Alkali-silica Reaction Products Research Articles

Influence of enforced carbonation on alkali-silica reaction: Performance and multi-scale mechanisms

Alkali-silica reaction (ASR) has emerged as one of the major concrete deterioration issues. Despite the extensive effort investments in ASR mitigation, the interaction between ASR and carbonation remains unclear. To investigate the role and mechanisms of early-age enforced carbonation in ASR, enforced carbonation conditions at 50 °C and 95 % RH with different CO2 concentrations up to 20 % were employed at two stages. The ASR mitigation efficacy was evaluated by monitoring the volume expansion and cracking behavior of mortars containing reactive aggregates. A comprehensive understanding was obtained by analyzing the evolutions of ASR gel's composition, molecular structure, and hygroscopicity. The results indicate that ASR can be effectively suppressed under carbonation with a 97.9 % decrease in ASR-induced volume expansion and fully suppressed cracking when the mortars were cured at 20 % CO2 for 30 days. The decreases in the Q3 polymerization sites and [K + Na]/Si ratios of the ASR products, as well as the 30.3 % reduction in water uptake capacity of ASR gels under carbonation, uncovered the underlying mechanisms for the mitigated ASR at multiple length scales.

Impact of exposure conditions on alkali-silica reaction in alkali-activated material systems

The influence of different testing methods on alkali-silica reaction (ASR) behavior is a result of the difference in the effect of exposure conditions on it. This study investigated the impact of temperature and the addition of alkali on ASR behavior in an alkali-activated material (AAM) system and compared it with an ordinary Portland cement (OPC) system. The results showed that the ASR in the AAM system was more independent of the exposure conditions than that in the OPC system because of its inherently high alkalinity and dense microstructure. More ASR products with higher Na:Si ratios were evident in the alkali-activated slag (AAS) mortars than in the OPC mortars. The accelerated test conditions developed for OPC systems could be applied to AAM systems. However, owing to the difference in the acceleration effect, comparing the ASR-induced expansion in OPC and AAM systems using the accelerated test method was misleading to some extent. This study contributes to the understanding of the ASR in AAM systems and is of considerable importance for the industrial application of AAMs.

Elucidating the role of magnesium in alkali-silica reaction: Performance and mechanisms

Alkali-silica reaction (ASR) is a major concrete deterioration causing volume expansion, cracking, and hence degradations in the strength and permeability of concrete structures. Despite a variety of ASR mitigation approaches having been investigated, the successful use of chemical admixture is limited to lithium salts, which have negative impacts on cement hydration and shrinkage behavior of concrete. To explore alternative substances for effective and economic ASR mitigation, the role of magnesium nitrate in mortars containing reactive aggregate is investigated at varying Mg/alkali ratios from 0 to 3.0 in terms of volume expansion, cracking behavior, silica dissolution, and modifications of ASR products. The results indicate that, at a Mg/alkali ratio of 0.74, the expansion and surface cracking density of the ASR-impacted mortar were decreased by 21.1 % and 39.1 %, respectively, which is superior to the efficacy of lithium nitrate. The 51.3 % less silica dissolution from aggregate, suppressed ASR gel formation, fewer Q3 polymerization sites in the silicate chains, less packed structure, and reduced alkali/Si ratio of the reaction products elucidate the underlying mechanisms across multiple length scales.

Thermodynamic modelling of alkali-silica reactions in blended cements

Experiments and field observations have shown that supplementary cementitious materials (SCMs) mitigate the risk of alkali-silica reaction (ASR) in Portland cement blends, but this is difficult to describe thermodynamically due to the lack of appropriate thermodynamic database information, and challenges in defining the kinetic factors. This paper examines how four well-known SCMs (namely ground granulated blast furnace slag, silica fume, metakaolin, and fly ash) mitigate ASR, by using a multi-stage simulation and a newly extended thermodynamic database. Calcium hydroxide consumption by SCM hydration, dilution of available alkalis, and alkali binding in calcium-alkali-aluminosilicate hydrate solid solutions, play pivotal roles in understanding the influence of SCMs in ASRs. Blending aluminium-rich SCMs in ASR-prone systems appears to act by preventing the dissolution of reactive silica in aggregates rather than by suppressing the formation of ASR products. This paper provides new thermodynamic insights into the mechanisms by which SCMs enhance concrete durability against ASR.

Initial Characteristics of Alkali-Silica Reaction Products in Mortar Containing Low-Purity Calcined Clay.

