How Does Concrete Work? Cement, Water and the Chemistry of Setting

Concrete is a composite: aggregate, meaning sand and gravel or crushed rock, glued together by cement paste. The cement is the active part, and the modern kind, Portland cement, is made by heating limestone and clay to about 1,450 degrees in a rotating kiln, which drives carbon dioxide out of the limestone and fuses the calcium, silicon, aluminium and iron oxides into lumps of clinker, which are ground with a little gypsum into the grey powder sold in bags. Joseph Aspdin, a Leeds bricklayer, patented the process in 1824 and named the product for its resemblance, when set, to the building stone of Portland in Dorset. The powder is a set of calcium silicates and aluminates that are unstable in the presence of water and waiting to react.

When water is added, the cement compounds dissolve and recombine into new ones, above all a fibrous gel called calcium silicate hydrate, which grows outward from each cement grain as tangled needles and plates that interlock with each other and with the aggregate. The reaction, hydration, gives off heat, which is why a large pour must be cooled and why the Hoover Dam had refrigeration pipes cast into it and would otherwise still be warm. It also continues for a long time: concrete reaches most of its strength in a month and goes on hardening for years, and it does so under water as well as in air, which is what makes it useful for harbours and foundations. Letting it dry out early stops the reaction and weakens it, so fresh concrete is kept damp, and the water-to-cement ratio is the single number that most decides its final strength. The main stages:

• •Mixing: cement, water, aggregate and admixtures combined into a workable slurry
• •Placing and compacting: poured into forms and vibrated to remove air
• •Setting: within hours the paste stiffens as the first crystals form
• •Curing: kept moist for days while hydration builds strength
• •Hardening: continuing for months and years as the last cement grains react

Concrete is strong in compression, resisting crushing at 30 to 80 megapascals, and weak in tension, cracking when pulled at about a tenth of that. A beam bends, and the underside of a bending beam is in tension, so plain concrete beams snap. The answer, developed by French gardeners and engineers in the 1850s to 1880s, is to cast steel bars into the parts that will be pulled, so that the steel carries the tension and the concrete the compression. Two properties make the partnership work: steel and concrete expand by almost exactly the same amount when heated, so they do not tear apart through the seasons, and the alkaline cement protects the steel from rusting. Prestressed concrete goes further, stretching the steel before the concrete sets so that the beam is squeezed in advance and never goes into tension at all, which is how motorway bridges span a hundred metres.

The alkaline protection is the weak point. Carbon dioxide from the air slowly neutralises the surface, and chloride from road salt or sea water penetrates the cover, and when either reaches the steel it rusts; rust occupies several times the volume of the iron it came from, and the expanding bars crack the concrete from inside, which lets in more water. Most concrete decay is corrosion of reinforcement, and the design life of an ordinary structure is fifty to a hundred years. The Romans had no steel, so their concrete could not fail that way, and their marine concrete, made with volcanic ash and seawater, has been found to grow new interlocking minerals over the centuries, which is why the harbour walls of Caesarea are intact and why researchers are copying the recipe.

Cement is about eight percent of global carbon dioxide emissions, more than aviation, for two reasons: the kiln burns fuel to reach 1,450 degrees, and the limestone gives off carbon dioxide as it turns to lime, which is chemistry and cannot be avoided by changing the fuel. The routes to cutting it are to use less cement per tonne of concrete, replacing part of it with fly ash, slag or calcined clay, which the industry already does; to capture the kiln's carbon dioxide, which the first full-scale plant in Norway began doing in 2025; to make cements from other rocks; and to let the concrete reabsorb some of the gas over its life, which it does, slowly, as it carbonates. None is a complete answer, and the material that built the modern world is one of the harder parts of it to make clean.

Concrete is sand and gravel bound by cement, a powder of calcium silicates made by burning limestone and clay, which hardens not by drying but by reacting with water to grow interlocking crystals that keep strengthening for years. Steel bars cast inside carry the tension that concrete cannot, protected by the cement's alkalinity until carbon dioxide or salt reaches them and they rust, and the limestone's own carbon dioxide makes cement one of the largest industrial sources of the gas.