Alkali–silica reaction

This hygroscopic gel swells and increases in volume when absorbing water: it exerts an expansive pressure inside the siliceous aggregate, causing spalling and loss of strength of the concrete, finally leading to its failure.

[8][non-primary source needed] It is a mineral acid-base reaction between NaOH or KOH, calcium hydroxide, also known as portlandite, or (Ca(OH)2), and silicic acid (H4SiO4, or Si(OH)4).

For simplification, after a complete exchange of the alkali cations with the calcium ions released by portlandite, the alkali-silica reaction in its ultimate stage leading to calcium silicate hydrate (C-S-H) could be schematically represented as follows: Here, the silicic acid H4SiO4, or Si(OH)4, which is equivalent to SiO2 · 2 H2O represents hydrous or amorphous silica for the sake of simplicity in aqueous chemistry.

The surface of solid silica in contact with water is covered by siloxane bonds (≡Si–O–Si≡) and silanol groups (≡Si–OH) sensitive to an alkaline attack by OH− ions.

Strained (deformed) quartz or chert exposed to freeze-thaw cycles in Canada and Nordic countries are also more sensitive to alkaline (high pH) solutions.

Osmotic processes (Chatterji et al., 1986, 1987, 1989) and the electrical double layer (EDL)[9] play also a fundamental role in the transport of water towards the concentrated liquid alkali gel, explaining their swelling behavior and the deleterious expansion of aggregates responsible of ASR damages in concrete.

When this latter is completely filled, and if the soluble but very viscous gel cannot be easily expelled from the silica network, the hydraulic pressure rises inside the attacked aggregate and leads to its fracture.

The hydro-mechanical expansion of the damaged siliceous aggregate surrounded by calcium-rich hardened cement paste is responsible for the development of a network of cracks in concrete.

In its turn, the regenerated NaOH can react with the amorphous silica aggregate, leading to an increased production of soluble sodium silicate.

Portlandite (Ca(OH)2) represents the main reserve of OH– anions in the solid phase as suggested by Davies and Oberholster (1988)[10] and emphasized by Wang and Gillott (1991).

The only way to avoid ASR in the presence of siliceous aggregates and water is to maintain the concentration of soluble alkali (NaOH and KOH) at the lowest possible level in concrete, so that the catalysis mechanism becomes negligible.

The alkali-silica reaction mechanism catalysed by a soluble strong base as NaOH or KOH in the presence of Ca(OH)2 (alkalinity buffer present in the solid phase) can be compared with the carbonatation process of soda lime.

Alkali hydroxides, NaOH and KOH, arise from the direct dissolution of Na2O and K2O oxides produced by the pyrolysis of the raw materials at high temperature (1450 °C) in the cement kiln.

There exist also other indirect sources of OH−, all related to the presence of soluble Na and K salts in the pore water of hardened cement paste (HCP).

SiO2 dissolution and Na2SiO3 formation (here, explicitly written in the ancient industrial metasilicate notation (based on the non-existing metasilicic acid, H2SiO3) to also illustrate the frequent use of this later in the literature): 2.

Condensation of silicate monomers or oligomers dispersed in a colloidal solution (sol) into a biphasic aqueous polymeric network of silicagel.

When pH slowly drops due to the progress of the silica dissolution reaction, the solubility of calcium hydroxide increases, and the alkali gel reacts with Ca2+ ions.

Its viscosity increases due to gelation process and its mobility (fluidity) strongly decreases when C-S-H phases start to precipitate after reaction with calcium hydroxide (portlandite).

Decompressed concrete cores can sometimes let observe fresh yellow liquid alkali exudations (viscous amber droplets) just after their drilling.

Following the same principle, the fabrication of low-pH cement also implies the addition of finely divided pozzolanic materials rich in silicic acid to the concrete mix to decrease its alkalinity.

Beside initially lowering the pH value of the concrete pore water, the main working mechanism of silica fume addition is to consume portlandite (the reservoir of hydroxyde (OH–) in the solid phase) and to decrease the porosity of the hardened cement paste by the formation of calcium silicate hydrates (C-S-H).

Silica fume is sufficiently dispersed during mixing operations of large batches of fresh concrete by the presence of coarse and fine aggregates.

As part of a study conducted by the Federal Highway Administration, a variety of methods have been applied to field structures suffering from ASR-affected expansion and cracking.

Massive structures such as dams pose particular problems: they cannot be easily replaced, and the swelling can block spillway gates or turbine operations.

This is why heavy aggregates must be systematically tested for ASR before nuclear applications such as radiation shielding or immobilization of strongly irradiating radioactive waste.

Another reason of concern for the possible accelerated development of ASR in the concrete of nuclear structures is the progressive amorphization of the silica contained in aggregates exposed to high neutron fluence.

[39] The only way to prevent, or to limit, the risk of ASR is to avoid one or several of the three elements in the critical triangle aggregate reactivity – cement alkali content – water: The American Society for Testing and Materials (ASTM International) has developed different standardized test methods for screening aggregates for their susceptibility to ASR: Other concrete prism methods have also been internationally developed to detect potential alkali-reactivity of aggregates or sometimes hardened concrete cores, e.g.: Alkali-aggregate reactions (AAR), both alkali-silica (ASR) and alkali-carbonate (ACR, involving dolomite) reactions, were identified in Canada since the years 1950s.

Characteristic crack pattern associated with the alkali–silica reaction affecting a concrete step barrier on a US highway. Note the typical fatty aspect of the silicagel exudations imbibing the concrete surface along the two sides of the cracks. [ 1 ] [ 2 ]
Typical crack pattern of the alkali-silica reaction (ASR). The gel exudations through the concrete cracks have a characteristic yellow color and a high pH. The fatty aspect of the exudations imbibing the concrete porosity along the cracks is also a distinctive feature of ASR.
The alkali-silica reaction as a chemical cycle process
Anhydrous and hydrated silica surface with siloxane and silanol groups along with surface bound water molecules.
Siloxane bond between two silicon atoms.
Chemical structure of a silanol group.
Surface of a concrete pillar of the building of the National Gallery of Canada at Ottawa presenting the typical crack pattern of the alkali-silica reaction (ASR). Note the typical fatty aspect of the yellow silicagel exudations imbibing the concrete surface along the two sides of the cracks.