Limestone into Cement

Limestone and Clay

Cement starts as rock. Limestone brings the lime; clay or shale brings silica, alumina and iron oxide; where one falls short, a little sand, iron ore or bauxite makes it up (USGS). The USGS puts a tonne of clinker at about 1.7 t of raw material, about 1.5 t of it limestone or a similar rock.

Real recipes are about four-⁠fifths limestone: a reference raw meal in a 2026 paper is 79 % limestone, about 19 % clay and sand and 2 % iron; other mixes run 83⁠–⁠86 % limestone. Ground together into a fine powder, they are the raw meal.

In the USGS’s typical clinker the lime is about 65 %, the silica 22 %, the alumina 6 % and the iron oxide 3 %. Three ratios steer the mix: the lime saturation factor (LSF), the lime there is against the most that the silica, alumina and iron can take up, usually 0.92⁠–⁠0.98 in modern clinker; the silica ratio, 2⁠–⁠3; the alumina ratio, about 1.6.

CaO 65 % · SiO₂ 22 % · Al₂O₃ 6 % · Fe₂O₃ 3 % · about 1.5 t of limestone per tonne of clinker

Five Stages of Cyclones

The raw meal goes first into the preheater tower. The EU’s reference document on best techniques describes four to six stages of cyclones stacked in a tower 50⁠–⁠120 m high, the top stage often two cyclones side by side. The hot gas from the kiln rises through them; the meal is fed into it just before the top stage.

Each cyclone flings the meal out of the gas; it drops into the duct of the stage below, is carried up again and flung out again, stage by stage, hotter each time. The meal spends only 20⁠–⁠90 seconds in the tower (USGS).

More stages recover more heat: in plants with a precalciner burning fossil fuels the gas leaves at about 280 °C with six stages, 310 °C with five and 350 °C with four (ECRA, 2017). The model has five, the top one a pair.

4⁠–⁠6 cyclone stages · a tower 50⁠–⁠120 m high · 20⁠–⁠90 s for the meal

Lime from Limestone

Limestone is mostly calcium carbonate. Heated far enough it breaks into lime and carbon dioxide: CaCO₃ → CaO + CO₂. 100 g of calcium carbonate leaves 56 g of lime; 44 g goes off as gas. The step takes in heat, about 178 kJ per mole (US National Bureau of Standards, 1969), some 1.78 MJ per kilogram (converted here).

From the same tables, the carbon dioxide over calcium carbonate reaches one atmosphere at about 885 °C (computed here); for practical burning the USGS gives about 950 °C or more.

A modern line does most of this in the precalciner, between the kiln and the cyclones. It runs at 850⁠–⁠950 °C and burns up to 65 % of all the fuel; its combustion air, the tertiary air, comes hot from the cooler through the duct over the kiln. The meal enters the kiln mostly calcined (75⁠–⁠95 % in the EU reference document).

CaCO₃ → CaO + CO₂ · about 178 kJ per mole · 850⁠–⁠950 °C in the precalciner

Three to Five Turns a Minute

The rotary kiln is a steel tube lined with refractory brick, sloping 2.5⁠–⁠4.5 % and turning 0.5⁠–⁠5 times a minute (EU reference document); the IEA puts it at about three to five. As it turns, the material slides and tumbles down towards the flame at the lower end.

Kilns with preheaters are typically 10 to 17 times as long as they are wide. One real kiln, at the Ste. Genevieve plant in Missouri, is 6.6 m across and 93 m long on three supports, and makes about 12,000 t of clinker a day (Global Cement, 2019). The material takes about 30⁠–⁠40 minutes to pass through the rotary kiln (EU reference document).

Each support is a riding ring round the shell, resting on a pair of rollers; a girth gear driven by a pinion turns the whole kiln. The model’s kiln is drawn about twice as thick as a real one and turns four times a minute.

slope 2.5⁠–⁠4.5 % · 0.5⁠–⁠5 turns a minute · one kiln: 6.6 m across, 93 m long, three supports

Only Partly Molten

At the lower end the flame is at about 2,000 °C; the material in the burning zone must be kept at 1,400⁠–⁠1,500 °C (EU reference document). Belite (C₂S) starts forming from about 700 °C, most of it at 1,100⁠–⁠1,200 °C; alite (C₃S) forms from belite and lime, and without a liquid that step is extremely slow (USGS).

