Alkali–silica reaction: Kinetics of chemistry of pore solution and calcium hydroxide content in cementitious system

This paper presents the results of the investigations on the chemistry of pore solutions, the contents of calcium hydroxide, and the expansions in mortars containing both reactive and non-reactive aggregates. In order to examine the effect of the temperature, experiments were performed at three different temperatures (23 °C, 38 °C and 55 °C).The compositions of the pore solution were measured at short time intervals for a period of up to 130 days in order to capture the kinetics of the chemistry of pore solution. The results showed that the changes in the concentrations of alkali ions can be best explained by the first order reaction. In addition, the proposed rate equation could reasonably simulate the changes in the actual concentrations of alkalis. Finally, the results in this paper suggest that the rate of the alkali–silica reaction in cementitious system containing highly reactive aggregate can be also expressed as the first order reaction.     

Alkali fixation of C–S–H in blended cement pastes and its relation to alkali silica reaction

Supplementary cementitious materials (SCM) are known to reduce or even stop expansion due to alkali silica reaction (ASR) in concretes with reactive aggregates. Studies indicate that the main reason for this is the decrease in alkalinity of the pore solution of the cement paste, which in turn is attributed to the change in composition of the C–S–H. In this paper we study the effect of aluminium and silicon incorporation in C–S–H on the composition of the pore solution in cement pastes containing SCMs. Different blended pastes of silica fume and metakaolin were cast, in order to obtain the same Si/Ca ratio of the C–S–H but with different aluminium contents. EDS micro analysis was made to determine the C–S–H compositions. In parallel pore solutions were extracted and analysed. It is found that the incorporation of aluminium does not increase the alkali fixation of the C–S–H found in real cementitious materials, suggesting that the greater effectiveness of SCMs containing alumina is due to other reasons.     

Comparison of the morphology of alkali–silica gel formed in limestones in concrete affected by the so-called alkali–carbonate reaction (ACR) and alkali–silica reaction (ASR)

The morphology of alkali–silica gel formed in dolomitic limestone affected by the so-called alkali–carbonate reaction (ACR) is compared to that formed in a siliceous limestone affected by alkali–silica reaction (ASR). The particle of dolomitic limestone was extracted from the experimental sidewalk in Kingston, Ontario, Canada that was badly cracked due to ACR. The siliceous limestone particle was extracted from a core taken from a highway structure in Quebec, affected by ASR. Both cores exhibited marked reaction rims around limestone particles. The aggregate particles were polished and given a light gold coating in preparation for examination in a scanning electron microscope. The gel in the ACR aggregate formed stringers between the calcite crystals in the matrix of the rock, whereas gel in ASR concrete formed a thick layer on top of the calcite crystals, that are of the same size as in the ACR aggregate.     

Determination of the elastic properties of amorphous materials: Case study of alkali–silica reaction gel

The gel formed during alkali–silica reaction (ASR) can lead to cracking and deterioration of a concrete structure. The elastic properties of the ASR gel using X-ray absorption and Brillouin spectroscopy measurements are reported. X-ray absorption was used to determine the density of the gel as a function of pressure, and the result yields an isothermal bulk modulus of 33 ± 2 GPa. Brillouin spectroscopy was applied to measure isentropic bulk (24.9–34.0 GPa) and shear moduli (8.7–10.1 GPa) of the gel. The range of values obtained is attributed to the variable composition of samples that were collected under field conditions. Results suggested that amorphous silica becomes expanded and compressible as it absorbs water molecules and alkali ions. This could explain high gel migration rates through the complex pore structures in concrete.     

