Cement and CO2


Cement production accounts for 5% of the world's greenhouse gases. Each tonne of cement produces 1 tonne of CO2, 0.6 from the lime and 0.4 from the fuel.

Geopolymer green cement is made from fly ash or boiler slag, producing very little CO2. In many ways it is superior to Portland cement.

A green cement made from Magnesium oxide absorbs CO2..

Other green cements are available and in development.

   Click on images to access source and more detailed information

Cement, Concrete, & CO2

Cement is a striking example of how difficult it is to spread good ideas, and how easily knowledge is lost.
First, three definitions. Cement is the binding agent, add sand and it produces mortar, add stones/aggregate as well and it is concrete. The most used cement is Portland cement.
The manufacture of each tonne of Portland cement produces one tonne of CO2.
2.5 billion tons of Portland cement is produced annually and it is responsible for 5% of global CO2 emissions.
Cement is the third largest contributor to global CO2 emissions after the burning of fossil fuels, and deforestation.
There are some old and new cements that don't give off CO2 during manufacture and these are known as green cements.

History of cement

The Assyrians, Egyptians and Babylonians used clay to hold bricks and rocks apart and distribute their load evenly to prevent the blocks below cracking through point loading.

The mud bricks were reinforced with straw.


The first concrete-like structures were built by the Nabataea traders or Bedouins who occupied and controlled a series of oases and developed a small empire in the regions of southern Syria and northern Jordan in around 6500 BC. They later discovered the advantages of hydraulic lime -- that is, cement that hardens underwater -- and by 700 BC, they were building kilns to supply mortar for the construction of rubble-wall houses, concrete floors, and underground waterproof cisterns. The cisterns were kept secret and were one of the reasons the Nabataea were able to thrive in the desert.

By about 5600 BC along the Danube River in the area of the former country of Yugoslavia, homes were built using a type of concrete for floors. Source...


The Egyptians discovered two cements, Lime, and gypsum.


Lime is limestone (calcium carbonate) that has been heated to drive off the CO2 producing lime.

Water is then added to produce calcium hydroxide, or slaked lime.

This is used as mortar. Over time it absorbs and reacts with CO2 from the atmosphere to convert back to calcium carbonate. 

Effective, but not terribly strong or waterproof.

Gypsum - plaster

The other discovery was gypsum or calcium sulfate crystals with two waters of crystallisation. It is heated to drive off three quarters of the water. When combined with water again, the crystals reform and set solid. This is the plaster of Paris used in wall boards or for casting as statues etc.








   An ancient Nabataea building



CaCO3   ---->  CaO + CO2 (heated)

CaO + H2O ---> Ca(OH)2  Hydrating or slaking the lime

Ca(OH)2 + CO2 ---> CaCO3 + H2O    Setting over time


Gypsum - Plaster

CaSO4.2H2O   ---> CaSO4.1/2H2O  (heated)  Making the plaster

CaSO4.1/2H2O + 1.5H2O  ---> CaSO4.2H2O  Setting


By 600 BC, the Greeks had discovered a natural pozzolan material that developed hydraulic properties when mixed with lime.


Roman cement

In 200 BC The Romans discovered that if lime was mixed with volcanic ash, it  would set hard when mixed with water. It would even set under water. 

The alkali lime reacts with the silicic acid found with natural glass - obsidian, in dry volcanic ash. The product of the reaction is calcium silicate hydrate which is insoluble in water.

The new cement sets in water so is called hydraulic cement and could be used in harbours and aqueducts etc.

The ash comes from  Mount Vesuvius near the village of Pozzuoli.

In other territories such as Britain the ash was replaced with pulverised pottery, tiles, or bricks.

Pozzolanic reaction in Roman cement

Ca(OH)2 + H4SiO4 → Ca2+ + H2SiO42- + 2 H2O → CaH2SiO4 · 2 H2O

After Rome

After the fall of the Roman empire, the method of making cement was largely lost, though military engineers and some stone masons probably still used the knowledge during the dark ages.  

In 1414 librarians discovered manuscripts describing those techniques and this rekindled interest in building with concrete.

The Pantheon in Rome is the largest non reinforced concrete dome ever built. It has survived almost 2,000 years so far.



