Cement

Powdered limestone and clay cooked at 1,450 C, which sets hard when mixed with water — the glue that holds concrete together.

なぜ重要なのか

Concrete is the most-used material on Earth after water, and cement making is roughly seven to eight percent of global CO2 emissions.

Where it is in the Earth

Cement is not made from a rare or exotic mineral. Its principal raw material is limestone, a sedimentary rock composed mainly of calcium carbonate (CaCO₃), which forms when calcium-rich shells and skeletal fragments accumulate on shallow sea floors and gradually compact and recrystallise over millions of years. Limestone is one of the most abundant rock types on the continental surface, and that abundance is precisely why cement can be manufactured in almost every country on Earth. The secondary raw material is a source of silica, alumina, and iron — typically clay, shale, or marl — which provides the compounds that give the final cement its hydraulic properties, meaning its ability to harden in the presence of water.

The deposits that matter are not rare occurrences but simply thick, pure, accessible limestone formations close to fuel and population. Purity matters: a limestone with a high calcium carbonate content and low amounts of magnesium, alkalis, or organic matter requires less blending and produces a more consistent clinker (the intermediate nodular product made inside the kiln before final grinding). Marl — a naturally occurring mixture of calcium carbonate and clay — is particularly convenient because it contains both the calcareous and aluminosilicate components in a single rock, reducing the need for a separate clay quarry. The map of cement production therefore tracks, broadly, the map of sedimentary basins, which is nearly everywhere people have built cities.

There is no meaningful concept of a mineral grade for limestone in the way that gold or copper ore is graded. What matters is the chemical composition — chiefly the calcium-to-silica ratio — and the proximity of adequate reserves to a kiln. Because limestone is heavy and cheap relative to its weight, transport costs dominate the economics of raw material supply, and virtually every large cement plant sits adjacent to or within a few kilometres of its own quarry.

Getting it out

Limestone for cement is quarried in open pits, almost without exception. The rock lies at or close to the surface over wide areas, which makes underground mining unnecessary and uneconomical. A typical quarry operates by drilling rows of holes into the rock face, loading them with explosives, and blasting to break the limestone into fragments small enough for primary crushing. Wheel loaders and large dump trucks then carry the broken rock to a primary crusher, where it is reduced to pieces roughly the size of a human fist before conveyor belts carry it to the cement plant.

Because limestone is abundant and the rock itself is the product — not a trace constituent within a host rock — there is very little waste in the conventional sense. The overburden, meaning the soil and non-limestone material above the deposit, must be stripped away before quarrying can begin, but once the limestone bench is exposed, nearly all of it can be used. Blending is the main operational challenge: the quarry manager must mix rock from different faces of the pit to keep the feed to the kiln within a narrow chemical specification. Where the natural rock is too high in silica or too low in calcium, a corrective material such as iron ore, bauxite, or pure calcium carbonate is added in small quantities. The scale of these quarries is large — a single plant typically consumes millions of tonnes of limestone per year — but because the resource is widespread and not geologically restricted, permitting and land access are the more common constraints on expansion, not geological scarcity.

What pulls on it

Almost all cement goes into concrete, which in turn goes into construction — buildings, roads, bridges, dams, ports, and the other fixed structures that underpin urbanisation. Demand therefore tracks population growth, economic development, and the rate at which societies build permanent infrastructure. Countries in early or middle stages of urbanisation consume cement at a far higher rate per person than mature economies, where most of the building stock already exists and activity is dominated by renovation rather than new construction. China's share of global production — roughly 1,700,000 thousand metric tonnes out of a world total of 3,800,000 thousand metric tonnes in 2025 — reflects the extraordinary pace of Chinese infrastructure and housing construction over recent decades, though that rate of growth has slowed as the country's urbanisation matures. India's figure of 470,000 thousand metric tonnes reflects a younger urbanisation curve still ascending.

Cement demand is almost entirely domestic. Because it is heavy, low in value per tonne, and produced from limestone that is available nearly everywhere, the economics of long-distance ocean freight are marginal. International trade flows mainly arise from regional imbalances — countries with insufficient production capacity importing from neighbours with surplus capacity — rather than from geological scarcity. The U.S. net import reliance figure of 21 percent for 2025 is notable precisely because it reflects a gap between domestic demand and domestic production capacity, filled largely by imports from Turkey, Canada, Vietnam, and Greece. For a material this bulky and this widely produced, that level of import dependence signals that domestic plant construction has not kept pace with demand rather than that the United States lacks the limestone to make cement.

