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107-424: This deleterious chemical reaction causes the expansion of the altered aggregate by the formation of a soluble and viscous gel of sodium silicate (Na 2 SiO 3 · n H 2 O , also noted Na 2 H 2 SiO 4 · n H 2 O , 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

214-439: A colloidal gel in which water is the dispersion medium. A three-dimensional solid results from the hydrophilic polymer chains being held together by cross-links. Because of the inherent cross-links, the structural integrity of the hydrogel network does not dissolve from the high concentration of water. Hydrogels are highly absorbent (they can contain over 90% water) natural or synthetic polymeric networks. Hydrogels also possess

321-431: A percolated network of particles in a fluid medium, providing mechanical properties , in particular the emergence of elastic behaviour. The particles can show attractive interactions through osmotic depletion or through polymeric links. Colloidal gels have three phases in their lifespan: gelation, aging and collapse. The gel is initially formed by the assembly of particles into a space-spanning network, leading to

428-457: 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

535-457: 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

642-544: A degree of flexibility very similar to natural tissue, due to their significant water content. As responsive " smart materials ," hydrogels can encapsulate chemical systems which upon stimulation by external factors such as a change of pH may cause specific compounds such as glucose to be liberated to the environment, in most cases by a gel-sol transition to the liquid state. Chemomechanical polymers are mostly also hydrogels, which upon stimulation change their volume and can serve as actuators or sensors . The first appearance of

749-408: A free energy difference density. The form of f gel ( λ 1 , λ 2 , λ 3 ) {\displaystyle f_{\text{gel}}(\lambda _{1},\lambda _{2},\lambda _{3})} naturally assumes two contributions of radically different physical origins, one associated with the elastic deformation of the polymer network, and

856-440: A good proxy ( indicator ) for the maximal concentration of OH in pore solution. This is why the total alkali equivalent content ( Na 2 O eq ) of cement can simply rely on the measurements of Na and K ( e.g. , by ICP-AES , AAS , XRF measurements techniques). The maturation process of the fluid alkali silicagel found in exudations into less soluble solid products found in gel pastes or in efflorescences

963-498: A more acute C−O−C angle of 111°. It can be appreciated that the siloxanes would have low barriers for rotation about the Si−O bonds as a consequence of low steric hindrance. This geometric consideration is the basis of the useful properties of some siloxane-containing materials, such as their low glass transition temperatures . The main route to siloxane functional group is by hydrolysis of silicon chlorides : The reaction proceeds via

1070-399: 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

1177-517: A number of tests." With an LD 50 in rats of >50 g/kg, they are virtually nontoxic. Questions remain however about chronic toxicity or the consequences of bioaccumulation since siloxanes can be long-lived. Findings about bioaccumulation have been largely based on laboratory studies. Field studies of bioaccumulation have not reached consensus. "Even if the concentrations of siloxanes we have found in fish are high compared to concentrations of classical contaminants like PCBs , several other studies in

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1284-441: A phase arrest. In the aging phase, the particles slowly rearrange to form thicker strands, increasing the elasticity of the material. Gels can also be collapsed and separated by external fields such as gravity. Colloidal gels show linear response rheology at low amplitudes. These materials have been explored as candidates for a drug release matrix. A hydrogel is a network of polymer chains that are hydrophilic, sometimes found as

1391-442: A polymer solution of concentration ϕ 0 {\displaystyle \phi _{0}} and volume V 0 {\displaystyle V_{0}} is mixed with a pure solvent of volume ( λ 1 λ 2 λ 3 − 1 ) V 0 {\displaystyle (\lambda _{1}\lambda _{2}\lambda _{3}-1)V_{0}} to become

1498-471: A relatively high concentration of H + {\displaystyle {\text{H}}^{+}} and salt cations inside the gel. But because the concentration of H + {\displaystyle {\text{H}}^{+}} is locally higher, it suppresses the further ionization of the acid sites. This phenomenon is the prediction of the classical Donnan theory. However, with electrostatic interactions, there are further complications to

1605-399: 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 OH 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

1712-542: 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 ( H 2 SiO 3 or SiO 2 ) is simply replaced in the reaction by the carbonic acid ( H 2 CO 3 or CO 2 ). 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

