Alkali Silica Gel Research Articles

The dam that fly ash built.

When the first concrete was poured in 1949 for the Hungry Horse Dam (Montana, USA), pozzolan cements had already been used in several major North American dams, including Grand Coulee on the Columbia River (diatomaceous earth explored but ultimately not used), Friant on the San Joaquin River and Altus on the North Fork Red River (pumicite) and Bonneville on the Columbia River and Davis on the Colorado River (calcined clay). But Hungry Horse Dam stands out as the first dam constructed using coal combustion fly ash. Utilising 2.4 million cubic metres of concrete, the dam is located on the South Fork Flathead River, one of the tributaries feeding one of the nation's major waterways, the Columbia River, and closely related to the adjacent Glacier National Park. In this respect, Hungry Horse is directly connected to two momentous periods in modern history - the massive adoption in the 1950s of coal as fuel for power plants, and the ongoing threats to fresh water supply and the rapid retreat of alpine glaciers due to global warming. Two concrete cores from this dam, one with fly ash and one without fly ash, are examined microscopically to explore the long-term suppression of alkali-aggregate reaction by fly ash. The core without fly ash exhibits clear evidence of alkali-aggregate reaction, manifested by sandstone coarse aggregate particles with darkened reaction rims. Sandstone coarse aggregate particles of the same lithology in the core with fly ash are without signs of alkali-aggregate reaction. A detailed examination of the darkened rims indicates that alkali-silica reaction products fill the narrow gaps between adjacent sand grains in the sandstone. This alkali-silica gel infilling allows for optical continuity between adjacent sand grains and is responsible for the classic darkened rim associated with the alkali-aggregate reaction.

Impact of Aggregate Grain Size on ASR-Induced Expansion.

Alkali-silica reaction (ASR) is a sequence of complex chemical processes, resulting in the formation of alkali silica gels with high swelling ability. ASR leads to the expansion of concrete and the degradation of its microstructure. The susceptibility of aggregates to alkali reaction depends, among other factors, on the type and origin of the aggregate, the presence of reactive forms of silica, the mineral composition, and the geometric properties of the aggregate, such as shape and grain size. This study aimed to investigate the impact of the grain size of polymineral post-glacial gravel aggregate, originating from the northern regions of Poland, on its susceptibility to ASR. The expansion of mortars made from polymineral aggregate and the cracking of grains and cement matrix due to the occurring reactions were analyzed. Based on the conducted research, it was observed that the expansion of mortars depends on the grain size of the aggregate. It was demonstrated that the fraction of reactive aggregate generating the most significant elongation of mortars is in the range of 1.0-2.0 mm. The reaction of silica with alkalis continued until the depletion of reactive components in the aggregate. The relationship between the progress of corrosive processes and the grain size of the aggregate was evident in the form of different linear elongation increments of mortars over time. The expansion of mortars was caused by the swelling ASR gel, inducing stress in the grain and the surrounding cementitious paste.

Diagnosis of durability-related problems in concrete structures through comprehensive analysis and non-destructive testing: a case study

ABSTRACT The present study investigated concrete durability problems at the National University of Sciences and Technology,Pakistan, through comprehensive analysis and non-destructive testing. During the visual inspection of concrete structures, different deterioration mechanisms such as alkali-silica aggregate reaction (ASR), carbonation, abrasion and drying shrinkage were found. After visual inspection, the data on ingredients used in the construction of various structures were collected, revealingthat different brands of cement used in construction contained high alkali-equivalent content. Fine aggregates were used from two different quarries during construction. The available literature and X-ray Diffraction (XRD) results showed that both the quarries’ aggregates have the potential for alkali-silica reactivity. Furthermore, the XRD results of gel-type material collected from mapped cracking indicated the presence of delhayelite, similar to alkali-silica gel, suggesting ASR. The XRD curves of the whitest powder collected from various sites showed the presence of sodium sulfate and calcium carbonate peaks, indicating salt hydration attacks. The carbonation rate in concrete ranges from 4.33 to 5.77 mm/year. The results of the rebound hammer and pulse velocity test indicated that concrete used in parking has low strength which is the main cause of abrasion. Based on these findings a few recommendations have been presented to avoid durability-related problems in future structures.