Demand could change sharply in two directions. On the upside, large infrastructure programmes — roads, housing, water systems — in South and Southeast Asia and Sub-Saharan Africa represent the continuation of a long-running trend. On the downside, a sustained shift toward lower-clinker blended cements, or the adoption of alternative binders, would reduce the volume of traditional Portland cement required per unit of construction output. Neither trend is fast-moving; the construction industry adopts new materials slowly because structural failure carries severe consequences and building codes change over years, not months.

Turning ore into product レベル 3

The journey from quarried limestone to finished cement passes through a sequence of high-temperature and mechanical steps, the most energy-intensive of which is the rotary kiln. Crushed and blended raw materials are first ground to a fine powder, then fed into a kiln — a rotating steel cylinder lined with refractory brick — where they are heated to approximately 1,450 °C. At that temperature, calcium carbonate decomposes into calcium oxide (CaO) and carbon dioxide in a reaction called calcination, and the calcium oxide then combines with silica, alumina, and iron oxides to form the mineral phases collectively known as clinker. It is this calcination step that releases the process CO₂ that makes cement manufacturing such a significant source of emissions, independent of the fuel combustion required to reach kiln temperature.

The clinker exits the kiln as dark, hard nodules and is rapidly cooled in a grate cooler to preserve its reactive mineral phases. It is then ground with a small addition of gypsum (calcium sulfate), which regulates the rate at which the cement sets when water is added. The resulting fine grey powder is ordinary Portland cement (OPC). Modern plants frequently substitute a portion of the clinker with supplementary cementitious materials (SCMs) — fly ash from coal-fired power stations, granulated blast-furnace slag from steelmaking, or natural pozzolans such as volcanic ash — to produce blended cements. These substitutions reduce the clinker-to-cement ratio, lowering both the energy consumption per tonne of finished product and the associated CO₂ emissions. The traded form of cement therefore ranges from pure clinker (shipped in bulk for grinding elsewhere) to fully blended bagged cement, and the distinction matters for interpreting production statistics because not all published figures account for SCM additions consistently.

The main losses in the process are thermal rather than material: the chemistry of clinker formation means that a tonne of clinker always requires somewhat more than a tonne of dry raw material because CO₂ is driven off during calcination. Fuel efficiency, measured as heat consumption per tonne of clinker, varies considerably between the dry process (the modern standard, in which raw materials are fed to the kiln dry) and the older wet process (where a slurry is used), with the wet process consuming substantially more energy per tonne. Most plants in high-income countries have converted, but wet-process capacity persists in some older industrial economies, contributing to disagreements in energy-intensity comparisons across regions.

Substitution and recycling レベル 3

Within concrete, cement can be partially displaced by supplementary cementitious materials. Fly ash, ground granulated blast-furnace slag (GGBS), and silica fume are all pozzolanic or latently hydraulic materials — they react with water or with the calcium hydroxide released during cement hydration to contribute to concrete strength. Their use is constrained by availability (fly ash supply is declining as coal-fired power generation contracts in some regions), by the slower early strength gain they produce relative to pure Portland cement, and by the conservatism of structural design codes. Geopolymer cements, which use an alkali activator to cure fly ash or slag without any Portland clinker, can achieve comparable strengths in laboratory conditions, but commercial adoption remains limited because the supply chains for activators such as sodium silicate are not organised at the scale cement requires, and consistency of feedstock is harder to guarantee.

Recycling of cement itself is not meaningful in the way that metal recycling is. When concrete is demolished, the crushed material — recycled concrete aggregate — can partially replace virgin aggregate in new mixes, but the cement paste that held the original structure together has already hydrated and cannot be reactivated. Some research programmes are investigating whether carbonated recycled cement paste can act as a supplementary material, but this is not practised at industrial scale. The more relevant circular flow is the continued use of industrial by-products as SCMs: every tonne of slag or fly ash incorporated into cement is a tonne of clinker not required, and the economics of this substitution are generally favourable because the by-products are available at low cost and reduce both raw material and energy spending. The limiting factor is simply that the supply of these by-products is determined by the steel and power industries, not by cement demand.