Cement hydrationBy the process of hydration - reaction with water - Portland cement, mixed with sand gravel and water, produces the synthetic rock we call concrete. Concrete is as essential a part of the modern world as are electricity or computers.Other pages on this web site describe how Portland cement is made and what is in it. Here, we'll discuss what happens when water is added to cement. Cement clinker is anhydrous - without water - having come from a hot kiln. Cement powder is also anhydrous if we ignore the small amount of water in any gypsum added at the clinker grinding stage.
The process by which cement reacts with water is termed 'hydration.' In cement, this involves many different reactions, often occurring at the same time. As the reactions proceed, the products of the cement hydration process gradually bond together the individual sand and gravel particles, and other components of the concrete, to form a solid mass.
The hydration process – reactionsPortland cement is composed largely of four types of minerals: alite, belite, aluminate (C3A) and a ferrite phase (C4AF). For more information on the composition of cement clinker, see the clinker pages. Also present are small amounts of clinker sulfate (sulfates of sodium, potassium and calcium) and also gypsum, which was added when the clinker was ground up to produce cement powder.
When cement and water are mixed together, the reactions which occur are mostly exothermic – heat is produced. We can get an indication of the rate at which the minerals are reacting by monitoring the rate at which heat is evolved using a technique called conduction calorimetry. An illustrative example of the heat evolution curve produced by cement is shown below.
Almost immediately on adding water, some of the clinker sulphates and gypsum dissolve, producing an alkaline, sulfate-rich solution. Soon after mixing, the (C3A) phase - the most reactive of the clinker minerals - reacts with the water to form an aluminate-rich gel (Stage I on the heat evolution curve above). The gel reacts with sulfate in solution to form small rod-like crystals of ettringite. (C3A) hydration is a strongly exothermic reaction but it does not last long, typically only a few minutes and is followed by a period of a few hours of relatively low heat evolution. This is called the dormant, or induction period (Stage II). The first part of the dormant period – up to perhaps half-way through - corresponds to when concrete can be placed. As the dormant period progresses, the paste becomes too stiff to be workable. At the end of the dormant period, the alite and belite in the cement start to hydrate, with the formation of calcium silicate hydrate and calcium hydroxide. This corresponds to the main period of cement hydration (Stage III), during which time concrete strengths increase. The cement grains react from the surface inwards, and the anhydrous particles become smaller. (C3A) hydration also continues, as fresh crystals become accessible to water. The period of maximum heat evolution occurs typically between about 10 and 20 hours after mixing and then gradually tails off. In a mix containing Portland cement as the only cementitious material, most of the strength gain has occurred within about a month. Where the cement has been partly-replaced by other materials, such as fly ash, strength growth may occur more slowly and continue for several months or even a year. Final strengths may exceed those from Portland-cement-only mixes.
Ferrite hydration also starts quickly as water is added, but then slows down, probably because a layer of iron hydroxide gel forms, coating the ferrite and acting as a barrier, preventing further reaction.
Cement hydration productsThe products of the reaction between cement and water are termed 'hydration products.' In concrete (or mortar or other cementitious materials) made using Portland cement only as the cementitious material there are four main types of hydration product:Calcium silicate hydrate: this is the main hydration product and is the main source of concrete strength. It is often abbreviated, using cement chemists' notation, to 'C-S-H,' the dashes indicating that no strict ratio of SiO2 to CaO is inferred. The Si/Ca ratio is somewhat variable but typically approximately 0.45-0.50. Calcium hydroxide - Ca(OH)2: often abbreviated, using cement chemists' notation, to 'CH.' CH is formed mainly from alite hydration. Alite has a Ca:Si ratio of 3:1 and C-S-H has a Ca/Si ratio of approximately 2:1, so excess lime is available from alite hydration and this produces CH. Ettringite: ettringite is present as rod-like crystals in the early stages of cement hydration. The chemical formula for ettringite is [Ca3Al(OH)6.12H2O]2.2H2O] or, mixing cement notation and normal chemistry notation, C3A.3CaSO4.32H2O. Monosulfate: monosulfate tends to occur in the later stages of hydration, after a few days. Usually, it replaces ettringite, either fully or partly. The chemical formula for monosulfate is C3A.CaSO4.12H2O. Both ettringite and monosulfate are compounds of C3A, CaSO4 (anhydrite) and water, in different proportions. AFm and AFt phases: monosulfate is one of a group of minerals called ‘AFm’ phases. Ettringite is a member of a group known as AFt phases. The general definitions of these phases are somewhat technical, but ettringite is an AFt phase because it contains three (t-tri) molecules of anhydrite when written as C3A.3CaSO4.32H2O and monosulfate is an AFm phase because it contains one (m-mono) molecule of anhydrite when written as C3A.CaSO4.12H2O. Important points to note about AFm and AFt phases are that:
Monosulfate gradually replaces ettringite in many concretes because the ratio of available alumina to sulfate increases with continued cement hydration. On mixing cement with water, most of the sulfate is readily available to dissolve, but much of the C3A is contained inside cement grains with no initial access to water. Continued hydration gradually releases alumina and the proportion of ettringite decreases as that of monosulfate increases.
Hydrogarnet: hydrogarnet forms mainly as the result of ferrite or C3A hydration. Hydrogarnets have a range of compositions, of which C3AH6 is the main phase forming from normal cement hydration and then only in small amounts. A wider range of hydrogarnet compositions can be found in autoclaved cement products.
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