1819-490: A solution with polymer concentration ϕ {\displaystyle \phi } and volume λ 1 λ 2 λ 3 V 0 {\displaystyle \lambda _{1}\lambda _{2}\lambda _{3}V_{0}} . The free energy density change in this mixing step is given as where on the right-hand side, the first term is the Flory–Huggins energy density of

1926-409: Is a semi-solid that can have properties ranging from soft and weak to hard and tough. Gels are defined as a substantially dilute cross-linked system, which exhibits no flow when in the steady state, although the liquid phase may still diffuse through this system. Gels are mostly liquid by mass , yet they behave like solids because of a three-dimensional cross-linked network within the liquid. It

2033-462: Is a common example of a hydrogel and has approximately the density of water. Polyionic polymers are polymers with an ionic functional group. The ionic charges prevent the formation of tightly coiled polymer chains. This allows them to contribute more to viscosity in their stretched state, because the stretched-out polymer takes up more space. This is also the reason gel hardens. See polyelectrolyte for more information. A colloidal gel consists of

2140-470: Is a solid formed from a gel by drying with unhindered shrinkage. Xerogels usually retain high porosity (15–50%) and enormous surface area (150–900 m /g), along with very small pore size (1–10 nm). When solvent removal occurs under supercritical conditions, the network does not shrink and a highly porous, low-density material known as an aerogel is produced. Heat treatment of a xerogel at elevated temperature produces viscous sintering (shrinkage of

2247-411: Is also often used to depict the alkali-silica reaction. However, the metasilicic acid, H 2 SiO 3 , or SiO(OH) 2 , is a hypothetic molecule which has never been observed, even in extreme diluted solutions because H 2 SiO 3 is unstable and continues to hydrate. Indeed, contrary to the hydration of CO 2 which consumes only one water molecule and stops at H 2 CO 3 ,

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2354-425: Is considered in analogy to carbonic acid ): 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

2461-419: Is derived from the words sil icon, ox ygen, and alk ane . In some cases, siloxane materials are composed of several different types of siloxane groups; these are labeled according to the number of Si−O bonds: Because silicones are heavily used in biomedical and cosmetic applications, their toxicology has been intensively examined. "The inertness of silicones toward warmblooded animals has been demonstrated in

2568-402: Is described hereafter. Four distinct steps are considered in this progressive transformation. 1. SiO 2 dissolution and Na 2 SiO 3 formation (here, explicitly written in the ancient industrial metasilicate notation (based on the non-existing metasilicic acid , H 2 SiO 3 ) to also illustrate the frequent use of this later in the literature): 2. Maturation of

2675-408: Is extruded around it. Siloxane In organosilicon chemistry , a siloxane is an organic compound containing a functional group of two silicon atoms bound to an oxygen atom: Si−O−Si . The parent siloxanes include the oligomeric and polymeric hydrides with the formulae H[OSiH 2 ] n OH and [OSiH 2 ] n . Siloxanes also include branched compounds ,

2782-400: Is high both elastically and electrostatically and hence suppress ionization. Even though this ionization suppression is qualitatively similar to that of Donnan prediction, it is absent without electrostatic consideration and present irrespective of ion partitioning. The combination of both effects as well as gel elasticity determines the volume of the gel at equilibrium. Due to the complexity of

2889-613: Is illustrated by the combustion of hexamethylcyclotrisiloxane : Strong base degrades siloxane group, often affording siloxide salts : This reaction proceeds by production of silanols. Similar reactions are used industrially to convert cyclic siloxanes to linear polymers. Polysiloxanes (silicones) , upon combustion in an inert atmosphere, generally undergo pyrolysis to form silicon oxycarbide or silicon carbide (SiC). By exploiting this reaction, polysiloxanes have been used as preceramic polymers in various processes including additive manufacturing. Polyvinyl siloxane (vinyl polysiloxane)

2996-437: Is just a polymer network, without solvent). This is so because the free energy penalty to stretch an ideal polymer segment N {\displaystyle N} monomers of size b {\displaystyle b} between crosslinks to an end-to-end distance R {\displaystyle R} is approximately given by This is the origin of both gel and rubber elasticity . But one key difference