An alkali-silica reaction (ASR) is a chemical process that leads to the formation of an expansive gel, potentially causing durability issues in concrete structures. This article investigates the properties and behaviour of ASR products in mortar with the addition of low-purity calcined clay as an additional material. This study includes an evaluation of the expansion and microstructural characteristics of the mortar, as well as an analysis of the formation and behaviour of ASR products with different contents of calcined clay. Expansion tests of the mortar beam specimens were conducted according to ASTM C1567, and a detailed microscopic analysis of the reaction products was performed. Additionally, their mechanical properties were determined using nanoindentation. This study reveals that with an increasing calcined clay content, the amount of the crystalline form of the ASR gel decreases, while the nanohardness increases. The Young's modulus of the amorphous ASR products ranged from 5 to 12 GPa, while the nanohardness ranged from 0.41 to 0.67 GPa. The obtained results contribute to a better understanding of how the incorporation of low-purity calcined clay influences the ASR in mortar, providing valuable insights into developing sustainable and durable building materials for the construction industry.

Atomic structure of nanocrystalline and amorphous ASR products

Limited information about the atomic structure of amorphous and nanocrystalline alkali–silica reaction (ASR) products is available. Here, reactive molecular simulations were employed to construct a model of these products based on defective shlykovite. To introduce disorder into the models, various annealing temperatures (ranging from ambient to temperatures above silicate dissolution) were tested. Structures obtained from annealing at 700 K reproduced the main peaks observed experimentally in X-ray diffraction patterns of “ASR-P1” phase, a nanocrystalline or amorphous alkali–silica reaction product recently identified in synthetic and field concrete. Pair distribution functions and structure factors computed for this structure also compared well with previous data reported for “amorphous” ASR products. These results demonstrate that what was previously considered “amorphous” ASR products might be related to nanocrystalline defective shlykovite structures.

The alkali–silica reaction of seawater-mixed mortar

The application of seawater concrete is receiving increasing attention due to the advantage of using seawater in alleviating water scarcity. One concern with seawater concrete applications is the potential for alkali–silica reaction (ASR) due to the elevated alkali concentration. In this paper, the ASR risk induced by mixed seawater in cement mortar incorporating non-reactive or reactive aggregates (RAs) was studied through short-term and long-term experiments. Experimental results indicate that seawater only increased ASR in the presence of RAs due to the high alkali concentration. Replacing cement with 70% ground granulated blast-furnace slag (GGBS) prevented expansion, although very few ASR products may be observed. SEM results indicated that seawater decreases the (Na + K)/Si ratio and increases the Ca/Si ratio of ASR products slightly. Compared with cement mortar, C–S–H in GGBS blended cement mortar has a higher (Na + K)/Si ratio, which indicates its higher alkali binding capacity.

Influences of aggregate gradation on alkali-silica reaction of seawater and sea sand concrete

The application of seawater and sea sand concrete (SWSSC) is beneficial for marine engineering, but the impact of aggregate gradation on its alkali-silica reaction (ASR) remains poorly understood. This study aims to bridge this gap by analyzing ASR products, pore characteristics and expansion rate of specimens. The test results reveal that ordinary concrete (OC) exhibits an inhibitory effect on ASR in comparison with to SWSSC. The 14-day expansion of ordinary concrete and SWSSC with the same aggregate gradation are 0.130 % and 0.212 %, showing potential and high risk of ASR, respectively. Moreover, particle size and gradation of aggregate are the key factors influencing the ASR degree of SWSSC. Compared with coarser aggregate, specimens with finer aggregate consume more K+ and Ca2+ ions, generate more ASR-P1, form less porosity, and produce a larger expansion. Additionally, specimens with gap-graded aggregates, as opposed to those with uniform or continuous gradation, show greater consumption of K+ and Ca2+ ions, increased ASR-P1 formation, more harmful pore formation, and a larger expansion. These results offer insights into optimizing aggregate gradation to reduce ASR in SWSSC and improve its durability.

