives including trass and
pozzolanas and did exhaustive market research on the available hydraulic limes, visiting their production sites,
and noted that the "hydraulicity" of the lime was directly related to the clay content of the limestone used to make
it. Smeaton was a civil engineer by profession, and took the idea no further.
In the South Atlantic seaboard of the United States, tabby relying on the oyster-shell middens of earlier Native
American populations was used in house construction from the 1730s to the 1860s.
In Britain particularly, good quality building stone became ever more expensive during a period of rapid growth,
and it became a common practice to construct prestige buildings from the new industrial bricks, and to finish
them with a stucco to imitate stone. Hydraulic limes were favored for this, but the need for a fast set time
encouraged the development of new cements. Most famous was Parker's "Roman cement". This was developed by
James Parker in the 1780s, and finally patented in 1796. It was, in fact, nothing like material used by the
Romans, but was a "natural cement" made by burning septaria – nodules that are found in certain clay deposits,
and that contain both clay minerals and calcium carbonate. The burnt nodules were ground to a fine powder. This
product, made into a mortar with sand, set in 5–15 minutes. The success of "Roman cement" led other
manufacturers to develop rival products by burning artificial hydraulic lime cements of clay and chalk. Roman
cement quickly became popular but was largely replaced by Portland cement in the 1850s.
19th century
Apparently unaware of Smeaton's work, the same principle was identified by Frenchman Louis Vicat in the first
decade of the nineteenth century. Vicat went on to devise a method of combining chalk and clay into an intimate
mixture, and, burning this, produced an "artificial cement" in 1817 considered the "principal forerunner"[4] of
Portland cement and "...Edgar Dobbs of Southwark patented a cement of this kind in 1811."
In Russia, Egor Cheliev created a new binder by mixing lime and clay. His results were published in 1822 in his
book A Treatise on the Art to Prepare a Good Mortar published in St. Petersburg. A few years later in 1825, he
published another book, which described various methods of making cement and concrete, and the benefits of
cement in the construction of buildings and embankments.
Portland cement, the most common type of cement in general use around the world as a basic ingredient of
concrete, mortar, stucco, and non-speciality grout, was developed in England in the mid 19th century, and usually
originates from limestone. James Frost produced what he called "British cement" in a similar manner around the
same time, but did not obtain a patent until 1822. In 1824, Joseph Aspdin patented a similar material, which he
called Portland cement, because the render made from it was in color similar to the prestigious Portland stone
quarried on the Isle of Portland, Dorset, England. However, Aspdins' cement was nothing like modern Portland
cement but was a first step in its development, called a proto-Portland cement. Joseph Aspdins' son William
Aspdin had left his father's company and in his cement manufacturing apparently accidentally produced calcium
silicates in the 1840s, a middle step in the development of Portland cement. William Aspdin's innovation was
counterintuitive for manufacturers of "artificial cements", because they required more lime in the mix (a problem
for his father), a much higher kiln temperature (and therefore more fuel), and the resulting clinker was very hard
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and rapidly wore down the millstones, which were the only available grinding
technology of the time. Manufacturing costs were therefore considerably
higher, but the product set reasonably slowly and developed strength quickly,
thus opening up a market for use in concrete. The use of concrete in
construction grew rapidly from 1850 onward, and was soon the dominant use
for cements. Thus Portland cement began its predominant role. Isaac Charles
Johnson further refined the production of meso-Portland cement (middle
stage of development) and claimed he was the real father of Portland
cement.
In the US the first large-scale use of cement was Rosendale cement, a natural cement mined from a massive
deposit of a large dolomite deposit discovered in the early 19th century near Rosendale, New York. Rosendale
cement was extremely popular for the foundation of buildings (e.g., Statue of Liberty, Capitol Building, Brooklyn
Bridge) and lining water pipes.
Sorel cement was patented in 1867 by Frenchman Stanislas Sorel. It was stronger than Portland cement but its
poor water resistance and corrosive qualities limited its use in building construction. The next development in the
manufacture of Portland cement was the introduction of the rotary kiln, which produced a stronger, more
homogeneous mixture and facilitated a continuous manufacturing process.
20th century
Calcium aluminate cements were patented in 1908 in France by Jules
Bied for better resistance to sulfates.
In the US, after World War One, the long curing time of at least a
month for Rosendale cement made it unpopular for constructing
highways and bridges, and many states and construction firms turned
to Portland cement. Because of the switch to Portland cement, by the
end of the 1920s only one of the 15 Rosendale cement companies had
survived. But in the early 1930s, builders discovered that, while The National Cement Share
Portland cement set faster, it was not as durable, especially for Company of Ethiopia's new plant in
Dire Dawa.
highways—to the point that some states stopped building highways
and roads with cement. Bertrain H. Wait, an engineer whose company
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had helped construct the New York City's Catskill Aqueduct, was impressed with the durability of Rosendale
cement, and came up with a blend of both Rosendale and Portland cements that had the good attributes of both.