3103-776: Is monomer volume, N {\displaystyle N} is polymer strand length and χ {\displaystyle \chi } is the Flory-Huggins energy parameter. Because in a network, the polymer length is effectively infinite, we can take the limit N → ∞ {\displaystyle N\to \infty } and f ( ϕ ) {\displaystyle f(\phi )} reduces to Substitution of this expression into f mix ( λ 1 , λ 2 , λ 3 ) {\displaystyle f_{\text{mix}}(\lambda _{1},\lambda _{2},\lambda _{3})} and addition of

3210-475: 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

3317-450: Is only partially by the classical Donnan theory. As a starting point we can neglect the electrostatic interactions among ions. Then at equilibrium, some of the weak acid sites in the gel would dissociate to form A − {\displaystyle {\text{A}}^{-}} that electrostatically attracts positive charged H + {\displaystyle {\text{H}}^{+}} and salt cations leading to

Alkali–silica reaction - Misplaced Pages Continue

3424-441: Is polymer volume fraction. Suppose the initial gel has a polymer volume fraction of ϕ 0 {\displaystyle \phi _{0}} , the polymer volume fraction after swelling would be ϕ = ϕ 0 / λ 1 λ 2 λ 3 {\displaystyle \phi =\phi _{0}/\lambda _{1}\lambda _{2}\lambda _{3}} since

3531-502: Is preferred here. However, the main advantage of the now deprecated, but still often used, industrial notation referring to the metasilicate anion ( SiO 3 ), which also does not exist in aqueous solution, is its greater simplicity and its direct similitude in notation with the carbonate ( CO 3 ) 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

3638-421: Is stretch by a factor of λ {\displaystyle \lambda } in all three directions. Under the affine network approximation, the mean-square end-to-end distance in the gel increases from initial R 0 2 {\displaystyle R_{0}^{2}} to ( λ R 0 ) 2 {\displaystyle (\lambda R_{0})^{2}} and

3745-474: Is stretched by factors λ 1 {\displaystyle \lambda _{1}} , λ 2 {\displaystyle \lambda _{2}} and λ 3 {\displaystyle \lambda _{3}} in the three orthogonal directions during swelling after being immersed in a solvent phase of initial volume V s 0 {\displaystyle V_{s0}} . The final deformed volume of gel

3852-411: Is that gel contains an additional solvent phase and hence is capable of having significant volume changes under deformation by taking in and out solvent. For example, a gel could swell to several times its initial volume after being immersed in a solvent after equilibrium is reached. This is the phenomenon of gel swelling. On the contrary, if we take the swollen gel out and allow the solvent to evaporate,

3959-440: Is the cross-linking within the fluid that gives a gel its structure (hardness) and contributes to the adhesive stick ( tack ). In this way, gels are a dispersion of molecules of a liquid within a solid medium. The word gel was coined by 19th-century Scottish chemist Thomas Graham by clipping from gelatine . The process of forming a gel is called gelation . Gels consist of a solid three-dimensional network that spans

4066-481: 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 (Na 2 O eq ) 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

4173-424: Is then λ 1 λ 2 λ 3 V 0 {\displaystyle \lambda _{1}\lambda _{2}\lambda _{3}V_{0}} and the total volume of the system is V 0 + V s 0 {\displaystyle V_{0}+V_{s0}} , that is assumed constant during the swelling process for simplicity of treatment. The swollen state of

4280-413: Is used to fill the plastic tubes containing the fibers. The main purpose of the gel is to prevent water intrusion if the buffer tube is breached, but the gel also buffers the fibers against mechanical damage when the tube is bent around corners during installation, or flexed. Additionally, the gel acts as a processing aid when the cable is being constructed, keeping the fibers central whilst the tube material

4387-453: Is used to make dental impressions and industrial impressions. The use of a poly-siloxane precursor in polymer derived ceramics allows the formation of ceramic bodies with complex shapes, although the significant shrinkage in pyrolysis needs to be taken into account. Trisiloxanes may be used as diffusion pump fluid. Cyclomethicones are a group of methyl siloxanes, a class of liquid silicones (cyclic polydimethylsiloxane polymers) that possess