Liquid appears at 1,338 °C, where the four clinker minerals and a melt can coexist. In an average clinker about 20 % is liquid near 1,340 °C and about 30 % at 1,400⁠–⁠1,450 °C, enough for alite to form fast; so 1,400⁠–⁠1,450 °C is the burning zone’s target (USGS).

The material does not melt through: partly molten, it sinters into nodules of clinker about 1⁠–⁠10 cm across. The burning zone is lined with basic brick such as magnesia-⁠spinel, under a crust of clinker coating; towards the feed end the lining is high-⁠alumina brick (RHI Magnesita). The glow in the section is drawn by temperature, as a schematic.

flame about 2,000 °C · material 1,400⁠–⁠1,500 °C · liquid from 1,338 °C · about 30 % liquid at 1,400⁠–⁠1,450 °C

Four Minerals

Clinker is mostly two calcium silicates, alite (close to C₃S, Ca₃SiO₅) and belite (close to C₂S), with aluminate (C₃A) and ferrite (close to C₄AF) between them. Cement chemists write C for CaO, S for SiO₂, A for Al₂O₃ and F for Fe₂O₃. Modern Portland cement holds about 50⁠–⁠70 % C₃S, 10⁠–⁠30 % C₂S, 3⁠–⁠13 % C₃A and 5⁠–⁠15 % C₄AF (USGS).

Polished and etched under a microscope, alite shows hexagonal outlines, belite rounded grains with a layered inside, ferrite bright white and aluminate grey (NIST). In three real clinkers the alite crystals were about 12⁠–⁠110 µm, the belite about 15⁠–⁠35 µm. The porcelain dish at the front holds nodules of clinker.

In 1929 Bogue gave a set of formulas for the minerals from the oxides; from 1940 they were in the US cement specification. By them the typical clinker of the first chapter holds about 53 % C₃S, 23 % C₂S, 11 % C₃A and 9 % C₄AF (computed here). Real minerals are not pure: in NIST’s three reference clinkers, X-⁠rays found 5⁠–⁠9 points more alite than the formulas give (computed here).

C₃S · C₂S · C₃A · C₄AF · Bogue for a typical clinker: about 53 / 23 / 11 / 9 %

Cooled Fast

The clinker drops into the cooler at about 1,450 °C (EU reference document). In a grate cooler, air is blown up through the bed of clinker lying on the grate. The USGS gives about 100 °C as safe for handling; a good grate cooler gets down to 80 °C above the air around it (ECRA, 2017).

The air comes off at 600⁠–⁠1,000 °C and goes back into the line: some as the burner’s secondary air, some through the tertiary air duct to the precalciner (ECRA, 2017).

Cooled fast, the minerals stay as they are. Measurements cited in a 1987 paper put the start of alite’s breakdown into belite and lime, on slow cooling, between 1,210 and 1,025 °C; large belite crystals can also turn into the γ form, swell and crumble the clinker.

in at about 1,450 °C · out at about 100 °C · air back at 600⁠–⁠1,000 °C

A Little Gypsum

Ground together with a little gypsum, clinker becomes Portland cement. The IEA says every cement holds about 4⁠–⁠5 % gypsum; the USGS gives 3⁠–⁠7 %. Under the European standard a CEM I Portland cement is 95⁠–⁠100 % clinker in its main constituents, the calcium sulfate counted apart.

The gypsum is there for the aluminate. Without sulfate, C₃A reacts almost the moment it meets water and the cement takes a “flash set”; with gypsum, a shell of ettringite grows round the aluminate grains and slows them down (USGS).

Ground cement grains average about 10 µm, and 85⁠–⁠95 % are under 45 µm (USGS). Many cements now carry other materials too: clinker was about 71 % of the world’s cement in 2022 (IEA).

clinker + about 4⁠–⁠5 % gypsum · grains about 10 µm · clinker about 71 % of world cement (2022)

Not by Drying

Cement does not harden by drying. The European standard defines it as a powder that, mixed with water, sets and hardens by hydration and keeps its strength even under water; “hydraulic”, the USGS writes, means it can set and harden under water, or with water to spare, through hydration.