Experiment study on the mechanical properties and alkali silica reaction (ASR) of mortar blended rice husk ash (RHA)

To ensure the serviceable popularization of RHA and avoid the alkali silica reaction of mortar blended RHA, the impact of RHA particle size and dosage on the mechanical strength and alkali silica reaction expansion of the mortar prepared with nonreactive silica sand and river sand has been evolutionarily investigated. The hydrates products of the mortar containing RHA were conducted by thermogravimetric (TG) and X-ray diffraction analysis (XRD). The microstructure of the mortar containing RHA was also explored by scanning electron microscope with an energy-dispersive X-ray spectroscopy analysis (SEM-EDX). The experiment results demonstrate that the compressive strength of mortar could be maximum enhanced by 21.1 % when RHA8.8 substituted 20 % of cement due to the pozzolanic effect and physical filling effect of RHA. Moreover, RHA with massive amorphous silica could react with Na ions in pore solution to form alkali-silica gel, thereafter resulting in expanding and destroying the microstructure of the mortar. The real ASR expansion value fabricated by RHA increased as the RHA particle size increased, which was mostly up to 0.0177 % when 20 % cement was replaced by RHA with a mean particle of 27.6 µm. Cracks noticeably emerged on the surface and edge of the mortar containing RHA with the particle size from 8.8 µm to 27.6 µm when RHA substituted 20 % of cement (by mass). Overall, the RHA particle size should be lower than 8.8 µm, while the substitution of cement by RHA also be below 10 % of cement, which can safely avoid the destructive expansion of RHA in an alkali environment.

Effect of aluminum on the structure of synthetic alkali-silica gels

The use of supplementary cementitious materials, such as fly ash, ground-granulated blast furnace slag (GGBFS), or metakaolin, in cement and concrete, is known to increase the amount of aluminum in the system and enhance its resistance against the alkali-silica reaction (ASR). It is, however, not yet sufficiently understood to what extent aluminum affects the structure of alkali-silica gels and how such a potential incorporation of aluminum contributes additionally to the beneficial effect of aluminum on ASR. In this study, various alkali-silica gels with different aluminum content were synthesized and structurally characterized. The results show that aluminum can be incorporated into the alkali-silica gel structure in four- and six-fold coordination, thus leading to a structural change. The incorporation of aluminum may increase the connectivity and the degree of polymerization within the gel framework. Based on the results, a 3D structural model for an aluminum-containing alkali-silica gel was created.

The ASR mechanism in concrete and the influence of lithium in mitigating it: A critical review

Alkali Silica Reaction (ASR) which is most familiarly known as ‘Concrete cancer ‘ is a serious trouble in concrete which has been a major challenge for all the researches. An alkali silica gel is formed when the reaction occurs between alkaline solution (OH–, Na+, K+ ions) and silica present in the aggregates. This gel sucks up adequate amount of moisture, it will enlarge and this is the cause of the cracks. This process is called “Alkali Silica Reaction”. ASR requires four vital conditions. Presence of silica in aggregates, existence of OH– ions in the alkaline solution of concrete, presence of dissolvable Ca, entry of moisture is the four vital condition. It can create significant cracking in concrete and results in various negative impacts on expansion, compressive strength, tensile strength, modulus of elasticity, fatigue and shear strength and as a consequence crucial structural complication can occur, which can even vigor the destruction of that specific structure. Elgeseter Bridge is one such example for that damage. Since it is a critical trouble, taking measures to mitigate them is indispensable. Various mitigating measures like limiting the alkali content of cement, limiting reactive silica content of the cement are taken. However, due to the dwindling availability of good performance aggregates, using lithium composites as an admixture to concrete mixes is one such technique to minimize this destructive reaction. The reaction between the alkalis and aggregates are reduced by altering chemical constitution of reaction products, damaging expansivity are minimized due to the presence of lithium ions. Thus, this paper gives detailed review about requirements of ASR, mechanism of ASR, their damages, their preventive measures, role of lithium in mitigating ASR and also about their various advantages.

Effect of Aluminum on the Structure of Synthetic Alkali-Silica Gels

Effect of Aluminum on the Structure of Synthetic Alkali-Silica Gels

Siliceous and dolomitic-bearing aggregates reaction in Tetramethylammonium hydroxide

Due to argillaceous dolomitic limestones usually contain more or less quartz or micro-crystalline quartz, based on existence of alkali-silica gels in concretes with dolomitic aggregates, lead to alkali-carbonation reaction (ACR) coexists with alkali-silica reaction (ASR) in concrete. This paper focuses on distinguish ASR are affected by the presence of Tetramethylammonium hydroxide (TMAH) solution to offer a better understanding of the effect of ADR on the mitigation process of alkali-carbonation reaction (ACR). In this work, X-ray diffraction (XRD), laser scanning confocal microscopy (LSCM) and scanning electron microscopy (SEM) were carried out analyses of products. The results presented strongly support that the TMAH has no react on reactive silica, and thus stop further attack on the reactive silica by hydroxyl lions. According to the results of the tests, the presence of TMAH is sufficient to promote the ADR in samples with dolomitic aggregates, formation products of brucite and calcite. Therefore, it was concluded that evidence to support the theory of TMAH can distinguish ASR and ADR when dolomitic aggregate bearing quartz.