Alkali–silica reaction - Misplaced Pages Continue

4494-689: The Oslo Fjord in Norway, Lake Pepin in the US, and Lake Erie in Canada have shown concentrations of siloxanes decrease at higher range in the food chain. This finding raises questions about which factors influence the bioaccumulation potential of siloxanes." Cyclomethicones are ubiquitous because they are widely used in biomedical and cosmetic applications. They can be found at high levels in American cities. They can be toxic to aquatic animals in concentrations often found in

4601-537: 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 Na 2 O and K 2 O in cement. Therefore, it

4708-456: 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 of the alkali hydroxides (function of the Na 2 O eq content of cement ) also contributes to

4815-447: The elastic properties and firmness of the organogel. Often, these systems are based on self-assembly of the structurant molecules. (An example of formation of an undesired thermoreversible network is the occurrence of wax crystallization in petroleum . ) Organogels have potential for use in a number of applications, such as in pharmaceuticals , cosmetics, art conservation, and food. A xerogel / ˈ z ɪər oʊ ˌ dʒ ɛ l /

4922-482: 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: 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

5029-577: The pozzolanic reaction . 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). The first category contains soluble Na and K salts whose corresponding anions can precipitate an insoluble calcium salts, e.g., Na 2 SO 4 , Na 2 CO 3 , Na 3 PO 4 , NaB(OH) 4 , Na 2 B 4 O 7 , ... . Hereafter, an example for calcium sulfate ( gypsum , anhydrite ) precipitation releasing sodium hydroxide : or,

5136-455: 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

5243-430: The 1930s with his founding publication in 1940. 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

5350-489: The C-S-H leading to precipitation of calcium carbonate and amorphous SiO 2 stylized as follows: As long as the alkali gel ( Na 2 SiO 3 ) has not yet reacted with Ca 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

5457-466: The HCP pore water or be slowly released from the solid phase (main buffer, or solid stock) by the dissolution of Ca(OH) 2 (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 OH into solution. Alkali hydroxides, NaOH and KOH, arise from

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5564-413: The affinity of silica for water and makes colloidal silica very hydrophilic . 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

5671-548: 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. Ca 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

5778-421: The alkali-silica reaction is self-perpetuating as the alkali hydroxides are continuously regenerated in the system. Therefore, portlandite is the main buffer of OH 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 the complete disparition of one of the reagents ( Ca(OH) 2 or SiO 2 ) involved in

5885-448: The calcified gel hardens, 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 or yellow efflorescences appear at the surface of concrete. When the relatively fluid alkali gel continues to exude below the hardened superficial gel layer, it pushes the efflorescences out of the crack surface making them to appear in relief. Because

5992-462: The carbonatation of portlandite by atmospheric CO 2 in concrete although the rate of propagation of the reaction front is there essentially limited by the CO 2 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

6099-423: The characteristics of low viscosity and high volatility as well as being skin emollients and in certain circumstances useful cleaning solvents. Unlike dimethicones , which are linear siloxanes that do not evaporate , cyclomethicones are cyclic : both groups consist of a backbone of [(CH 3 ) 2 SiO] n . They are used in many cosmetic products including deodorants and antiperspirants which need to coat

6206-454: The charged nature of H + {\displaystyle {\text{H}}^{+}} and A − {\displaystyle {\text{A}}^{-}} , electrostatic interactions with other ions in the systems. This is effectively a reacting system governed by acid-base equilibrium modulated by electrostatic effects, and is relevant in drug delivery , sea water desalination and dialysis technologies. Due to

6313-504: The compounds’ hydrophobicity , low thermal conductivity , and high flexibility. Siloxanes generally adopt structures expected for linked tetrahedral (" sp -like") centers. The Si−O bond length is 1.64 Å (vs Si–C distance of 1.92 Å) and the Si-O-Si angle is rather open at 142.5°. By contrast, the C−O distance in a typical dialkyl ether is much shorter at 1.414(2) Å with

6420-459: 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 (H 4 SiO 4 , 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,

6527-405: The coupled acid-base equilibrium, electrostatics and network elasticity, only recently has such system been correctly recreated in computer simulations . Some species secrete gels that are effective in parasite control. For example, the long-finned pilot whale secretes an enzymatic gel that rests on the outer surface of this animal and helps prevent other organisms from establishing colonies on

