The alkali–silica reaction (ASR), also commonly known as concrete cancer, is a deleterious internal swelling reaction that occurs over time in concrete between the highly alkaline cement paste and the reactive amorphous (i.e., non-crystalline) silica found in many common aggregates, given sufficient moisture.
This deleterious chemical reaction causes the expansion of the altered aggregate by the formation of a soluble and viscous gel of sodium silicate (Na2SiO3, also noted Na2H2SiO4, or N-S-H (sodium silicate hydrate), depending on the adopted convention). 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.
ASR can lead to serious cracking in concrete, resulting in critical structural problems that can even force the demolition of a particular structure. The expansion of concrete through reaction between cement and aggregates was first studied by Thomas E. Stanton in California during the 1930s with his founding publication in 1940.
Chemistry
To attempt to simplify and to stylize a very complex set of various reactions, the whole ASR reaction, after its complete evolution (ageing process) in the presence of sufficient Ca2+ cations available in solution, could be compared to the pozzolanic reaction which would be catalysed by the undesirable presence of excessive concentrations of alkali hydroxides (NaOH and KOH) in the concrete. 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:
Ca(OH)2 + H4SiO4 → Ca2+ + H2SiO42− + 2 H2O → CaH2SiO4
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.
Indeed, the term silicic acid has traditionally been used as a synonym for silica, SiO2. Strictly speaking, silica is the anhydride of orthosilicic acid, Si(OH)4. SiO2↓ + 2 H2O Si(OH)4
An ancient industrial notation referring to , metasilicic acid, is also often used to depict the alkali-silica reaction. However, the metasilicic acid, , or , is a hypothetic molecule which has never been observed, even in extreme diluted solutions because is unstable and continues to hydrate.
Indeed, contrary to the hydration of CO2 which consumes only one water molecule and stops at H2CO3, the hydration of SiO2 consumes two water molecules and continues one step further to form H4SiO4. The difference in hydration behaviour between SiO2 and CO2 is explained by thermodynamic reasons (Gibbs free energy) and by bond energy or steric hindrance around the central atom of the molecule.
This is why the more correct geochemical notation referring to the orthosilicic acid really existing in dilute solution is preferred here. However, the main advantage of the now deprecated, but still often used, industrial notation referring to the metasilicate anion (), which also does not exist in aqueous solution, is its greater simplicity and its direct similitude in notation with the carbonate () system.
One will also note that the NaOH and KOH species (alkali hydroxides, also often simply called alkali to refer to their strongly basic character) which catalyze and accelerate the silica dissolution in the alkali-silica reaction do not explicitly appear in this simplified representation of the ultimate reaction with portlandite, because they are continuously regenerated from the cation exchange reaction with portlandite. As a consequence, they disappear from the global mass balance equation of the catalyzed reaction.
Silica dissolution mechanism
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 ions.
The presence of these oxygen-bearing groups very prone to form hydrogen bonds with water molecules explains the affinity of silica for water and makes colloidal silica very hydrophilic.
Siloxane bonds may undergo hydrolysis and condensation reactions as schematically represented hereafter:
≡Si–O–Si≡ + ↔ ≡Si–OH + HO–Si≡ =Si=O + ↔ =
On the other hand, silanol groups can also undergo protonation/deprotonation: ≡Si–OH ↔ ≡Si– + .
These equilibria can be shifted towards the right side of the reaction leading to silica dissolution by increasing the concentration of the hydroxide anion (OH–), i.e., by increasing the pH of the solution.
Alkaline hydrolysis of siloxane bonds occurs by nucleophilic substitution of OH– onto a silicon atom, while another –O–Si group is leaving to preserve the tetravalent character of Si atom:
≡Si–O–Si≡ + → ≡Si–OH + –O–Si≡ =Si=O + → =
Deprotonation of silanol groups: ≡Si–OH + → ≡Si– + .
In the pH range 0 – 7, the solubility of silica is constant, but above pH=8, the hydrolysis of siloxane bonds and deprotonation of silanol groups exponentially increase with the pH value. This is why glass easily dissolves at high pH values and does not withstand extremely basic NaOH/KOH solutions. Therefore, NaOH/KOH is released during cement hydration attacks and dissolves the tridimensional network of silica present in the aggregates. Amorphous or poorly crystallized silica, such as cryptocrystalline chalcedony or chert present in flints (in chalk) or rolled river gravels, is much more soluble and sensitive to alkaline attack by OH– anions than well crystallized silica such as quartz. Strained (deformed) quartz or chert exposed to freeze-thaw cycles in Canada and Nordic countries are also more sensitive to alkaline (high pH) solutions.