In England during the industrial revolution cements were made by firing natural deposits with a mixture of clay and chalk. The quality depended on the mix of the day.

Repeated structural failure of the Eddystone Lighthouse off the coast of Cornwall, England, led John Smeaton, a British engineer, to conduct experiments with mortars in both fresh and salt water. In 1756, these tests led to the discovery that cement made from limestone containing a considerable proportion of clay would harden under water.

Making use of this discovery, he rebuilt the Eddystone Lighthouse in 1759. It stood for 126 years before replacement was necessary.

In 1824, Joseph Aspdin, a bricklayer and mason in Leeds, England, took out a patent on a hydraulic cement that he called portland cement because its color resembled the stone quarried on the Isle of Portland off the British coast. Aspdin's method involved the careful proportioning of limestone and clay, pulverizing them, and burning the mixture into clinker, which was then ground into finished cement.

Portland cement is a chemical combination of calcium, silicon, iron, and aluminum.

A hydraulic cement is one that sets under water.


Portland cement

Cement is made by heating a mixture of limestone and clay, or shale. CO2 is driven off the limestone and water is driven off the shale. More CO2 comes from the burning of the fuel for heating.

The clay provides silica, alumina, and ferric oxide necessary for the reaction.

Portland cement is made with 60% CaO, 40%SiO and some Al2O3, Fe2O3 and SO3. The source of calcium is limestone, which is mainly calcium carbonate, 

In manufacturing a tonne of Portland cement approximately 0.60 tonne CO2 from the calcination reaction and, approximately 0.40 tonne CO2 is produced from fossil fuels used to generate the energy to heat the materials to 1400 degrees C. In total the manufacture of a tonne of OPC therefore emits approximately 1 tonne CO2.  Source..

The chemistry....


Reducing the CO2 from lime production

When limestone is heated to make lime, CO2 is driven off. Because it is mixed with flue gas it would be expensive to capture and dispose of.

A new process drives the CO2 off with steam. The CO2 is 98% pure and can be buried.

Calix has reinvented the calcining process with the development of its innovative Catalytic Flash Calcination (CFC) reactor. We are deploying this new reactor technology at our full-scale commercial production facility in Victoria, Australia and in conjunction with partners. 

Instead of directly heating minerals in the traditional way, we heat them indirectly and in the presence of steam. This separates the processes of calcination and combustion, delivering profound benefits in terms of mineral properties and carbon emissions.  Source: Calix

Geopolymer green cement from blast furnace slag

In the 1950s Ukrainian scientist Victor Glukhovsky invented geopolymer cement that  does not give off CO2 during manufacture. It uses alumino-silicates in slag from a blast furnace.

Most of the concrete poured between 1962 and 84 in the Ukraine was this new cement, and it has now been examined and found to have lasted longer than Portland cement would have. Ref....


Geopolymer cement from fly ash

As well as slag, fly ash from coal fired power stations can be used to produce the geopolymer.

Alkali is used to activate the hydration process by reacting with aluminium and silicon compounds from the slag and/or ash.

These molecules then link together, forming much longer molecules…they’re the geopolymers.

The geopolymers themselves link together, creating a vast three dimensional network. That’s what gives the cement its strength. And no carbon dioxide is produced.

Up to 80% of the cement's strength can be reached in only four hours at room temperature.

Apart from the obvious environmental effects, geopolymer concrete has been shown to have:

High fire, acid, pH, salt and freeze‐thaw resistance,
Higher unconfined compressive strength
Less shrinkage than normal concrete.

The geopolymer concrete also solidifies the heavy metal ions which come from the fly‐ash and slag so that it effectively immobilises and blocks them from the environment.

At present ‘geopolymer green cement’ is 62% more expensive per cubic meter than Portland cement though the reason is not obvious. : 

Company making geoploymer green cement: Zeobond



Fly ash is mainly composed of vitrified (amorphous) alumina-silicate melt in addition to a small amount of crystalline minerals, such as quartz, mullite, mica etc. There is a high degree of polymerization at which tetrahedral silicate is bridged with oxygen. Alkali  activates hydration of fly ash.




