Alkali–silica reaction
The alkali–silica reaction (ASR), more commonly known as "concrete cancer", is a deleterious 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 (NaSiO, also noted NaHSiO, 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 Ca 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)), and silicic acid (HSiO, or Si(OH)). 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 (C-S-H) could be schematically represented as following: Here, the silicic  acid HSiO, or Si(OH), which is equivalent to SiO · 2 HO 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, SiO. Strictly speaking, silica is the anhydride of orthosilicic acid, Si(OH). 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 CO  which consumes only one water molecule and stops at HCO, the hydration of SiO consumes two water molecules and continues one step further to form HSiO. The difference in hydration behaviour between SiO and CO is explained by thermodynamic reasons (Gibbs free energy) 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 (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 hydrophile. Siloxane bonds may undergo hydrolysis and condensation reactions as schematically represented hereafter: 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 (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: 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 (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 alkali's, one systematically refers to NaOH and KOH basic hydroxides, or their corresponding oxides NaO and KO 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 (NaO) 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 (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) or silicic acid: HSiO), but an older industrial notation also exists (HSiO, hemihydrated silica (does not exist), by analogy with carbonic acid): 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 (NaOH / KOH) responsible for the dissolution of the silica (silicic acid) at high pH: 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, 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 (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)) 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) (alkalinity buffer present in the solid phase) can be compared with the carbonatation process of soda lime. The silicic acid (HSiO or  SiO) is simply replaced in the reaction by the carbonic acid (HCO or CO). In the presence of water or simply ambient moisture, the strong bases, NaOH or KOH, readily dissolve in their hydration water (hygroscopic substances, deliquescence phenomenon) 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 (function of the NaO content of cement) also contributes to the carbonatation of portlandite by atmospheric CO in concrete although the rate of propagation of the reaction front is there essentially limited by the CO diffusion within the concrete matrix less porous. The soda lime carbonatation reaction can be directly translated into the ancient industrial notation of silicate (referring to the never observed metasilicic acid) simply by substituting a C atom by a Si atom in the mass balance equations ("i.e.", by replacing a carbonate by a metasilicate anion). 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 (soda lime or alkali-silica reaction), it is formally possible to write the same reactions sets by simply replacing the CO anions by HCO and the SiO anions by HSiO, 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 (HCP) from the family of Portland cement (pure OPC, with BFS, or  with cementitious additions, FA or SF). Direct sources. anions can be directly present in the HCP pore water or be slowly released from the solid phase (main buffer, or solid stock) by the dissolution of (portlandite) 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 (1450 °C) 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 (water evaporation) 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 carbonatation, soluble NaOH and KOH are continuously regenerated and released into solution when the soluble alkali silicate reacts with to precipitate insoluble calcium silicate. As suggested by Davies and  Oberholster (1988), the alkali-silica reaction is self-perpetuating as the alkali hydroxides are continuously regenerated in the system. Therefore, portlandite is the main buffer of in the solid phase. As long as the stock of hydroxides in the solid phase is not exhausted, the alkali-silica reaction can continue to proceed until complete disparition of one of the reagents ( or ) 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 (HCP). 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 (gypsum, anhydrite) 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 (2000) and Bérubé "et al." (2005): 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 (the culprit is their counter ion ) 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 (HCP)? Beside portlandite, other hydrated solid phases are present in HCP. The main phases are the calcium silicate hydrates (C-S-H) (the ""glue"" in cement paste), calcium sulfo-aluminate phases (AFm and AFt , ettringite) and hydrogarnet. C-S-H phases are less soluble (~ 10 M) than portlandite (CH) (~ 2.2 10 M at 25 °C) 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 (C-A-H), or related phases (C-A-S-H, AFm, AFt), 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 (with portlandite, CH) or anion exchange reactions (with phases related to C-A-H), 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 (indicator) 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 ("e.g.", by ICP-AES , AAS, XRF measurements techniques). 