Alkali–silica reaction


The alkali–silica reaction, more commonly known as "concrete cancer", is a swelling reaction that occurs over time in concrete between the highly alkaline cement paste and the reactive non-crystalline silica found in many common aggregates, given sufficient moisture.
This reaction causes the expansion of the altered aggregate by the formation of a soluble and viscous gel of sodium silicate. 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 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 in the concrete. It is a mineral acid-base reaction between NaOH or KOH, calcium hydroxide, also known as portlandite, or, and silicic acid. For simplifying, 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 could be schematically represented as following:
Here, the silicic acid H4SiO4, or Si4, 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, Si4.
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 continue 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 and by bond energy or steric hindrance around the central atom of the molecule.
That 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 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 and silanol groups 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 hydrophile.
Siloxane bonds may undergo hydrolysis and condensation reactions as schematically represented hereafter:
group.
On the other hand, silanol groups can also undergo protonation/deprotonation:
These equilibria can be shifted towards the right side of the reaction leading to silica dissolution by increasing the concentration of hydroxide anion, 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:
Deprotonation of silanol groups:
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 pH. This is why glass easily dissolves at high pH and does not withstand extremely basic NaOH/KOH solutions. Therefore, NaOH/KOH released during cement hydration attacks and dissolves the tridimensional network of silica present in the aggregates. Amorphous or poorly crystallized silica, as cryptocrystalline chalcedony or chert present in flints or rolled river gravels, is much more soluble and sensitive to alkaline attack by OH– anions than well crystallized silica such as quartz. Strained quartz or chert exposed to freeze-thaw cycles in Canada and Nordic countries are also more sensitive to alkaline solutions.
The species responsible for silica dissolution is the hydroxide anion. 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, 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 Mendeleev table. When speaking of alkali's, 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 in cement, because 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 and the electrical double layer 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 at very high pH. It can be represented as follows using the classical geochemical notation for representing silica by the fully hydrated dissolved silica, but an older industrial notation also exists :
The sum, or 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 hydroxydes responsible for the dissolution of the silica at high pH:
Without the presence of dissolved NaOH or KOH, responsible for the high pH 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, 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. It is the hydro-mechanical expansion of the damaged siliceous aggregate surrounded by calcium-rich hardened cement paste which 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 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 represents the main reserve of OH– anions in the solid phase as emphasized by Wang and Gillott. 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 at the lowest possible level in concrete, so that the catalysis mechanism becomes negligible.

Analogy with the soda lime and concrete carbonation

The alkali-silica reaction mechanism catalysed by a soluble strong base as NaOH or KOH in the presence of Ca2 can be compared with the carbonation process of soda lime. The silicic acid is simply replaced in the reaction by the carbonic acid.
In the presence of water or simply ambient moisture, the strong bases, NaOH or KOH, readily dissolve in their hydration water and this greatly facilitates the catalysis process because the reaction in aqueous solution occurs much faster than in the dry solid phase. The moist NaOH impregnates the surface and the porosity of calcium hydroxide grains with a high specific surface area. Soda lime is commonly used in closed-circuit diving rebreathers and in anesthesia systems.
The same catalytic effect by the alkali hydroxides also contributes to the carbonation of portlandite by atmospheric CO2 in concrete although the rate of propagation of the reaction front is there essentially limited by the CO2 diffusion within the concrete matrix less porous.
The soda lime carbonation reaction can be directly translated into the ancient industrial notation of silicate simply by substituting a C atom by a Si atom in the mass balance equations. This gives the following set of reactions also commonly encountered in the literature to schematically depict the continuous regeneration of NaOH in ASR:
If NaOH is clearly deficient in the system under consideration, it is formally possible to write the same reactions sets by simply replacing the CO32- anions by HCO3− and the SiO32- anions by HSiO3−, the principle of catalysis remaining the same, even if the number of intermediate species differs.

Main sources of in hardened cement paste

One can distinguish several sources of hydroxide anions in hardened cement paste from the family of Portland cement.

Direct sources

anions can be directly present in the HCP pore water or be slowly released from the solid phase by the dissolution of when its solubility increases when high pH value starts to drop. Beside these two main sources, ions exchange reactions and precipitation of poorly soluble calcium salts can also contribute to release into solution.
Alkali hydroxides, NaOH and KOH, arise from the direct dissolution of and oxides produced by the pyrolysis of the raw materials at high temperature in the cement kiln. The presence of minerals with high Na and K contents in the raw materials can thus be problematic. The ancient wet manufacturing process of cement, consuming more energy that the modern dry process, had the advantage to eliminate much of the soluble Na and K salts present in the raw material.
As previously described in the two sections dealing respectively with ASR catalysis by alkali hydroxides and soda lime carbonation, soluble NaOH and KOH are continuously regenerated and released into solution when the soluble alkali silicate reacts with to precipitate insoluble calcium silicate. Therefore, portlandite is the main buffer of in the solid phase. As long as the stock of hydroxide in the solid phase is not exhausted, the alkali-silica reaction can continue to proceed until complete disparition of one of the reagents involved in the pozzolanic reaction.