Investigating the effect of cationic emulsified bitumen as an admixture of roller-compacted concrete pavement on the progress of alkali-silica reaction

Alkali silica reaction (ASR) is one of the mechanisms that can reduce the durability of concrete. ASR causes concrete to swell, thereby creating internal stresses, which result in micro-cracking. This study investigated the impact of using rapid setting cationic bitumen emulsion (CRS) as an additive in roller-compacted concrete (RCC) on the progress of Alkali silica reaction (ASR). For this purpose, RCC specimens with different bitumen contents by weight of cement were prepared and subjected to ASR simulation conditions for one year. Length change, ultrasonic pulse velocity, and compressive strength tests were performed at the different ages (3, 7, 28, 90, 180, and 360 days). In addition, water absorption (WA) test, and Semi-circular bending (SCB) test at notch depths 19, 25 and 32 mm in loading mode I were performed, and thereby, the ASR-induced damage in terms of change in critical strain energy release rate (ΔJC) and water absorption (ΔWA) were also assessed. The results showed that as the bitumen content increased, the intensity of ASR progression decreased. In the control specimen (EAC0), a 0.1% increase in length, a 50% decrease in fracture toughness, and a 25% increase in water absorption due to ASR progress were observed, while for the specimen with 8% bitumen (EAC8), the results of 0.05% increase in length and 11% decrease in fracture toughness and 8% increase in water absorption were revealed. Scanning Electron Microscopy (SEM) and Energy Dispersive Spectrometry (EDS) were performed to examine the microstructure of the specimens, as well. These examinations exhibited the fast progress of ASR and the extensive production of alkali-silica gel in the specimens without bitumen. However, in the mixture EAC8, aggregates near the bitumen-containing area revealed no trace of gel production, indicating that bitumen protects aggregates against ASR. One- and two-way analyzes of variance (ANOVA) performed at a 95% confidence level also confirmed the effectiveness of bitumen in preventing or reducing the intensity of ASR-induced damage in roller-compacted concrete.

Alkali-silica reaction in calcium aluminate cement mortars induced by deicing salts solutions

Calcium aluminate cement (CAC) mortars and concrete may be used as high-performance repairing materials for pavement restoration in airfields and roads. During recent years alkali-silica reaction (ASR) was reported in Portland cement concrete subjected to the action of organic deicers. Present paper reports the results of the investigation on the influence of organic and inorganic alkali deicers on ASR potential in CAC mortars containing fused silica as model reactive aggregate. It was shown for the first time, that concentrated solutions of sodium and potassium acetates, as well as sodium chloride, can cause ASR in CAC mortars. Sodium and potassium acetate resulted in much larger expansions comparing to sodium chloride and sodium hydroxide solutions used as a reference. Differences in alkali-silica gel were found depending on the solution which samples were soaked in. It was shown that ASR could proceed in a system without calcium hydroxide and an external source of hydroxyl ions.

Comparison of the Alkali-Silica Reactivity of North Cyprus and South Cyprus aggregates; preliminary studies using RILEM method

Alkali-silica reaction (ASR) is regarded as one of the most deleterious concrete durability problems, known to cause severe deteriorations in reinforced concrete structures all around the world. ASR involves the reaction of alkaline concrete pore solution with silica minerals in the aggregates and as a result, hydrous alkali-silica gel is produced. Expansion caused by this gel upon absorbing moisture results in serious deterioration in concrete. Although the susceptibility of South Cyprus aggregates to ASR has been previously studied to a very limited extent in the past, no scientific information on the ASR susceptibility of North Cyprus aggregates are available in the related literature. Beşparmak (Pentadaktylos) Mountains (North Cyprus) and from Troodos Mountains (South Cyprus) are positioned close to each other; however, aggregates obtained from both mountains are known to differ in composition. This difference in composition has the potential to yield ASR performances varying significantly. The aim of this study was to carry out preliminary investigations on the alkali-silica reactivity performance of both North and South Cyprus aggregates under same conditions, in a systematic and comparative manner. Aggregates obtained from both mountains are tested in combination with CEM I and CEM II (with supplementary cemenetitious materials) under the exposure conditions of RILEM method AAR-2. Preliminary results showed that North Cyprus aggregates are potentially reactive when used with CEM II, where South Cyprus aggregates tested under same conditions are detected to have much higher levels of reactivity.

