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.

Because concrete gains its strength by a chemical reaction rather than by drying, you cannot judge it by looking, so builders test it at two stages. The first happens on site as the truck arrives, with the slump test. Fresh concrete is packed into a metal cone open at both ends, the cone is lifted away, and the pile is measured to see how far it sags. A stiff mix barely moves and a wet one collapses. The test checks that the batch has the workability that was ordered, and a mix that slumps far too much often has extra water added, which will cost strength later.

The second stage checks strength directly. Samples from the same delivery are poured into moulds, cubes in Britain and cylinders in the United States, and cured in water under controlled conditions. They are then crushed in a hydraulic press at set ages, commonly seven and twenty-eight days, and the load at which each one fails gives the compressive strength. Twenty-eight days became the standard reference because most of the strength has developed by then, and the seven-day result gives an early warning. If the cubes fail below the specified strength, engineers may drill cores from the actual structure to check it.

The dome of the Pantheon in Rome, completed under the emperor Hadrian in the 120s AD, is about 43 metres across and made of unreinforced concrete, and it is still the largest dome of its kind in the world. Its builders understood that their material was strong when squeezed and weak when pulled, and they designed the structure to keep it squeezed. The concrete was mixed with different aggregates at different heights, heavy stone near the base, lighter volcanic rock higher up and pumice near the top, so the dome grows lighter as it rises and presses down less on the parts below.

The shape helps in the same way. The dome is around six metres thick at its base and thins to little more than a metre at the edge of the open oculus at the top, about eight metres wide, which removes weight where it would push outward hardest. The sunken coffers inside remove more weight. The danger in a dome is the tendency of its lower part to spread outward, which puts that zone in tension, and the Romans countered it with massive supporting walls and stepped rings of concrete around the outside of the base. Cracks did form, but the dome still stands in compression.

Small cracks are normal in concrete, but they let in water and air, which can corrode the steel reinforcement inside and slowly weaken a structure. Researchers have been developing concrete that seals its own cracks. One approach, developed at Delft University of Technology in the Netherlands by microbiologist Henk Jonkers, mixes in dormant bacteria along with food for them.

When a crack lets in water, the bacteria wake up, consume the food and produce limestone, which fills the gap. The bacteria chosen can survive the harsh, alkaline conditions inside concrete for years. Other researchers use capsules of healing agents that break open when a crack passes through them. Self-healing materials could extend the life of bridges and tunnels and reduce costly repairs.

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.