Type Blast furnace slag Electric arc furnace slag Ordinary cement
Component Oxidizing slag Reducing slag
CaO 41.7 22.8 55.1 64.2
SiO2 33.8 12.1 18.8 22.0
T-Fe 0.4 29.5 0.3 3.0
MgO 7.4 4.8 7.3 1.5
Al2O3 13.4 6.8 16.5 5.5
S 0.8 0.2 0.4 2.0
P2O5 <0.1 0.3 0.1 -
MnO 0.3 7.9 1.0



Green Cement - Magnesium

Magnesium phosphate cements made from magnesia (MgO) and the phosphate in animal faeces( or fermented plant material) were used in the great wall of China. 

Magnesia reacts with soluble phosphates to precipitate almost totally insoluble magnesium phosphate.

Ancient times in Europe, India, and China, among other countries. The Great Wall of China and many of the stupas in India, still standing today, were all made with magnesium-based cements.

Blends of magnesium oxide were used in ancient times in Germany, France, Mexico and Latin America, Switzerland, India, China and New Zealand, among other countries. Ref..

(these words have been used in many websites so not sure of the original source or accuracy)

these cements develop considerably greater compressive and tension strengths compared to Portland cement.


Magnesium oxide when combined with clay and cellulose creates cements. Portland cement does not bind to cellulose.

Magnesium cements on sale are: 

Grancrete - sets in 10-15 minutes

Bindan - Magnesium Oxide with phosphate

Ceratech - patching cements


Geoswan - MgO fibre boards (probably cellullose fibres)

All magnesium based cements use less energy and release less CO2 during manufacture.

MgO + H2O = Mg(OH)2
3Mg(OH)2 + 2H3PO4 = Mg3((PO)4)2 + 6H2O

Green Cement - Novacem

Novacem make a cement  based on Magnesium that absorbs 100 KG of CO2 per tonne.

Novacem has developed a new class of cement which will offer performance and cost parity with ordinary Portland cement, but with a carbon negative footprint. It is uniquely positioned to meet the challenge of reducing cement industry carbon emissions.


Our cement is based on magnesium oxide (MgO) and hydrated magnesium carbonates. Our production process uses accelerated carbonation of magnesium silicates under elevated levels of temperature and pressure (i.e. 180oC/150bar). The carbonates produced are heated at low temperatures (700oC) to produce MgO, with the CO2 generated being recycled back in the process. The use of magnesium silicates eliminates the CO2 emissions from raw materials processing. In addition, the low temperatures required allow use of fuels with low energy content or carbon intensity (i.e. biomass), thus further reducing carbon emissions. Additionally, production of the carbonates absorbs CO2; they are produced by carbonating part of the manufactured MgO using atmospheric/industrial CO2. Overall, the production process to make 1 tonne of Novacem cement absorbs up to 100 kg more CO2 than it emits, making it a carbon negative product.

From Novacem website.


Koalin or China clay is an alumino slilcate that reacts with lime to form a cement. It is added to cement 5-20% to change it's properties. It represents a large saving in CO2 production.

A new cement made from kaolin, Kaocem claims to cut CO2 by 80%.

(It is not clear how this reaction works)


Marine cement

Calera is producing/developing a green cement by passing CO2 from flue gas through sea water. This produces calcium and magnesium carbonate thereby capturing nearly a tonne of CO2 for every tonne of cement manufactured.

"Calera’s Substirute Cementitious Material is produced to perform similarly to fly ash and can be used to replace cement in concrete mixtures thereby avoiding the release of CO2"

They say the process mimics the process marine life use to make shells and corals.

(Calera is vague on how the calcium carbonate become a cement. Their website looks as if it was written by their PR dept. with no technical knowledge.)


Portland cement blends

From wikipedia..

Portland blast furnace cement

Up to 70 % 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 flyash cement

Contains up to 30 % fly ash. 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

Contains 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.

Portland silica fume cement.

Addition of silica fume (colloidal silica) can yield exceptionally high strengths, and cements containing 5–20 % silica fume are occasionally produced. However, silica fume is more usually added to Portland cement at the concrete mixer.



​Research is being carried out into reducing the quantity of cement used by increasing its strength with carbon nanotubes.