Investigating the pozzolanic reaction of post-consumption glass powder and the role of portlandite in the formation of sodium-rich C-S-H

The use of glass powder (GP) as an alternative SCM offers a viable opportunity to partially substitute OPC, and therefore provides economic and environmental benefits. Moreover, the predominant siliceous amorphous phase provides the required component for its use as pozzolanic material. In this work, the mechanism and the products of the finely ground GP reaction with lime are investigated. The released silica, from glass dissolution, reacts with calcium hydroxide (CH) to form C-(N)-S-H with different compositions depending on the system. In the studied CH-GP binder systems, the fineness of GP ensures a higher surface for silica to react, leaving time for the pozzolanic reaction to take place. However, the glass continues to react even after the consumption of CH, which may lead to the apparition of alkali-silica gels around the particles in the long term.

Visualization of internal crack growth due to alkali–silica reaction using digital image correlation

This study visualized the development of micro-cracks in concrete due to the progression of the alkali–silica reaction (ASR) in two dimensions using a digital image correlation (DIC) technique. First, a preliminary experiment to determine the optimum specimen size for DIC was conducted on three specimens with different thicknesses. The results show that the ASR neither caused macro-level expansion nor attenuation of the ultrasonic propagation velocity in concrete specimens with a thickness of 10 mm. This was attributed to the leakage of the alkali–silica gel from the surface. On the other hand, we found that specimens with a thickness of 30 mm or greater were suitable for visualizing strain distributions on the concrete surface. Second, we examined the effects of the mechanical properties of aggregates on the ASR-induced crack propagation in concrete. The specimen dimensions were chosen based on the results of the preliminary experiment, and the mixing ratios selected ensured the manifestation of pessimum effects. The degree of concrete expansion depended on the type of non-reactive aggregate used in the mixture. Furthermore, the ASR-induced expansion of concrete was significantly reduced with the introduction of an artificial lightweight aggregate as the non-reactive aggregate, and the mechanical properties of the aggregate(s) were found to affect the ASR-induced crack growth.

HPC simulations of alkali-silica reaction-induced damage: Influence of alkali-silica gel properties

The alkali-silica reaction (ASR), a common cause of internal concrete degradation, is characterized by the formation of hydrophilic gel pockets inside the aggregates. These gel pockets swell by absorbing water, thus causing expansion and cracking of the concrete. In this work we have used high-performance computing simulations to investigate how the various gel properties, such as the Young's modulus, influence the mechanical degradation process of ASR. It is found that the relationship between the concrete expansion and the loss of elastic modulus is solely affected by the density of gel pockets and does not depend on the material properties of the gel. The ASR model developed in this work, however, overestimates the loss of elastic modulus. This can be explained by the fact that the visco-elasticity of the cement paste and the load-bearing capacity of the ASR-induced cracks are not considered in the model.

Variations in the composition and relations between alkali-silica gels and calcium silicate hydrates in highway concrete

Alkali-aggregate reactions (AAR) are a form of severe concrete deterioration that can be found anywhere in the world. The rock type of aggregate is the crucial factor for AAR occurrence as well as for the chemical and morphological nature of their deleterious products – alkali-silica gels (ASG). The paper focuses on highway cement concrete pavement with aggregate composed of metamorphosed volcano-sedimentary complex rocks, such as proterozoic chert, black shale, spilite, and green schist, and alluvial sandy gravels. These petrographic types of aggregate were found to be potentially hazardous. The microchemical composition of ASG was analysed and compared to calcium silicate hydrates present in the vicinity of one another. The qualitative elemental composition of both is the same. Gradual variations of ASG within a single position and among different positions in cement concrete pavement slabs were monitored and compared.

Determining alkali leaching during accelerated ASR performance testing and in field exposed cubes using cold water extraction (CWE) and µXRF

Considerable leaching of alkali metals was documented, both in concrete samples exposed to accelerated laboratory testing and field samples. CWE allowed to quantitatively determine the free alkali metal profiles as a function of the depth. However, CWE cannot account for the changes in the paste content towards a cast surface leading to a seemingly increase in alkali metals. The μXRF allowed to distinguish paste and aggregates. It allowed thereby to determine qualitative Na and K profiles in the cement paste phase of the concrete samples. The laboratory exposed samples showed a clear leaching profile into a depth of about 15 mm after 21 weeks of exposure at 60°C. Corresponding numbers for the 12 years field exposed cube were 50-60 mm. Alkali sorption by alkali silica gel was detected using the μXRF. For the laboratory exposed samples, the prisms prepared with Portland fly ash cement leached less alkali compared to the prisms prepared with ordinary Portland cement, as expected. The leaching in the middle of the prisms estimated based on the μXRF profiles agreed rather well with the level of alkali leaching determined based on the analysis of the leachate (i.e. the water below the samples during exposure).