Influences of composition and reaction conditions on molecular structure, hygroscopicity, and swelling behavior of alkali-silica reaction gels

Alkali-silica reaction (ASR) gel, a product of the reactions between the dissolved silica from aggregate and the alkalis from cement, governs the destructiveness of ASR and concrete deterioration. The influences of chemical composition and reaction conditions of ASR gel on its structure and properties remain unclear. In this work, the correlations between the molecular structure, hygroscopicity, and swelling behavior of ASR products were uncovered by investigating six CaO–SiO2-M2O systems with varying alkali/Si and Ca/Si ratios under two reaction conditions. The results indicate that the low-alkali gels have a layered silicate structure dominated by Q1 and Q2 sites similar to calcium silicate hydrate (C–S–H), whereas the high-alkali gels showed coexistence of tobermorite-type C–S–H and alkali-silicate hydrates featured with Q3 polymerization. The 29Si nuclear magnetic resonance (NMR) results showed that the mean chain length (MCL) and degree of polymerization (DP) decrease with alkali/Si and Ca/Si ratios but increase with moisture. Under 97 % RH, decreased crystallinity and enhanced formation of ASH and Q3 sites are obtained along with increased polymerization and d-spacing. The moisture uptake and swelling of the high-alkali gels showed a reverse correlation with the Ca/Si ratio, which confirms the role of calcium in suppressing the formation of ASH and the dominant role of Q3 sites in determining the hygroscopicity of ASR products.

Effects of desalinated sea sand on the alkali–silica reaction of seawater and sea sand concrete

The application of seawater and sea sand concrete (SWSSC) can reduce the construction period and cost of island infrastructure, but it may also bring the risk of alkali–silica reaction (ASR) owing to the presence of alkali ions in seawater and sea sand. To compare the characteristics of ASR between SWSSC and seawater and desalinated sea sand (DSS) concrete, the properties and the ASR products of mortar bars with different DSS content were studied. When the DSS proportion increased from 0% to 100%, the sodium (Na+), potassium (K+) and calcium (Ca2+) ion concentration of the specimens decreased by 22.6, 2.0 and 45.1 mg L−1, respectively, the pH decreased by 0.05 and the expansion of mortar bars was reduced by 0.16%. Desalination of sea sand could not eliminate the risk of ASR of SWSSC completely. The 14 days expansion of mortar bars with 100% DSS was 0.13%, and the precursors of ASR-P1 were observed by SEM. The experimental results of micriscopic tests all showed that with the increase of DSS proportion, the content of Na-shlykovite and ASR-P1 were reduced. A small amount of magnesium (Mg) in ASR products was detected. This study can provide a basis for the application of SWSSC in island infrastructure.

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.

Evaluation of the biotransformation of alkali-silica reaction products by Alkalihalobacillus clausii and Bacillusthuringiensis

Currently, there are several ways to reduce the possibility of the occurrence of the alkali-silica reaction (ASR), when measures are adopted that attenuate the favorable conditions for its occurrence, avoiding possible damage to concrete structures. However, after ASR has been installed in the concrete structure, does not exist a completely efficient and economically viable solution that attenuate the expansive effects of ASR. In the present research, the ability of Alkalihalobacillus clausii (formerly Bacillus clausii) and Bacillus thuringiensis microorganisms to biotransform ASR products was studied, and thus contribute to an initial study and proposition of an alternative, to solve problems caused by expansions in concrete structures affected by ASR. The ASR products were synthesized under controlled laboratory conditions, from sources of amorphous (pyrex) silica and inoculated with microorganisms for a period of 40 days, being submitted to microstructural characterization using the XRD, DTG/TG and FTIR techniques in periods of 10, 20, 30 and 40 days. The results showed that the A. clausii and B. thuringiensis bacteria were able to biotransform the ASR products. XRD's showed that there was biotransformation of the elements contained in the samples due to the reduction in the degree of crystallinity of the samples inoculated with bacteria. Thermal analyzes showed that there was biotransformation of ASR products from reducing weight loss in samples inoculated with bacteria compared to non-inoculated samples. The FTIR's showed that the bacteria are capable of biotransforming the ASR products, reducing the vibration bands associated with the ASR products to values greater than 98 % of the initial product. The FTIR analyzes also showed that the diversity of elements present in the mineralogy of the aggregates and that are part of the metabolism of the bacteria can influence, depending on their availability, in the efficiency of the biotransformation process of the ASR products.