A thermodynamic and kinetic model for paste–aggregate interactions and the alkali–silica reaction

A new conceptual model is developed for ASR formation based on geochemical principles tied to aqueous speciation, silica solubility, kinetically controlled mineral dissolution, and diffusion. ASR development is driven largely by pH and silica gradients that establish geochemical microenvironments between paste and aggregate, with gradients the strongest within the aggregate adjacent to the paste boundary (i.e., where ASR initially forms). Super-saturation of magadiite and okenite (crystalline ASR surrogates) occurs in the zone defined by gradients in pH, dissolved silica, Na⁺, and Ca^2 +. This model provides a thermodynamic rather than kinetic explanation of why quartz generally behaves differently from amorphous silica: quartz solubility does not produce sufficiently high concentrations of H₄SiO₄ to super-saturate magadiite, whereas amorphous silica does. The model also explains why pozzolans do not generate ASR: their fine-grained character precludes formation of chemical gradients. Finally, these gradients have interesting implications beyond the development of ASR, creating unique biogeochemical environments.     

Alkali–silica reaction (ASR)—performance testing: Influence of specimen pre-treatment,exposure conditions and prism size on alkali leaching and prism expansion

Whether or not concrete prism tests developed for assessment of alkali–silica reactivity of aggregates might be suitable for general ASR performance testing of concrete has been evaluated. This paper discusses how variations in specimen pre-treatment, ASR exposure conditions and prism size influence the rate and amount of alkali leaching and prism expansion, together with a discussion of consequences for ASR test procedures. Furthermore, results from some complementary tests are included.Generally, a remarkably high proportion of the in-mixed alkalis were leached out of the concrete prisms during the ASR exposure. For prisms exposed to 60 °C, the rate and amount of alkali leaching is the main controlling factor for the prism expansion. For less permeable concretes exposed to 38 °C, lack of internal moisture and lower rate of diffusion contributes to reduce the rate and extent of ASR expansion (reported in a separate paper).     

Polyurethaneurea–silica nanocomposites: Preparation and investigation of the structure–property behavior

Nanocomposites consisting of thermoplastic polyurethane–urea (TPU) and silica nanoparticles of various size and filler loadings were prepared by solution blending and extensively characterized by Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), thermal analysis, tensile tests, and nanoindentation. TPU copolymer was based on a cycloaliphatic diisocyanate and poly(tetramethylene oxide) (PTMO-2000) soft segments and had urea hard segment content of 20% by weight. TPU/silica nanocomposites using silica particles of different size (29, 74 and 215 nm) and at different loadings (1, 5, 10, 20 and 40 wt. %) were prepared and characterized. Solution blending using isopropyl alcohol resulted in even distribution of silica nanoparticles in the polyurethane–urea matrix. FTIR spectroscopy indicated strong interactions between silica particles and polyether segments. Incorporation of silica nanoparticles of smaller size led to higher modulus and tensile strength of the nanocomposites, and elastomeric properties were retained. Increased filler content of up to about 20 wt. % resulted in materials with higher elastic moduli and tensile strength while the glass transition temperature remained the same. The fracture toughness increased relative to neat TPU regardless of the silica particle size. Improvements in tensile properties of the nanocomposites, particularly at intermediate silica loading levels and smaller particle size, are attributed to the interactions between the surface of silica nanoparticles and ether linkages of the polyether segments of the copolymers.     

Cobalt–silica interaction and the dehydrogenation of 2-propanol

The interaction between silica and cobalt was studied on supported catalysts with low silica loading. Below a threshold cobalt level of 0.41 wt%, the catalysts were inactive for dehydrogenation of 2-propanol at 450 K. Inactivity was attributed to irreducibility of cobalt ions. Samples that were impregnated at a level below the threshold, dried, calcined, then reimpregnated below the threshold level, redried and recalcined such that the total cobalt content exceeded the threshold, were inactive. These results are not consistent with a model in which a portion of the cobalt interacts with specific silica sites, forming an irreducible species. Rather, they suggest that strongly interacting cobalt ions are incorporated into the silica surface.     

