Alkali-aggregate reaction
Encyclopedia
Alkali–aggregate reaction is a term mainly referring to a reaction which occurs over time in concrete
between the highly alkaline cement
paste and non-crystalline silicon dioxide
, which is found in many common aggregate
s. This reaction can cause expansion of the altered aggregate, leading to spalling and loss of strength of the concrete.
The alkali–silica reaction is the most common form of alkali–aggregate reaction.
Two other types are:
The Pozzolanic reaction
which occurs in the setting of the mixture of slaked lime and pozzolanic materials
has also features similar to the alkali–silica reaction, mainly the formation of calcium silicate hydrate
(CSH).
Concrete
Concrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...
between the highly alkaline cement
Portland cement
Portland cement is the most common type of cement in general use around the world because it is a basic ingredient of concrete, mortar, stucco and most non-specialty grout...
paste and non-crystalline silicon dioxide
Silicon dioxide
The chemical compound silicon dioxide, also known as silica , is an oxide of silicon with the chemical formula '. It has been known for its hardness since antiquity...
, which is found in many common aggregate
Construction Aggregate
Construction aggregate, or simply "aggregate", is a broad category of coarse particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates. Aggregates are the most mined material in the world...
s. This reaction can cause expansion of the altered aggregate, leading to spalling and loss of strength of the concrete.
More accurate terminology
The alkali–aggregate reaction is a general, but relatively vague, expression which can lead to confusion. Actually, it is recommended to refer to a more precise definition such as one of the following:- Alkali–silica reaction (ASR, main article to read first);
- Alkali–silicate reaction, and;
- Alkali–carbonate reaction.
The alkali–silica reaction is the most common form of alkali–aggregate reaction.
Two other types are:
- the alkali–silicate reaction, in which layer silicate minerals (clay mineralsClay mineralsClay minerals are hydrous aluminium phyllosilicates, sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations. Clays have structures similar to the micas and therefore form flat hexagonal sheets. Clay minerals are common weathering products and low...
), sometimes present as impurities, are attacked, and ; - the alkali–carbonate reaction, which is an uncommon attack on certain argillaceous dolomitic limestoneDolomiteDolomite is a carbonate mineral composed of calcium magnesium carbonate CaMg2. The term is also used to describe the sedimentary carbonate rock dolostone....
s, likely involving the expansion of the mineral BruciteBruciteBrucite is the mineral form of magnesium hydroxide, with the chemical formula Mg2. It is a common alteration product of periclase in marble; a low-temperature hydrothermal vein mineral in metamorphosed limestones and chlorite schists; and formed during serpentinization of dunites...
(Mg(OH)2.
The Pozzolanic reaction
Pozzolanic reaction
The Pozzolanic reaction is the chemical reaction that occurs in hydraulic cement, a mixture of slaked lime with amorphous siliceous materials , forming non-water-soluble calcium silicate hydrates...
which occurs in the setting of the mixture of slaked lime and pozzolanic materials
Pozzolan
A pozzolan is a material which, when combined with calcium hydroxide, exhibits cementitious properties. Pozzolans are commonly used as an addition to Portland cement concrete mixtures to increase the long-term strength and other material properties of Portland cement concrete, and in some cases...
has also features similar to the alkali–silica reaction, mainly the formation of calcium silicate hydrate
Calcium silicate hydrate
Calcium Silicate Hydrate is the main product of the hydration of Portland cement and is primarily responsible for the strength in cement based materials.-Preparation:...
(CSH).