It was highly durable and had a much faster setting time. Wait convinced the New York Commissioner of
Highways to construct an experimental section of highway near New Paltz, New York, using one sack of
Rosendale to six sacks of Portland cement. It was a success, and for decades the Rosendale-Portland cement
blend was used in highway and bridge construction.
Cementitious materials have been used as a nuclear waste immobilizing matrix for more than a half-century.
Technologies of waste cementation have been developed and deployed at industrial scale in many countries.
Cementitious wasteforms require a careful selection and design process adapted to each specific type of waste to
satisfy the strict waste acceptance criteria for long-term storage and disposal.
Modern cements
Modern hydraulic development began with the start of the Industrial Revolution (around 1800), driven by three
main needs:
Hydraulic cement render (stucco) for finishing brick buildings in wet climates
Hydraulic mortars for masonry construction of harbor works, etc., in contact with sea water
Development of strong concretes
Components of Cement:
Comparison of Chemical and Physical Characteristics[a]
Al2O3 6.9 23 18 12 —
Content (%)
Fe2O3 3 11 6 1 —
CaO 63 5 21 40 <1
MgO 2.5 — — — —
SO3 1.7 — — — —
Specific
15,000–
surface[d] 370 420 420 400
30,000
(m2/kg)
Specific
3.15 2.38 2.65 2.94 2.22
gravity
General
Primary Cement Cement Cement Property
use in
binder replacement replacement replacement enhancer
concrete
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Modern cements are often Portland cement or Portland cement blends, but industry also uses other cements.
Portland cement
Portland cement is by far the most common type of cement in general use around the world. This cement is made
by heating limestone (calcium carbonate) with other materials (such as clay) to 1,450 °C (2,640 °F) in a kiln, in a
process known as calcination that liberates a molecule of carbon dioxide from the calcium carbonate to form
calcium oxide, or quicklime—which then chemically combines with the other materials in the mix to form calcium
silicates and other cementitious compounds. The resulting hard substance, called 'clinker', is then ground with a
small amount of gypsum into a powder to make ordinary Portland cement, the most commonly used type of
cement (often referred to as OPC). Portland cement is a basic ingredient of concrete, mortar, and most non-
specialty grout. The most common use for Portland cement is to make concrete. Concrete is a composite material
made of aggregate (gravel and sand), cement, and water. As a construction material, concrete can be cast in
almost any shape, and once it hardens, can be a structural (load bearing) element. Portland cement may be grey
or white.
Portland blast-furnace slag cement, or Blast furnace cement (ASTM C595 and EN 197-1 nomenclature
respectively), contains up to 95% ground granulated blast furnace slag, with the rest Portland clinker and a little
gypsum. All compositions produce high ultimate strength, but as slag content is increased, early strength is
reduced, while sulfate resistance increases and heat evolution diminishes. Used as an economic alternative to
Portland sulfate-resisting and low-heat cements.
Portland-fly ash cement contains up to 40% fly ash under ASTM standards (ASTM C595), or 35% under EN
standards (EN 197-1). The fly ash is pozzolanic, so that ultimate strength is maintained. Because fly ash addition
allows a lower concrete water content, early strength can also be maintained. Where good quality cheap fly ash is
available, this can be an economic alternative to ordinary Portland cement.
Portland pozzolan cement includes fly ash cement, since fly ash is a pozzolan, but also includes cements
made from other natural or artificial pozzolans. In countries where volcanic ashes are available (e.g., Italy, Chile,
Mexico, the Philippines), these cements are often the most common form in use. The maximum replacement
ratios are generally defined as for Portland-fly ash cement.
Portland silica fume cement. Addition of silica fume can yield exceptionally high strengths, and cements
containing 5–20% silica fume are occasionally produced, with 10% being the maximum allowed addition under
EN 197-1. However, silica fume is more usually added to Portland cement at the concrete mixer.
Masonry cements are used for preparing bricklaying mortars and stuccos, and must not be used in concrete.
They are usually complex proprietary formulations containing Portland clinker and a number of other ingredients
that may include limestone, hydrated lime, air entrainers, retarders, waterproofers and coloring agents. They are
formulated to yield workable mortars that allow rapid and consistent masonry work. Subtle variations of Masonry
cement in the US are Plastic Cements and Stucco Cements. These are designed to produce a controlled bond with
masonry blocks.
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Expansive cements contain, in addition to Portland clinker, expansive clinkers (usually sulfoaluminate
clinkers), and are designed to offset the effects of drying shrinkage normally encountered in hydraulic cements.