Composition–rheology relationships in alkali–silica reaction gels and the impact on the gel's deleterious behavior

Alkali–silica reaction (ASR) continues to be a major challenge to the durability of concrete structures. This is in part because the relationships between the composition, properties, and behavior of ASR gels in concrete are poorly understood. Gels with high pore solution pH, osmotic pressure, and rheological (e.g., yield stress) and swelling properties are the most deleterious. In this paper, the effects of the composition (primarily Ca/Si and Na/Si) of synthetic ASR gels on these characteristics are investigated, and regression analyses are done on the data. The pessimum combination of osmotic and rheological properties was found in the case of gels having intermediate calcium and high sodium contents (i.e., Ca/Si = 0.2 and Na/Si ≈ 0.85), leading to the highest estimated swelling pressures. These gels also developed the most alkaline pore solutions. While highest yield stresses were observed in the gels with low calcium and low sodium, they showed negligible osmotic and swelling pressures.     

Application of micro X-ray diffraction to investigate the reaction products formed by the alkali–silica reaction in concrete structures

Alkali–silica reaction (ASR) is one of the most important deterioration mechanisms in concrete leading to substantial damages of structures worldwide. Synchrotron-based micro-X-ray diffraction (micro-XRD) was employed to characterize the mineral phases formed in micro-cracks of concrete aggregates as a consequence of ASR. This high spatial resolution technique enables to directly gain structural information on ASR products formed in a 40-year old motorway bridge damaged due to ASR.Micro-X-ray-fluorescence was applied on thin sections to locate the reaction products formed in veins within concrete aggregates. Micro-XRD pattern were collected at selected points of interest along a vein by rotating the sample. Rietveld refinement determined the structure of the ASR product consisting of a new layered framework similar to mountainite and rhodesite.It is conceivable that understanding the structure of the ASR product may help developing new technical treatments inhibiting ASR.     

Influence of the interfacial reaction on interfacial adhesion in polyarylacetylene resin–silica glass composites

The influence of interfacial reaction on interfacial adhesion in silica glass/polyarylacetylene resin composites was studied. In order to achieve chemical reaction at the interface, vinyltrimethoxysilane was grafted onto silica glass surface to react with polyarylacetylene resin. The reaction between polyarylacetylene resin and vinyltrimethoxysilane was proved based on the model reaction between phenylacetylene and vinyltrimethoxysilane. At the same time, the modified silica glass surface characteristics were evaluated by contact-angle measurements and surface energy determination. The interfacial adhesion in silica glass/polyarylacetylene resin composites was evaluated by shear strength testing and fracture morphology analysis. It was concluded that polyarylacetylene resin reacted with vinyltrimethoxysilane. Furthermore, due to the reaction between polyarylacetylene resin and vinyltrimethoxysilane at the interface, the interfacial adhesion in composites was significantly increased. The improvement in interfacial adhesion was solely attributed to the interfacial reaction.     

Alkali–silica reaction (ASR)—performance testing: Influence of specimen pre-treatment,exposure conditions and prism size on concrete porosity,moisture state and transport properties

Whether or not concrete prism tests (CPTs) developed for assessment of alkali–silica reactivity of aggregates might be suitable for general ASR performance testing of concrete has been evaluated. This paper presents the background for the choice of test procedures and results on how variations in specimen pre-treatment, ASR exposure conditions and prism size influence concrete porosity, moisture state and transport properties. Results from measurements of alkali leaching and prism expansions during the ASR exposure are presented in a separate paper, together with discussion of consequences for ASR test procedures.For ordinary Portland cements and with water-to-cementitious-materials ratio (w/cm) 0.45 and higher it was found that the internal moisture state is sufficiently high in all the assessed procedures to produce ASR expansion. However, for less permeable concretes lack of internal moisture and lower rate of diffusion can significantly reduce the rate and extent of ASR expansion during laboratory performance testing.     