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6634-512: 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

6741-451: The defining feature of which is that each pair of silicon centres is separated by one oxygen atom. The siloxane functional group forms the backbone of silicones [−R 2 Si−O−SiR 2 −] n , the premier example of which is polydimethylsiloxane (PDMS). The functional group R 3 SiO− (where the three Rs may be different) is called siloxy . Siloxanes are manmade and have many commercial and industrial applications because of

6848-427: The direct dissolution of Na 2 O and K 2 O 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

6955-515: The elastic energy of one stand can be written as where R ref {\displaystyle R_{\text{ref}}} is the mean-square fluctuation in end-to-end distance of one strand. The modulus of the gel is then this single-strand elastic energy multiplied by strand number density ν = ϕ / N b 3 {\displaystyle \nu =\phi /Nb^{3}} to give This modulus can then be equated to osmotic pressure (through differentiation of

7062-439: The elastic nature of the gel, the dispersion of A − {\displaystyle {\text{A}}^{-}} in the system is constrained and hence, there will be a partitioning of salts ions and H + {\displaystyle {\text{H}}^{+}} inside and outside the gel, which is intimately coupled to the polyelectrolyte degree of ionization. This ion partitioning inside and outside

7169-576: The environment. The cyclomethicones D 4 and D 5 are bioaccumulative in some aquatic organisms, according to one report. In the European Union, D 4 , D 5 and D 6 have been deemed hazardous as per the REACH regulation. They were characterized as substances of very high concern (SVHC) due to their PBT ( persistent , bioaccumulative and toxic) and vPvB (very persistent and very bioaccumulative) properties. Canada regulates D 4 under

7276-416: The final swollen gel, the second is associated with the initial gel and the third is of the pure solvent prior to mixing. Substitution of ϕ = ϕ 0 / λ 1 λ 2 λ 3 {\displaystyle \phi =\phi _{0}/\lambda _{1}\lambda _{2}\lambda _{3}} leads to Note that the second term is independent of

7383-404: The free energy) to give the same equation as we found above. Consider a hydrogel made of polyelectrolytes decorated with weak acid groups that can ionize according to the reaction is immersed in a salt solution of physiological concentration. The degree of ionization of the polyelectrolytes is then controlled by the pH {\displaystyle {\text{pH}}} and due to

7490-425: 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

7597-415: The gel is analogous to the partitioning of ions across a semipemerable membrane in classical Donnan theory, but a membrane is not needed here because the gel volume constraint imposed by network elasticity effectively acts its role, in preventing the macroions to pass through the fictitious membrane while allowing ions to pass. The coupling between the ion partitioning and polyelectrolyte ionization degree

7704-627: The gel is now completely characterized by stretch factors λ 1 {\displaystyle \lambda _{1}} , λ 2 {\displaystyle \lambda _{2}} and λ 3 {\displaystyle \lambda _{3}} and hence it is of interest to derive the deformation free energy as a function of them, denoted as f gel ( λ 1 , λ 2 , λ 3 ) {\displaystyle f_{\text{gel}}(\lambda _{1},\lambda _{2},\lambda _{3})} . For analogy to

7811-478: The gel would shrink to roughly its original size. This gel volume change can alternatively be introduced by applying external forces. If a uniaxial compressive stress is applied to a gel, some solvent contained in the gel would be squeezed out and the gel shrinks in the applied-stress direction. To study the gel mechanical state in equilibrium, a good starting point is to consider a cubic gel of volume V 0 {\displaystyle V_{0}} that

7918-424: 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

8025-475: The historical treatment of rubber elasticity and mixing free energy, f gel ( λ 1 , λ 2 , λ 3 ) {\displaystyle f_{\text{gel}}(\lambda _{1},\lambda _{2},\lambda _{3})} is most often defined as the free energy difference after and before the swelling normalized by the initial gel volume V 0 {\displaystyle V_{0}} , that is,

8132-466: The hydration of SiO 2 consumes two water molecules and continues one step further to form H 4 SiO 4 . The difference in hydration behaviour between SiO 2 and CO 2 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 Si(OH) 4 really existing in dilute solution