This cement can make concrete for floor slabs (up to 60 m square) without contraction joints.
White blended cements may be made using white clinker (containing little or no iron) and white
supplementary materials such as high-purity metakaolin. Colored cements serve decorative purposes. Some
standards allow the addition of pigments to produce colored Portland cement. Other standards (e.g., ASTM)
don't allow pigments in Portland cement, and colored cements are sold as blended hydraulic cements.
Very finely ground cements are cement mixed with sand or with slag or other pozzolan type minerals that are
extremely finely ground together. Such cements can have the same physical characteristics as normal cement but
with 50% less cement, particularly due to their increased surface area for the chemical reaction. Even with
intensive grinding they can use up to 50% less energy (and thus less carbon emissions) to fabricate than ordinary
Portland cements.
Other cements
Pozzolan-lime cements are mixtures of ground pozzolan and lime. These are the cements the Romans used, and
are present in surviving Roman structures like the Pantheon in Rome. They develop strength slowly, but their
ultimate strength can be very high. The hydration products that produce strength are essentially the same as
those in Portland cement.
Slag-lime cements—ground granulated blast-furnace slag is not hydraulic on its own, but is "activated" by
addition of alkalis, most economically using lime. They are similar to pozzolan lime cements in their properties.
Only granulated slag (i.e., water-quenched, glassy slag) is effective as a cement component.
Supersulfated cements contain about 80% ground granulated blast furnace slag, 15% gypsum or anhydrite and a
little Portland clinker or lime as an activator. They produce strength by formation of ettringite, with strength
growth similar to a slow Portland cement. They exhibit good resistance to aggressive agents, including sulfate.
Calcium aluminate cements are hydraulic cements made primarily from limestone and bauxite. The active
ingredients are monocalcium aluminate CaAl2O4 (CaO · Al2O3 or CA in Cement chemist notation, CCN) and
mayenite Ca12Al14O33 (12 CaO · 7 Al2O3, or C12A7 in CCN). Strength forms by hydration to calcium aluminate
hydrates. They are well-adapted for use in refractory (high-temperature resistant) concretes, e.g., for furnace
linings.
Calcium sulfoaluminate cements are made from clinkers that include ye'elimite (Ca4(AlO2)6SO4 or C4A3S in
Cement chemist's notation) as a primary phase. They are used in expansive cements, in ultra-high early strength
cements, and in "low-energy" cements. Hydration produces ettringite, and specialized physical properties (such
as expansion or rapid reaction) are obtained by adjustment of the availability of calcium and sulfate ions. Their
use as a low-energy alternative to Portland cement has been pioneered in China, where several million tonnes per
year are produced. Energy requirements are lower because of the lower kiln temperatures required for reac-
tion, and the lower amount of limestone (which must be endothermically decarbonated) in the mix. In ad-
dition, the lower limestone content and lower fuel consumption leads to a CO2 emission around half that
associated with Portland clinker. However, SO2 emissions are usually significantly higher.
"Natural" cements corresponding to certain cements of the pre-Portland era, are produced by burning
argillaceous limestones at moderate temperatures. The level of clay components in the limestone (around
30–35%) is such that large amounts of belite (the low-early strength, high-late strength mineral in Portland
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cement) are formed without the formation of excessive amounts of free lime. As with any natural material, such
cements have highly variable properties.
Geopolymer cements are made from mixtures of water-soluble alkali metal silicates, and aluminosilicate mineral
powders such as fly ash and metakaolin.
Polymer cements are made from organic chemicals that polymerise. Producers often use thermoset materials.
While they are often significantly more expensive, they can give a water proof material that has useful tensile
strength.
Safety issues
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.[40]
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.
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China, representing an increasing share of world cement
consumption, remains the main engine of global growth. By
2012, Chinese demand was recorded at 2160 Mt, representing
58% of world consumption. Annual growth rates, which reached
16% in 2010, appear to have softened, slowing to 5–6% over 2011
and 2012, as China's economy targets a more sustainable growth
rate.
The performance in North America and Europe over the 2010–12 period contrasted strikingly with that of China,
as the global financial crisis evolved into a sovereign debt crisis for many economies in this region and recession.
Cement consumption levels for this region fell by 1.9% in 2010 to 445 Mt, recovered by 4.9% in 2011, then dipped
again by 1.1% in 2012.
The performance in the rest of the world, which includes many emerging economies in Asia, Africa and Latin
America and representing some 1020 Mt cement demand in 2010, was positive and more than offset the declines
in North America and Europe. Annual consumption growth was recorded at 7.4% in 2010, moderating to 5.1%
and 4.3% in 2011 and 2012, respectively.
As at year-end 2012, the global cement industry consisted of 5673 cement production facilities, including both
integrated and grinding, of which 3900 were located in China and 1773 in the rest of the world.
