Hydration
Cement, lime and plaster set by reacting with water rather than by drying — which is why concrete hardens under the sea.
Concrete does not dry. This is the most widely held wrong belief about any common material, and almost every practical mistake made with concrete follows from it.
What happens is a chemical reaction. The calcium silicates in Portland cement react with water to grow an interlocking mass of calcium silicate hydrate — a gel of needles and sheets that knits the aggregate together — and the water is consumed into the product rather than evaporating from it.
Three things follow immediately, and all three are counterintuitive. Concrete sets perfectly well under water, and always has. It must be kept wet for days after pouring, because concrete that dries out stops hydrating and never reaches its strength. And adding water to make it easier to place permanently weakens it, because the extra water leaves pores behind when it eventually does evaporate — the water-to-cement ratio is the single most important number in concrete, and it is the one most often quietly increased on site.
Gypsum plaster and lime mortar set by related but different chemistry. Plaster rehydrates back to the mineral it was calcined from, in minutes rather than weeks. Lime mortar mostly does not hydrate at all: it hardens by absorbing carbon dioxide from the air over months and years, slowly turning back into limestone.
Why it behaves as it does
The reaction runs for a very long time. Concrete reaches most of its strength in twenty-eight days, which is why that is the number every specification uses, and it continues gaining slowly for years — a hundred-year-old dam is stronger than it was when it was signed off.
It is also exothermic, and at scale that is a structural problem rather than a curiosity. A large pour generates enough heat that the interior expands while the surface cools and contracts, and the resulting stress cracks it. Mass concrete is therefore poured in lifts, made with cement blended with slag or fly ash that hydrates more slowly, and in extreme cases cooled by pipework cast into it — which is what was done at the Hoover Dam, where the heat would otherwise have taken over a century to dissipate.
The lime cycle is worth stating whole, because it is a loop. Limestone is burned to quicklime, driving carbon dioxide off. Quicklime is slaked with water to slaked lime. Slaked lime, used as mortar, absorbs carbon dioxide from the air and becomes limestone again. Every atom returns to where it started, and the carbon released in the kiln is reabsorbed by the wall over the following decades — which is a real and frequently overstated part of the carbonation argument in modern cement accounting.
How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.
How this connects
Where a connection has been confirmed by an outside reference, that reference is named beside it.
produces
- Concrete — material · it does not dry — the calcium silicates react with the mix water and grow an interlocking hydrate gel, which is why it sets under water and must be kept wet to reach strength
- Reinforced concrete — material · the same reaction around a steel cage, and the concrete's alkalinity is what passivates the steel and stops it rusting — until carbonation or chloride reaches it
- Gypsum plaster — material · rehydrating back into the mineral it was calcined from, in minutes rather than weeks, which is why plaster is mixed in small batches
- Lime mortar — material · and mostly it does not hydrate at all: it hardens by absorbing carbon dioxide from the air over months and years, turning back into the limestone it was burned from
takes as input
- Portland cement — material · the reactive component — everything else in a concrete mix is aggregate, water, or an admixture adjusting how this reaction runs
- Water — compound · consumed into the product rather than evaporated from it, and the water-to-cement ratio is the single most important number in concrete — and the one most often quietly increased on site
- Quicklime — compound · slaked with water to make the lime putty that mortar and plaster are mixed from
Sources
- Material WorldOur own writing
- Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)