8239-418: The hydrogel structure to obtain nanocomposites with tailored functionality. Nanocomposite hydrogels can be engineered to possess superior physical, chemical, electrical, thermal, and biological properties. Many gels display thixotropy – they become fluid when agitated, but resolidify when resting. In general, gels are apparently solid, jelly-like materials. It is a type of non-Newtonian fluid . By replacing

8346-495: 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. The ASR reaction significantly differs from

8453-432: The initial formation of silanols (R 3 Si−OH): The siloxane bond can then form via a silanol + silanol pathway or a silanol + chlorosilane pathway: Hydrolysis of a silyldichloride can afford linear or cyclic products. Linear products are terminated with silanol groups: Cyclic products have no silanol termini: The linear products, polydimethylsiloxane (PDMS), are of great commercial value. Their production requires

8560-488: The initial state. On the other hand, the mixing term f mix ( λ 1 , λ 2 , λ 3 ) {\displaystyle f_{\text{mix}}(\lambda _{1},\lambda _{2},\lambda _{3})} is usually treated by the Flory-Huggins free energy of concentrated polymer solutions f ( ϕ ) {\displaystyle f(\phi )} , where ϕ {\displaystyle \phi }

8667-446: 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? Na and K cations in themselves cannot attack silica (the culprit is their counter ion OH ), 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

8774-485: The liquid with gas it is possible to prepare aerogels , materials with exceptional properties including very low density, high specific surface areas , and excellent thermal insulation properties. A gel is in essence the mixture of a polymer network and a solvent phase. Upon stretching, the network crosslinks are moved further apart from each other. Due to the polymer strands between crosslinks acting as entropic springs , gels demonstrate elasticity like rubber (which

8881-430: The network contribution leads to This provides the starting point to examining the swelling equilibrium of a gel network immersed in solvent. It can be shown that gel swelling is the competition between two forces, one is the osmotic pressure of the polymer solution that favors the take in of solvent and expansion, the other is the restoring force of the polymer network elasticity that favors shrinkage. At equilibrium,

8988-427: The number of monomers remains the same while the gel volume has increased by a factor of λ 1 λ 2 λ 3 {\displaystyle \lambda _{1}\lambda _{2}\lambda _{3}} . As the polymer volume fraction decreases from ϕ 0 {\displaystyle \phi _{0}} to ϕ {\displaystyle \phi } ,

9095-448: The other with the mixing of the network with the solvent. Hence, we write We now consider the two contributions separately. The polymer elastic deformation term is independent of the solvent phase and has the same expression as a rubber, as derived in the Kuhn's theory of rubber elasticity : where G 0 {\displaystyle G_{0}} denotes the shear modulus of

9202-420: 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 ,

9309-426: The picture. Consider the case of two adjacent, initially uncharged acid sites HA {\displaystyle {\text{HA}}} are both dissociated to form A − {\displaystyle {\text{A}}^{-}} . Since the two sites are both negatively charged, there will be a charge-charge repulsion along the backbone of the polymer than tends to stretch the chain. This energy cost

9416-402: 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 : H 4 SiO 4 ), while in an older industrial notation the non-existing (H 2 SiO 3 , hemihydrated silica,

9523-426: 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

9630-489: The production of dimethylsilicon dichloride . Starting from trisilanols, cages are possible, such as the species with the formula (RSi) n O 3 n /2 with cubic ( n  = 8) and hexagonal prismatic ( n  = 12) structures. The cubic cages are cubane-type clusters , with silicon centers at the corners of a cube oxygen centres spanning each of the twelve edges. Oxidation of organosilicon compounds, including siloxanes, gives silicon dioxide . This conversion

9737-458: The rates of the gel drying and of the carbonation reactions 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 encountered 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. Gel A gel

9844-428: 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

9951-482: 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. The alkali-silica reaction mechanism catalysed by

10058-519: The same reactions sets by simply replacing the CO 3 anions by HCO 3 and the SiO 3 anions by HSiO 3 , the principle of catalysis remaining the same, even if the number of intermediate species differs. One can distinguish several sources of hydroxide anions ( OH ) in hardened cement paste (HCP) from the family of Portland cement (pure OPC , with BFS , or with cementitious additions, FA or SF ). OH anions can be directly present in