Total cement capacity worldwide was recorded at 5245 Mt in 2012, with 2950 Mt located in China and 2295 Mt in
the rest of the world.
China
"For the past 18 years, China consistently has produced more cement than any other country in the world. [...]
(However,) China's cement export peaked in 1994 with 11 million tonnes shipped out and has been in steady
decline ever since. Only 5.18 million tonnes were exported out of China in 2002. Offered at $34 a ton, Chinese
cement is pricing itself out of the market as Thailand is asking as little as $20 for the same quality."
In 2006, it was estimated that China manufactured 1.235 billion tonnes of cement, which was 44% of the world
total cement production. "Demand for cement in China is expected to advance 5.4% annually and exceed 1
billion tonnes in 2008, driven by slowing but healthy growth in construction expenditures. Cement consumed in
China will amount to 44% of global demand, and China will remain the world's largest national consumer of
cement by a large margin."
In 2010, 3.3 billion tonnes of cement was consumed globally. Of this, China accounted for 1.8 billion tonnes.
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Environmental impacts
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.
CO2 emissions
Carbon concentration in cement spans from ≈5% in cement structures
to ≈8% in the case of roads in cement.[53] Cement manufacturing
releases CO2 in the atmosphere both directly when calcium carbonate
is heated, producing lime and carbon dioxide, and also indi-
rectly through the use of energy if its production involves the
emission of CO2. The cement industry produces about 10% of global
man-made CO2 emissions, of which 60% is from the chemical process,
and 40% from burning fuel. A Chatham House study from 2018 es-
timates that the 4 billion tonnes of cement produced annually Global carbon emission by type to
account for 8% of worldwide CO2 emissions. 2004. Attribution: Mak Thorpe
The majority of carbon dioxide emissions in the manufacture of Portland cement (approximately 60%) are
produced from the chemical decomposition of limestone to lime, an ingredient in Portland cement clinker. These
emissions may be reduced by lowering the clinker content of cement. They can also be reduced by alternative
fabrication methods such as the intergrinding cement with sand or with slag or other pozzolan type minerals to a
very fine powder.
To reduce the transport of heavier raw materials and to minimize the associated costs, it is more economical to
build cement plants closer to the limestone quarries rather than to the consumer centers.
In certain applications, lime mortar reabsorbs some of the CO2 as was released in its manufacture, and has a
lower energy requirement in production than mainstream cement. Newly developed cement types from
Novacem[60] and Eco-cement can absorb carbon dioxide from ambient air during hardening.
As of 2019 carbon capture and storage is about to be trialled, but its financial viability is uncertain.
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and also selenium are often found as trace elements in common metal sulfides (pyrite (FeS2), zinc blende (ZnS),
galena (PbS), ...) present as secondary minerals in most of the raw materials. Environmental regulations exist in
many countries to limit these emissions. As of 2011 in the United States, cement kilns are "legally allowed to
pump more toxins into the air than are hazardous-waste incinerators."
Clinker is manufactured by heating raw materials inside the main burner of a kiln to a temperature of 1450 °C.
The flame reaches temperatures of 1800 °C. The material remains at 1200 °C for 12–15 seconds at 1800 °C for
5–8 seconds (also referred to as residence time). These characteristics of a clinker kiln offer numerous benefits
and they ensure a complete destruction of organic compounds, a total neutralization of acid gases, sulphur oxides
and hydrogen chloride. Furthermore, heavy metal traces are embedded in the clinker structure and no by-
products, such as ash of residues, are produced.
The EU cement industry already uses more than 40% fuels derived from waste and biomass in supplying the
thermal energy to the grey clinker making process. Although the choice for this so-called alternative fuels (AF) is
typically cost driven, other factors are becoming more important. Use of alternative fuels provides benefits for
both society and the company: CO2-emissions are lower than with fossil fuels, waste can be co-processed in an
efficient and sustainable manner and the demand for certain virgin materials can be reduced. Yet there are large
differences in the share of alternative fuels used between the European Union (EU) member states. The societal
benefits could be improved if more member states increase their alternative fuels share. The Ecofys study[68]
assessed the barriers and opportunities for further uptake of alternative fuels in 14 EU member states. The Ecofys
study found that local factors constrain the market potential to a much larger extent than the technical and
economic feasibility of the cement industry itself.
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Green cement
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. One is
Geopolymer cement.
New manufacturing processes for producing green cement are being researched with the goal to reduce, or even
eliminate, the production and release of damaging pollutants and greenhouse gasses, particularly CO2.
Growing environmental concerns and the increasing cost of fuels of fossil origin have resulted in many countries
in a sharp reduction of the resources needed to produce cement and effluents (dust and exhaust gases).
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 conventional construction materials.
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