Combining nonlinear acoustics and physico-chemical analysis of aggregates to improve alkali–silica reaction monitoring

Very few chemical, physical and mechanical parameters appear suitable to monitor progressive damage caused by alkali–silica reaction (ASR) in concrete with the proper level of sensitivity and/or specificity. The purpose of the experimental work presented in this paper is to handle this limitation by proposing a non-conventional approach based on the combined use of nonlinear acoustics and physico-chemical analysis to assess the damage caused by ASR.The study was first carried out on laboratory concrete specimens kept in conditions promoting ASR (100% R.H. and 38 °C), as well as on laboratory concrete specimens submitted to thermal damage (thermal shock). For both types of damage, nonlinear acoustics allowed detecting and tracking the early evolution of micro-cracking in concrete with a higher sensitivity than other non-destructive parameters (i.e. dynamic Young's modulus or ultrasonic pulse velocity). The physico-chemical approach allowed distinctions to be made between types of damage by assessing granular swelling in the case of ASR, while no physical change in aggregates was detected regarding thermal damage.The procedure was then applied to concrete cores extracted from a large concrete field structure affected by ASR and submitted to residual expansion tests. Results confirmed the sensitivity of the nonlinear parameter to track residual damage in concrete and confirmed (with physico-chemical analysis) that ASR specifically contributed to the residual expansion and damage observed.     

Effects of uniaxial stress on alkali–silica reaction induced expansion of concrete

asr affected concrete in real structures is usually subject to loads which affect the macroscopic expansion of the material. An experimental study was undertaken using sensors embedded in reactive and non-reactive samples loaded on modified creep frames. Numerical analysis was used to link micro-structural damage under the load to expansion. We show that load influences the micro-crack propagation, which changes the shape of the expansion curve.     

Synthesis and characterization of novel bifunctional mesoporous silica catalyst and its scope for one-pot deacetalization–knoevenagel reaction

Bifunctional (both sulfonic and amine functionalized) mesoporous silica catalysts were prepared by hydrothermal method. (3-[2-(2-amino ethyl amino) ethyl amino] propyl trimethoxysilane) and (3-mercaptopropyl) trimethoxysilane respectively were used as amine and sulfonic sources. The synthesised bifunctional materials were fully characterized by BET, P-XRD, ²⁹Si CPMAS-NMR, FTIR, XPS and SEM spectral analysis. FTIR and NMR results endorse the successful grafting of organic amines and sulfonic group by surface silanol of silica. The presence of of N and S groups in the catalyst material was confirmed by the XPS spectra. Catalytic activity of the bifunctional material was tested for one-pot deacetalization–knoevenagel reaction, which gave excellent yield (95 %). Catalyst is reusable with little loss of activity up to 4 runs (<6 %).     

The wall-jet electrode and the study of electrode reaction mechanisms: The EC′ (catalytic) mechanism

The theory of EC (catalytic) reactions at the wall-jet electrode is developed using a computational procedure based on the Expanding Grid Backwards Implicit method. In particular, it is shown that the variation of the transport limited current with solution flow rate provides a means of characterising the EC mechanism. A working curve is presented which shows how the effective number of electrons transferred depends on a normalized rate constant and this permits the analysis of experimental data and the deduction of the homogeneous rate constant of the catalytic chemical reaction, for arbitrary electrode geometry, without recourse to further computation.     

Influences of heat-treatment on the microstructure and properties of silica–titania composite aerogels

Silica–titania composite aerogels were synthesized via ambient pressure drying by using water glass and titanium tetrachloride as raw materials. The influences of heat-treatment at different temperature with different heating rate on the microstructure and properties of the composite aerogels were investigated by differential thermal analyzer, Fourier transform infrared spectrometer, X-ray diffraction, nitrogen adsorption–desorption, scanning electron microscope and transmission electron microscope analysis. The results indicate that the silica–titania composite aerogels heat-treated at 250 °C exhibited highest specific surface area, pore volume and average pore diameter. When the heat-treatment temperature was higher than 450 °C, the –CH₃ groups on the surface of silica–titania composite aerogels would transform into –OH groups gradually, and in the meantime, the composite aerogels network structure would be destroyed gradually and the crystallinity of TiO₂ would be improved with the increase of heat-treatment temperature. Particularly, heat-treatment at temperatures above 750 °C would cause serious damage to the network structure of the composite aerogels. The adsorption/photocatalytic activity experiments showed that the composite aerogels heat-treated at 550 °C exhibit highest darkroom adsorption efficiency, and the 650 °C-heat-treated samples exhibited highest efficiency for removing the Rhodamine B from water.     

