Alkali-silica reaction

A reaction between the cement pore fluid in concrete with certain types of silica, which may occur in concrete aggregate. Susceptible types of silica include strained or microcrystalline quartz, glass, cristobalite, tridymite, opal, flint, chert. A gel is formed which exerts an expansive force within the concrete and which may lead to widespread cracking and failure of the concrete.

Bogue calculation

Calculation to give the approximate proportions of the four main clinker minerals in Portland cement, requiring the clinker bulk analysis and the free lime content, the free lime being subtracted from the total CaO before calculating the mineral proportions. Approximately given by:

C₃S=4.07CaO - 7.60SiO₂ - 6.72Al₂O₃ - 1.43Fe₂O₃

C₂S=-3.07CaO + 8.60SiO₂ + 5.07Al₂O₃ + 1.08Fe₂O₃

C₃A=2.65Al₂O₃ - 1.69Fe₂O₃

C₄AF=3.04Fe₂O₃.

Calcium hydroxide

Ca(OH)₂ or CH, Probably forms mainly in regions in concrete initially occupied by water on mixing.

Cement Chemistry Notation

An arcane but convenient notation where single letters represent oxides:

C=CaO, S=SiO₂, F=Fe₂O₃, A=Al₂O₃, N=Na₂O, K=K₂O

Sulfate (SO₃) is denoted by an 'S' with a short horizontal line above the S, spoken as 'S-bar.' Carbonate (CO₃) is similarly denoted by 'C-bar.' HTML restrictions mean this cannot be shown here, but see the downloadable pdf file.

C₃S represents tricalcium silicate. Alite in cement clinker is C₃S with impurities.

C₂S represents dicalcium silicate. Belite in cement clinker is C₂S with impurities.

C₃A represents tricalcium aluminate. Aluminate phase in cement clinker is C₃A with impurities.

C₄AF represents tetracalcium aluminoferrite. Ferrite phase in cement clinker may approximate to this composition but the proportions of C, A and F may vary considerably. Ferrite also contains impurities.

C-S-H represents calcium silicate hydrate. The '-' indicate no exact stoichiometric proportions are intended.

CH represents calcium hydroxide.

AFt (Al2O3-Fe2O3-tri) represents a group of calcium sulphoaluminate hydrates. AFt has the general formula: [Ca3(Al,Fe)(OH)6.12H2O]2.X3.xH2O where X represents a doubly-charged anion or, sometimes, two singly-charged anions. Ettringite is the most common and important member of the AFt group (X in this case denoting sulfate.)

AFm (Al2O3-Fe2O3-mono) represents another group of calcium aluminate hydrates with the general formula: [Ca2(Al,Fe)(OH6)].X.nH2O where X represents a singly charged anion or 'half' a doubly-charged anion. X may be one of many anions; the most important in Portland cement hydration are hydroxyl, sulfate and carbonate.

Clinker mineral proportions

Proportions of the four main minerals in Portland cement clinker (alite - C3S; belite ­ C2S; aluminate - C3A and ferrite - C4AF are commonly calculated using the Bogue calculation. This uses assumed pure mineral compositions and requires the bulk analysis of the clinker for CaO, SiO2, Al2O3 and Fe2O3, and the unreacted lime in the cement ('free lime'). See Bogue calculation.

EDX

Energy-dispersive X-ray microanalysis (sometimes also known as EDS: Energy-dispersive X-ray Spectroscopy and EDAX.)

Ettringite

C3A.3CaSO4.32H2O - usually forms at an early stage in Portland cement hydration. Mature concretes usually contain little or no ettringite. Ettringite is commonly found in smaller or larger quantities in concrete subjected to sulfate attack. See AFt phase in 'Cement Chemistry Notation.'

Free lime

Unreacted lime (CaO) in Portland cement clinker; usually less than about 2%.

Hydrogarnet

Hydrated mineral of variable composition: often referred to as C3AH6 but this is only one end member of a solid solution series bounded by C3AH6, C3FH6, C3AS3, C3FS3. Occurs in concrete in sites formerly occupied by ferrite phase. Also a significant phase in autoclaved concrete products, especially when made using pulverised fuel ash (PFA).

In-situ hydration product

Hydration product which has formed within the volume formerly occupied by a cement grain - usually this is only visible by SEM for the larger grains. In-situ hydration product is usually relatively pure C-S-H. Also known as 'inner product.' See also ‘Undesignated product.’

SEM

Scanning Electron Microscope; also Scanning Electron Microscopy.

Stratlingite

An AFm phase with an aluminosilicate anion, composition C₂ASH₈.

Sulfate (sulphate) attack

Concrete made from Portland cement is susceptible to attack by solutions which contain sulfate. There are different forms of sulfate attack.

A typical form might be where, for example, sulfate-containing groundwater reacts with concrete foundations.

Delayed ettringite formation (DEF) is a particular form of sulfate attack which may occur in concrete cured at high temperatures.

Thaumasite formation is another particular type of sulfate attack ('TSA' - thaumasite form of sulfate attack) in which thaumasite -(Ca3Si(OH)6(SO4)(CO3).12H2O) - forms through a combination of sulfate attack and carbonation. TSA can attack the surface regions of concrete foundations but more commonly is found in masonry and render. Usually thought to require cold conditions to form.

Undesignated product

Hydration product forming the matrix containing all other hydration products in the concrete apart from in-situ hydration product. Strictly, the space originally occupied by water on mixing but in practice also includes the volumes formerly occupied by small cement grains, the outlines of which later are obscured. Also known as 'outer-product' and 'undifferentiated product.' See also ‘In-situ hydration product’.