Alkali-silica reaction and microstructure of concrete subjected to combined chemical and physical exposure conditions

Salt solutions are used to ensure safe driving conditions during winter. NaCl deicer is the most often used brine in Polish climatic zone. The chemical effects of this type of chloride-based deicer in wetting and drying (WD) and temperature cycles on concrete need to be better understood. This research was focus to study the microstructure of air-entrained pavement concrete after combined chemical (10% of NaCl) and physical (WD and 60°C) exposure conditions. The adopted WD and temperature regime was designed to verify the hypothesis that regularly alternating wetting and drying cycles with external alkali supply from deicer salt will provoke the Alkali-Silica Reaction (ASR). The aggregates varied their origin and mineralogical composition. The microscopic examination was carried out on concrete specimens using SEM with EDX. The microscopic analysis has shown that main reason for concrete deterioration during cyclic chemical and physical exposure conditions was both physical influence - WD cycles and the chemical influence – ASR (primarily, the fine aggregate which lead to form of alkali-silica gel). The expansive gel was shown to be capable of destroying the test specimens. Also differences in mineralogical composition of coarse aggregates influenced on the concrete prism expansion due to ASR.

Fundamentals of alkali-silica gel formation and swelling: Condensation under influence of dissolved salts

The current theory of ASR gel formation in concrete between silicic acid anions with cations and is unable to explain the differences in concentration required for gel formation between e.g. potassium and sodium or the effect of calcium on the gel formation. In this paper, a new gelation mechanism is explained by means of a condensation between silicic acid anions. The role of salts on the ASR gel formation and gel time is described by their charge screening capacity of the silicic acid, resulting in the decreasing concentration range for gelation: Ca<Mg<Cs<Rb<K<Na<Li. A Cation Gelling Strength (CGS) is defined to assess the cation strength when different cations are present. Silica gel formation will moreover in many cases form a calcium silica precipitate rather than a gel that will be difficult to distinguish from CSH formed during hydration.

Assessment of the Alteration of Granitic Rocks and its Influence on Alkalis Release

Several concrete structures had shown signs of degradation some years after construction due to internal expansive reactions. Among these reactions there are the alkali-aggregate reactions (AAR) that occur between the aggregates and the concrete interstitial fluids which can be divided in two types: the alkali-silica reaction (ASR) and alkali-carbonate reaction (ACR). The more common is the ASR which occurs when certain types of reactive silica are present in the aggregates. In consequence, an expansive alkali-silica gel is formed leading to the concrete cracking and degradation. Granites are rocks composed essentially of quartz, micas and feldspars, the latter being the minerals which contain more alkalis in their structure and thus, able to release them in conditions of high alkalinity. Although these aggregates are of slow reaction, some structures where they were applied show evidence of deterioration due to ASR some years or decades after the construction. In the present work, the possible contribution of granitic aggregates to the interstitial fluids of concrete by alkalis release was studied by performing chemical attack with NaOH and KOH solutions. Due to the heterogeneity of the quarries in what concerns the degree of alteration and/or fracturing, rock samples with different alteration were analysed. The alteration degree was characterized both under optical microscope and image analysis and compared with the results obtained from the chemical tests. It was concluded that natural alteration reduces dramatically the releasable alkalis available in the rocks.

Effect of lithium nitrate on the reaction between opal aggregate and sodium and potassium hydroxides in concrete over a long period of time

AbstractAlkali-silica reaction (ASR) is a reaction between amorphous or poorly crystallized siliceous phase, present in aggregates, and sodium and potassium hydroxides in the pore solution of concrete. Chemical admixtures such as lithium compounds are known to have high potential of inhibiting ASR. The aim of this study was to determine the effect of lithium nitrate on ASR in mortars containing high reactive opal aggregate over a long period of time. Mortar bar expansion tests were performed and microstructures of mortar bars were observed by scanning electron microscopy coupled with an energy dispersive X-ray microanalyser. Results from this study showed that effectiveness of lithium nitrate in mitigating ASR was limited over a long period of time. A larger amount of ASR gel which was formed in the presence of lithium nitrate indicated that the deterioration processes intensify within longer periods of time, which so far has not been observed in literature. Microscopic observation confirmed the presence of alkali-silica gel and delayed ettringite in mortars with lithium nitrate.