Stability of cenospheres in lightweight cement composites in terms of alkali–silica reaction

This paper presents an experimental study on characteristics and stability of cenospheres used in lightweight cement composites. ASTM C227 and C1260 tests were used to evaluate if cenospheres are potentially deleterious due to alkali–silica reaction (ASR). Natural sand was used as control. Examination by scanning electron microscope with energy-dispersive X-ray spectroscopy and analyses by X-ray diffractometer and thermogravimetry were conducted on samples with cenospheres after 9-month C227 and C1260 tests to better understand the behavior of cenospheres exposed to high alkaline environments and higher temperatures in these tests. Results indicate that cenospheres are not potentially deleterious due to ASR. Expansion of the mortar specimens tested to ASTM C227 and C1260 seems to be affected by the pozzolanic reactivity of cenospheres. Fine cenospheres showed limited pozzolanic reactivity at 28–30 °C and 38 °C, but exhibited significant pozzolanic reactivity at 80 °C with aluminum tobermorite [Ca₅Si₅Al(OH)O₁₇? 5H₂O] identified as the main reaction product.     

A study of the acidic and catalytic properties of pure and sulfated zirconia–titania and zirconia–silica mixed oxides

Protonated pyridine (PyH⁺) was not found on ZrO₂ (Z) or ZrO₂–TiO₂ (ZT), but was detected on sulfated oxides (ZS, ZTS) by IR spectroscopy. In contrast, ZrO₂–SiO₂ samples containing about 30–80 mol% ZrO₂ showed Brønsted acidity both in nonsulfated (ZS) and sulfated (ZSS) forms. The total acidity was determined by NH₃TPD. Introduction of sulfate ions increased the sitespecific catalytic activity (TOF) in the conversion of cyclopropane or nhexane. The effect of sulfate ions was more significant on samples rich in zirconia. Results suggest that Zr is homogeneously distributed in ZS samples rich in silica. Zirconiabound dimeric sulfate, generating strong acidity, could not be formed in these preparations due to the absence of fairly large ZrO₂ domains.     

Synthesis and characterization of microporous silica–alumina membranes

The development of microporous ceramic thin layers is of prime interest for sensors or gas separation membranes working at high temperature. Microporous silica membranes can be easily prepared by the sol–gel process. However the microporosity of pure silica is rapidly modified by steam at high temperature. One way to improve hydrothermal stability is to use mixed-oxide membranes. In this work, microporous silica–alumina membranes were prepared by a simple and robust sol–gel method. Tetraethoxysilane was mixed with an acidic alumina hydrosol. Urea was added for preparing the alumina hydrosol, for controlling the mixed-oxide network polycondensation and also as porogen agent. FTIR and ²⁷Al NMR spectroscopic analyses showed that for Si/Al molar ratios up to 6/1, homogeneous mixed oxides were obtained with a random distribution of Al and Si atoms in the oxide lattice based on tetrahedral units. The derived supported layers were crack-free as demonstrated by scanning electron microscopy (SEM) observations. Their microporosity was investigated using ellipsoporosimetry (EP) with films supported on flat dense substrates. He, N₂ and CO₂ permeance measurements were performed for membranes deposited on porous tubular substrates. The measured values of He/N₂ and He/CO₂ ideal selectivities are in agreement with the microporous nature of the prepared layers.