Alite reacts with water to give calcium silicate hydrate (C-⁠S-⁠H) gel and calcium hydroxide; the textbook shorthand is 2 C₃S + 6 H → C₃S₂H₃ + 3 CH, H for water (USGS), and real gel holds about 1.7⁠–⁠1.8 calcium to each silicon. The C-⁠S-⁠H is the glue. Belite reacts the same way, only slower: at 28 days about 80 % of the alite has reacted, about 40 % of the belite.

So the strength grows day by day. Standard mortar of one CEM I 42.5 R cement reached 16.8 MPa at 1 day, 28.8 MPa at 2, 43.6 MPa at 7 and 57.0 MPa at 28 (supplier’s data, in a 2020 paper). The cube at the front is hardened cement mortar.

by hydration, not drying · 2 C₃S + 6 H → C₃S₂H₃ + 3 CH · about 57 MPa at 28 days (one CEM I 42.5 R)

Carbon Dioxide Twice

Making clinker sends out carbon dioxide twice. Once from the limestone itself: a clinker of 65 % lime, all of it from calcium carbonate, gives off 0.51 t per tonne (USGS; the IPCC’s default, with the kiln dust, is 0.52 t). And once from the fuel burnt to heat it.

In the industry’s 2006 figures (GNR) a tonne of clinker gave off 866 kg on average, typically about 540 kg of it from the raw material and so about 330 kg from the fuel (converted here). A kiln’s thermal efficiency only touches the fuel’s share; the stone’s stays. A cleaner fuel still leaves the carbon in the stone to come out.

Andrew (2019) puts the world’s process emissions from cement at 1.50 billion tonnes of CO₂ in 2018, about 4 % of the emissions from fossil fuels; the IEA (2018) puts the whole cement industry at about 7 % of the world’s CO₂. How carbon dioxide warms the air is on the greenhouse page.

per tonne of clinker: about 0.54 t from the stone, about 0.33 t from the fuel (GNR 2006) · process CO₂ about 4 % of fossil-⁠fuel CO₂

Named after a Stone

The Romans used lime and volcanic ash. Vitruvius wrote of a powder from round Mount Vesuvius that, mixed with lime and rubble, set even under the sea. That was not Portland cement.

On 21 October 1824 Joseph Aspdin was granted British patent No. 5022 for a cement “which I call Portland cement”, after the set mortar’s likeness to Portland stone. His method only called for burning until the carbonic acid was driven off, short of a sintered clinker; his son William Aspdin is generally credited with burning it hot enough to make an alite clinker from 1842, with I. C. Johnson following.

Ransome patented a rotary kiln in 1885; it was first made to work in the United States around 1890, and there, in 1892, a little gypsum was found to slow the set. The first multi-⁠stage cyclone preheater started at Beckum in Germany in 1953; precalciners first ran in Japan in the late 1960s.

patent 1824 · clinker from 1842 · rotary kiln patent 1885 · cyclone preheater 1953

Four Billion Tonnes

The USGS estimates the world made about 4.0 billion tonnes of cement in 2024: China about 1.9 billion, close to half (47.5 %, converted here), India about 450 million, Vietnam about 110 million, the United States about 86 million. Andrew (2019) puts it at more than half a tonne per person a year.

Cement is not concrete. Concrete is cement, water, sand and gravel or crushed stone mixed together; by the USGS’s volumes, the cement powder is only about 7⁠–⁠15 % of it. The paste of cement and water binds the stones into an artificial rock; with sand alone, it is mortar.

A tonne of cement makes 3.4⁠–⁠3.8 m³ of concrete weighing 7⁠–⁠9 t (USGS). Along the front, left to right: limestone, raw meal, clinker, cement and a cube of hardened mortar.

about 4.0 billion t in 2024 · China about 1.9 billion t · cement powder about 7⁠–⁠15 % of concrete by volume

The Kiln in Parts

The preheater tower and the precalciner lift together, and the hood lifts off; the kiln’s shell parts into three sections between its riding rings and slides apart along its axis, each still cut open so its lining and bed show. The grate cooler stays.

Rest the pointer on a part and it lifts on its own, named; otherwise the parts stand apart together.

preheater tower · precalciner · hood · inlet section · middle section · burning zone · grate cooler