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 (here, explicitly written in the ancient industrial metasilicate notation (based on the non-existing metasilicic acid, ) to also illustrate the frequent use of this later in the literature): 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 (sol) into a biphasic aqueous polymeric network of silicagel. divalent cations released by calcium hydroxide (portlandite) when the pH starts to slightly drop may influence the gelation process. 3. Cation exchange with calcium hydroxide (portlandite) and precipitation of amorphous calcium silicate hydrates (C-S-H) accompanied by NaOH regeneration: 4. Carbonation of the C-S-H leading to precipitation of calcium carbonate and amorphous SiO 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 (amber liquid droplets) 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 (fluidity) strongly decreases when C-S-H phases start to precipitate after reaction with calcium hydroxide (portlandite). 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 (liquid gel expulsion rate through open cracks), 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 (viscous amber droplets) 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: Structural effects of ASR. The cracking caused by ASR can have several negative impacts on concrete, including: Mitigation. ASR can be mitigated in new concrete by several approaches: 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 (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). 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. 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. Some methods, such as the application of silanes, have shown significant promise, especially when applied to elements such as small columns and highway barriers. 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 (walls, slabs) of a structure is possible, and is followed by the installation of a watertight membrane, the evolution of the reaction can be slowed down, and sometimes stopped, due to the lack of water needed to continue fueling 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 (water present under the liquid form, or under the vapor state). 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. Heavy aggregates for nuclear shielding concrete. Two types of heavy aggregates are commonly used for nuclear shielding concrete in order to efficiently absorb gamma-rays: baryte (, density = 4.3 – 4.5) and various types of iron oxides, mainly magnetite (, density = 5.2) and hematite (, density =  5.3). The reason is their high density favorable to gamma attenuation. Both types of aggregates need to be checked for ASR as they may contain reactive silica impurities under a form or another. As elevated temperature in the range of may be reached in the concrete of the primary confinement wall around nuclear reactors, particular attention has to be paid to the selection of aggregates and heavy aggregates to avoid alkali-silica reaction promoted by reactive silica impurities and accelerated by the high temperature to which concrete is exposed. In some hydrothermal deposits, baryte is associated with silica mineralization and  can also contain reactive cristobalite while oxy-hydroxides of Fe(III), in particular ferrihydrite, exhibit a strong affinity for dissolved silica present in water and may constitute an ultimate sink for . This explains how microcrystalline silica can progressively accumulate in the mineral gangue of iron oxides. Dissolved silica (), and its corresponding silicate anion (), strongly sorbs onto hydrous ferric oxides (HFO) and ferric oxides hydrated surface (>Fe–OH) by ligand exchange: In this ligand exchange reaction, a silicate anion (also often more simply written as ) is making a nucleophilic substitution onto a >Fe–OH ferrol surface group of HFO and  ejects a hydroxide anion while taking its place onto the ferrol group. This mechanism explains the formation of strong inner sphere complexes of silica at the surface of iron oxy-hydroxides and iron oxides. The surface of iron oxides becomes progressively coated with silica and a silica gangue forms at the surface of iron oxide ores. This explains why some iron ores are rich in silica and may therefore be sensitive to the alkali-silica reaction. Very low level of reactive silica in heavy aggregates are sufficient to induce ASR. 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. This process is also known as metamictization and is known to create amorphous halo's in minerals like zircon rich in uranium and thorium when their crystal structure is submitted to intense alpha-particles internal bombardment and becomes amorph (metamict state). The loss of mechanical properties of heavily neutron-irradiated concrete component such  as the biological shield of a reactor at the end of the service life of a nuclear power plant is expected to be due to radiation-induced swelling of aggregates, which leads to volumetric expansion of the concrete. 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: Methods for testing potential alkali reactivity. 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.": Known affected structures. Canada. Alkali-aggregate reactions (AAR), both alkali-silica (ASR) and alkali-carbonate (ACR, involving dolomite) reactions, were identified in Canada since the years 1950's.