Indirect sources

There exist also other indirect sources of, all related to the presence of soluble Na and K salts in the pore water of hardened cement paste.
The first category contains soluble Na and K salts whose corresponding anions can precipitate an insoluble calcium salts, e.g.,,,,,,....
Hereafter, an example for calcium sulfate precipitation releasing sodium hydroxide:
or, the reaction of sodium carbonate with portlandite, also important for the catalysis of the alkali–carbonate reaction as emphasized by Fournier and Bérubé and Bérubé et al. :
However, not all Na or K soluble salts can precipitate insoluble calcium salts, such as, e.g., NaCl-based deicing salts:
As calcium chloride is a soluble salt, the reaction cannot occur and the chemical equilibrium regresses to the left side of the reaction.
So, a question arises: can NaCl or KCl from deicing salts still possibly play a role in the alkali-silica reaction? and cations in themselves cannot attack silica and soluble alkali chlorides cannot produce soluble alkali hydroxide by interacting with calcium hydroxide. So, does it exist another route to still produce hydroxide anions in the hardened cement paste ?
Beside portlandite, other hydrated solid phases are present in HCP. The main phases are the calcium silicate hydrates , calcium sulfo-aluminate phases and hydrogarnet. C-S-H phases are less soluble than portlandite and therefore are expected to play a negligible role for the calcium ions release.
An anion-exchange reaction between chloride ions and the hydroxide anions contained in the lattice of some calcium aluminate hydrates, or related phases, is suspected to also contribute to the release of hydroxide anions into solution. The principle mechanism is schematically illustrated hereafter for C-A-H phases:
As a simple, but robust, conclusion, the presence of soluble Na and K salts can also cause, by precipitation of poorly soluble calcium salt or anion exchange reactions, the release of anions into solution. Therefore, the presence of any salts of Na and K in cement pore water is undesirable and the measurements of Na and K elements is a good proxy for the maximal concentration of in pore solution. This is why the total alkali equivalent content of cement can simply rely on the measurements of Na and K.

Alkali gel evolution and ageing

The maturation process of the fluid alkali silicagel found in exudations into less soluble solid products found in gel pastes or in efflorescences is described hereafter. Four distinct steps are considered in this progressive transformation.
1. dissolution and formation :
2. Maturation of the alkali gel: polymerisation and gelation by the sol–gel process. Condensation of silicate monomers or oligomers dispersed in a colloidal solution into a biphasic aqueous polymeric network of silicagel. divalent cations released by calcium hydroxide when the pH starts to slightly drop may influence the gelation process.
3. Cation exchange with calcium hydroxide and precipitation of amorphous calcium silicate hydrates accompanied by NaOH regeneration:
4. Carbonation of the C-S-H leading to precipitation of calcium carbonate and amorphous SiO2 stylized as follows:
As long as the alkali gel has not yet reacted with ions released from portlandite dissolution, it remains fluid and can easily exude from broken aggregates or through open cracks in the damage concrete structure. This can lead to visible yellow viscous liquid exudations at the surface of affected concrete.
When pH slowly drops due to the progress of the silica dissolution reaction, solubility of calcium hydroxide increases and the alkali gel reacts with ions. Its viscosity increases due to gelation process and its mobility strongly decreases when C-S-H phases start to precipitate after reaction with calcium hydroxide. At this moment, the calcified gel becomes hard, hindering therefore the alkali gel transport in concrete.
When the C-S-H gel is exposed to atmospheric carbon dioxide, it undergoes a rapid carbonation and white/yellow efflorescences appear at the surface of concrete. When the relatively fluid alkali gel continue to exude below the hardened superficial gel layer, it pushes the efflorescences out of the crack surface making them to appear in relief. Because the gel drying and carbonation reactions rates are faster than the gel exudation velocity, in most of the cases, fresh liquid alkali exudates are not frequently encounterered at the surface of civil engineering concrete structures. Decompressed concrete cores can sometimes let observe fresh yellow liquid alkali exudations just after their drilling.

Mechanism of concrete deterioration

The mechanism of ASR causing the deterioration of concrete can thus be described in four steps as follows:
  1. The very basic solution attacks the siliceous aggregates, converting the poorly crystallised or amorphous silica to a soluble but very viscous alkali silicate gel.
  2. The consumption of NaOH / KOH by the dissolution reaction of amorphous silica decreases the pH of the pore water of the hardened cement paste. This allows the dissolution of Ca2 and increases the concentration of Ca2+ ions into the cement pore water. Calcium ions then react with the soluble sodium silicate gel to convert it into solid calcium silicate hydrates. The C-S-H forms a continuous poorly permeable coating at the external surface of the aggregate.
  3. The penetrated alkaline solution converts the remaining siliceous minerals into bulky soluble alkali silicate gel. The resulting expansive pressure increases in the core of the aggregate.
  4. The accumulated pressure cracks the aggregate and the surrounding cement paste when the pressure exceeds the tolerance of the aggregate.