10165-637: 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. 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. The presence of these oxygen-bearing groups very prone to form hydrogen bonds with water molecules explains

10272-454: The silicic acid H 4 SiO 4 , or Si(OH) 4 , which is equivalent to SiO 2 · 2 H 2 O 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 2 . Strictly speaking, silica is the anhydride of orthosilicic acid, Si(OH) 4 . An ancient industrial notation referring to H 2 SiO 3 , metasilicic acid ,

10379-401: The skin but not remain tacky afterward. Dow is a major producer of cyclomethicones. Cyclomethicones, like all siloxanes, degrade by hydrolysis, producing silanols . These silanols are produced at such low levels that they do not interfere with hydrolytic enzymes. Even though some cyclomethicones structurally resemble crown ethers , they bind metal ions only weakly. The word siloxane

10486-408: 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

10593-421: 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 Ca(OH) 2 to precipitate insoluble calcium silicate. As suggested by Davies and Oberholster (1988),

10700-467: The stretching factors λ 1 {\displaystyle \lambda _{1}} , λ 2 {\displaystyle \lambda _{2}} and λ 3 {\displaystyle \lambda _{3}} and hence can be dropped in subsequent analysis. Now we make use of the Flory-Huggins free energy for a polymer-solvent solution that reads where v c {\displaystyle v_{c}}

10807-424: The subject discusses the use of hydrogels for nucleus pulposus replacement, cartilage replacement, and synthetic tissue models. Many substances can form gels when a suitable thickener or gelling agent is added to their formula. This approach is common in the manufacture of a wide range of products, from foods to paints and adhesives. In fiber optic communications, a soft gel resembling hair gel in viscosity

10914-412: The surface of affected concrete. 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 Ca 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,

11021-537: The surface of these whales' bodies. Hydrogels existing naturally in the body include mucus , the vitreous humor of the eye, cartilage , tendons and blood clots . Their viscoelastic nature results in the soft tissue component of the body, disparate from the mineral-based hard tissue of the skeletal system. Researchers are actively developing synthetically derived tissue replacement technologies derived from hydrogels, for both temporary implants (degradable) and permanent implants (non-degradable). A review article on

11128-436: The term 'hydrogel' in the literature was in 1894. An organogel is a non-crystalline , non-glassy thermoreversible ( thermoplastic ) solid material composed of a liquid organic phase entrapped in a three-dimensionally cross-linked network. The liquid can be, for example, an organic solvent , mineral oil , or vegetable oil . The solubility and particle dimensions of the structurant are important characteristics for

11235-493: The two effects exactly cancel each other in principle and the associated λ 1 {\displaystyle \lambda _{1}} , λ 2 {\displaystyle \lambda _{2}} and λ 3 {\displaystyle \lambda _{3}} define the equilibrium gel volume. In solving the force balance equation, graphical solutions are often preferred. In an alternative, scaling approach, suppose an isotropic gel

11342-640: The volume of a liquid medium and ensnares it through surface tension effects. This internal network structure may result from physical bonds such as polymer chain entanglements (see polymers ) (physical gels) or chemical bonds such as disulfide bonds (see thiomers ) (chemical gels), as well as crystallites or other junctions that remain intact within the extending fluid. Virtually any fluid can be used as an extender including water ( hydrogels ), oil, and air ( aerogel ). Both by weight and volume, gels are mostly fluid in composition and thus exhibit densities similar to those of their constituent liquids. Edible jelly

11449-645: The xerogel due to a small amount of viscous flow) which results in a denser and more robust solid, the density and porosity achieved depend on the sintering conditions. Nanocomposite hydrogels or hybrid hydrogels, are highly hydrated polymeric networks, either physically or covalently crosslinked with each other and/or with nanoparticles or nanostructures. Nanocomposite hydrogels can mimic native tissue properties, structure and microenvironment due to their hydrated and interconnected porous structure. A wide range of nanoparticles, such as carbon-based, polymeric, ceramic, and metallic nanomaterials can be incorporated within

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