The species responsible for silica dissolution is the hydroxide anion (OH–). The high pH conditions are said to be alkaline and one also speaks of the alkalinity of the basic solutions. For the sake of electroneutrality, (OH–) anions need to be accompanied by positively charged cations, Na+ or K+ in NaOH or KOH solutions, respectively. Na and K both belong to the alkali metals column in the Periodic Table. When speaking of alkalis, one systematically refers to NaOH and KOH basic hydroxides, or their corresponding oxides Na2O and K2O in cement. Therefore, it is the hydroxide, or the oxide, component of the salt which is the only relevant chemical species for silica dissolution, not the alkali metal in itself. However, to determine the alkali equivalent content (Na2Oeq) in cement, because of the need to maintain electroneutrality in solids or in solution, one directly measures the contents of cement in Na and K elements and one conservatively considers that their counter ions are the hydroxide ions. As Na+ and K+ cations are hydrated species, they also contribute to retain water in alkali-silica reaction products.
Osmotic processes (Chatterji et al., 1986, 1987, 1989) and the electrical double layer (EDL) 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.
Catalysis of ASR by dissolved NaOH or KOH
The ASR reaction significantly differs from the pozzolanic reaction by the fact that it is catalysed by soluble alkali hydroxides (NaOH / KOH) at very high pH. It can be represented as follows using the classical geochemical notation for representing silica by the fully hydrated dissolved silica (Si(OH)4 or silicic acid: H4SiO4), while in an older industrial notation the non-existing (H2SiO3, hemihydrated silica, is considered in analogy to carbonic acid):
2 Na(OH) + H4SiO4 → Na2H2SiO4
The so-produced soluble alkali silicagel can then react with calcium hydroxide (portlandite) to precipitate insoluble calcium silicate hydrates (C-S-H phases) and regenerate NaOH, thus continuing the initial silica dissolution reaction:
Na2H2SiO4 + Ca(OH)2 → CaH2SiO4 + 2 NaOH
The combination of the two above mentioned reactions gives a general reaction resembling the pozzolanic reaction, but it is important to keep in mind that this reaction is catalysed by the undesirable presence in cement, or other concrete components, of soluble alkaline hydroxides (NaOH / KOH) responsible for the dissolution of the silica (silicic acid) at high pH:
Ca(OH)2 + H4SiO4 → CaH2SiO4
Without the presence of dissolved NaOH or KOH, responsible for the high pH (~13.5) of the concrete pore water, the amorphous silica of the reactive aggregates would not be dissolved and the reaction would not evolve. Moreover, the soluble sodium or potassium silicate is very hygroscopic and swells when it absorbs water. When the sodium silicate gel forms and swells inside a porous siliceous aggregate, it first expands and occupies the free porosity. 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. When the sodium silicate expelled from the aggregate encounters grains of portlandite present in the hardened cement paste, an exchange between sodium and calcium cations occurs and hydrated calcium silicate (C-S-H) precipitates with a concomitant release of NaOH. In its turn, the regenerated NaOH can react with the amorphous silica aggregate, leading to an increased production of soluble sodium silicate. When a continuous rim of C-S-H completely envelops the external surface of the attacked siliceous aggregate, it behaves as a semi-permeable barrier and hinders the expulsion of the viscous sodium silicate while allowing the NaOH / KOH to diffuse from the hardened cement paste inside the aggregate. This selective barrier of C-S-H contributes to increase the hydraulic pressure inside the aggregate and aggravates the cracking process. It is the expansion of the aggregates which damages concrete in the alkali-silica reaction.
Portlandite (Ca(OH)2) represents the main reserve of OH– anions in the solid phase as suggested by Davies and Oberholster (1988) and emphasized by Wang and Gillott (1991). As long as portlandite, or the siliceous aggregates, has not become completely exhausted, the ASR reaction will continue. The alkali hydroxides are continuously regenerated by the reaction of the sodium silicate with portlandite and thus represent the transmission belt of the ASR reaction driving it to completeness. It is thus impossible to interrupt the ASR reaction. 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.
Analogy with the soda lime and concrete carbonatation
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. The silicic acid (H2SiO3 or SiO2) is simply replaced in the reaction by the carbonic acid (H2CO3 or CO2).
{| border = 0 |- | align = "right" | (1)|| || align = "right" | CO2 + 2 NaOH || || → || || Na2CO3 + H2O|| ||(CO2 trapping by soluble NaOH) |- | align = "right" | (2)|| || align = "right" | Na2CO3 + Ca(OH)2 || || → || || CaCO3 + 2 NaOH || ||(regeneration of NaOH after reaction with lime) |-