    Structural effects of ASR

The cracking caused by ASR can have several negative impacts on concrete, including:
  1. Expansion: The swelling nature of ASR gel increases the chance of expansion in concrete elements.
  2. Compressive strength: The effect of ASR on compressive strength can be minor for low expansion levels, to relatively higher degrees at larger expansion. points out that the compressive strength is not very accurate parameter to study the severity of ASR; however, the test is done because of its simplicity.
  3. Tensile strength / Flexural capacity: Researches show that ASR cracking can significantly reduce the tensile strength of concrete; therefore reducing the flexural capacity of beams. Some research on bridge structures indicate about 85% loss of capacity as a result of ASR.
  4. Modulus of elasticity/UPV: The effect of ASR on elastic properties of concrete and ultrasound pulse velocity is very similar to tensile capacity. The modulus of elasticity is shown to be more sensitive to ASR than pulse velocity.
  5. Fatigue: ASR reduces the load bearing capacity and the fatigue life of concrete.
  6. Shear strength: ASR enhances the shear capacity of reinforced concrete with and without shear reinforcement.

    Mitigation

ASR can be mitigated in new concrete by several complementary approaches:
  1. Limit the alkali metal content of the cement. Many standards impose limits on the "Equivalent Na2O" content of cement.
  2. Limit the reactive silica content of the aggregate. Certain volcanic rocks are particularly susceptible to ASR because they contain volcanic glass and should not be used as aggregate. The use of calcium carbonate aggregates is sometimes envisaged as an ultimate solution to avoid any problem. However, while it may be considered as a necessary condition, it is not a sufficient one. In principle, limestone is not expected to contain a high level of silica, but it actually depends on its purity. Indeed, some siliceous limestones may be cemented by amorphous or poorly crystalline silica and can be very sensitive to the ASR reaction, as also observed with some Tournaisian siliceous limestones exploited in quarries in the area of Tournai in Belgium. In Canada, the Spratt siliceous limestone is also particularly well known in studies dealing with ASR and is commonly used as the Canadian ASR reference aggregate. So, the use of limestone as aggregate is not a guarantee against ASR in itself.
  3. Add very fine siliceous materials to neutralize the excessive alkalinity of cement with silicic acid by deliberately provoking a controlled pozzolanic reaction at the early stage of the cement setting. Convenient pozzolanic materials to add to the mix may be, e.g., pozzolan, silica fume, fly ash, or metakaolin. These react preferentially with the cement alkalis without formation of an expansive pressure, because siliceous minerals in fine particles convert to alkali silicate and then to calcium silicate without formation of semipermeable reaction rims.
  4. Another method to reduce the ASR is to limit the external alkalis that come in contact with the system.
In other words, as it is sometimes possible to fight fire with fire, it is also feasible to combat the ASR reaction by itself. A prompt reaction initiated at the early stage of concrete hardening on very fine silica particles will help to suppress a slow and delayed reaction with larger siliceous aggregates on the long term. 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 and to decrease the porosity of the hardened cement paste by the formation of calcium silicate hydrates. However, silica fume has to be very finely dispersed in the concrete mix, because agglomerated flakes of compacted silica fume can themselves also induce ASR if the dispersion process is insufficient. This can be the case in laboratory studies made on cement pastes alone in the absence of aggregates. However, most often, in large concrete batches, silica fume is sufficiently dispersed during mixing operations 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. Some methods, such as the application of silanes, have shown significant promise, especially when applied to elements such as small columns and highway barriers, whereas other methods, such as the topical application of lithium compounds, have shown little or no promise in reducing ASR-induced expansion and cracking.

Curative treatment

There are no curative treatments in general for ASR affected structures. Repair in damaged sections is possible, but the reaction will continue. In some cases, when a sufficient drying of thin components of a structure is possible, and is followed by the installation of a watertight membrane, the evolution of the reaction can be slow down, and sometimes stopped, because the lack of water to continue to fuel the reaction. Indeed, water plays a triple role in the alkali-silica reaction: solvent for the reaction taking place, transport medium for the dissolved species reacting, and finally also reagent consumed by the reaction itself.
However, concrete at the center of thick concrete components or structures can never dry because water transport in saturated or unsaturated conditions is always limited by diffusion in the concrete pores. The water diffusion time is thus proportional to the square of its transport distance. As a consequence, the water saturation degree inside thick concrete structures often remains higher than 80%, a level sufficient to provide enough water to the system and to maintain the alkali-silica reaction on going.
Massive structures such as dams pose particular problems: they cannot be easily replaced, and the swelling can block spillway gates or turbine operations. Cutting slots across the structure can relieve some pressure, and help restore geometry and function.

Prevention of the risk

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 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.:

Australia

Alkali-aggregate reactions, both alkali-silica and alkali-carbonate reactions, were identified in Canada since the years 1950's.