Effect of cement composition and the storage conditions on the morphology of ASR products in concrete

AbstractUnder natural conditions, the alkali silica reaction (ASR) is a slow degradation process in concrete. Therefore, accelerated laboratory test methods are used to estimate the alkali‐reactivity of aggregates and concrete performance regarding a damaging ASR. In this study, concrete cubes were stored in a fog chamber at 40 °C/100% RH and at outdoor conditions in Munich climate for up to 9 years. Opaline sandstone/flint (25 vol.%) was used as reactive aggregate. 15 wt.% of OPC was replaced by fly ash and compared with the reference mix with 100 wt.% OPC. The cracks widths of the cubes were measured regularly. To investigate the effect of accelerating test conditions on the morphology of reaction products, the concrete microstructure, pore solution composition and ARS products formed under accelerated and field conditions were investigated. The present study shows that accelerated test methods consider only partial aspects of the complex mechanisms of ASR in concretes containing pozzolanic fly ash. Storage conditions have a direct effect on the morphology of the ASR products, the porosity and the pore solution composition of the concrete. When testing blended binders under accelerating conditions, opposing mechanisms are achieved and the chemical and physical processes can be changed significantly.

Alkali silica reaction in concrete - Revealing the expansion mechanism by surface force measurements

Alkali-silica reaction (ASR) is a major cause for concrete deterioration worldwide. However, the mechanism leading to cracking has not been identified yet. In this study, the extended Surface Force Apparatus (eSFA) has been used to determine the surface forces of alkali-silica solutions between two atomically smooth mica surfaces. The setup imitates the situation present in reactive concrete aggregates with negatively charged silicate surfaces and negatively charged polynuclear silica in solution. The distance of strong electrostatic repulsion increases with increasing concentration of dissolved silica, leading to the buildup of pressure up to 6 MPa. When the precipitation of ASR products in confined conditions is triggered by the addition of CaCl2 to the alkali-silica solution, the resulting solidification pressure forces the mica platelets apart and reaches 6–13 MPa. The eSFA experiments shows that solidification pressure is the mechanism leading to aggregate cracking and expansion of ASR-affected concrete.

Mitigation of alkali-silica reaction in blast-furnace slag-based alkaline activated material through incorporation of alum water treatment residue

Alkaline-activated materials (AAMs) have been developed as an alternative binder for cement in an effort to reduce the carbon footprint. However, due to the high alkaline contents in AAMs, these materials, particularly ground granulated blast-furnace slag (GGBS)-based AAM, may have a high potential of detrimental alkali-silica reaction (ASR) when amorphous silica-rich aggregates are used. This study utilised alum water treatment residue (AWTR), a waste from drinking water treatment, as an additional precursor in slag-based AAM to mitigate ASR in mortars with high reactive glass aggregate. The results showed that incorporating 40% AWTR in the precursor improved the compressive strength from 47 to 64 MPa and reduced ASR-induced expansion from 0.9% to 0.2% after 28 days of the accelerated test. The characteristics of the ASR gels were analysed using backscattered electron images coupled with energy-dispersive x-ray spectroscopy. Microscopic observations revealed that the ASR products formed in two stages. In the earlier stage, amorphous ASR gels precipitated surrounding the aggregates, exerting pressure on them and the matrix, resulting in initial cracking. In the latter stage, ASR products extruded into the open cracks, and crystalline products were formed, causing additional damage to the matrix. The quantitative elemental analysis demonstrated that incorporating AWTR could slow down silica dissolution by forming an Al layer in the aggregate surface, prevent the ASR gel calcification and reduce the growth of the harmful crystalline ASR products.

Internal alkali transport in recycling concrete and its impact on alkali-silica reaction

Recycling concrete (RC) produced with recycled aggregates (RA) obtained from demolished concrete structures bears the risk of an alkali-silica reaction (ASR). In this study, the interplay between aggregate reactivity and alkalis contributed by the cement and the RA is investigated. Three sets of concrete mixtures are produced using RA and natural aggregates, both reactive and non-reactive ones. Caesium either present in the RA or in the mixing water of the RC is used to trace the transport of alkalis and their incorporation in ASR products. The result shows that alkali released by RA can intensify ASR. On the other hand, ASR in RA with a low alkali content but reactive aggregate particles can be triggered by the alkali provided by the new cement paste of the RC. The highest risk of ASR in RC is caused by RA from a source concrete already affected by ASR.

Phase evolution and mechanical-hydroscopic properties of alkali-silica reaction gels modified by magnesium nitrate

Alkali-silica reaction (ASR)'s destructiveness is governed by the compositions and properties of ASR products, while methods of converting these hygroscopic-expansive gel-like products into innocuous phases remain unexploited. In this study, the influence of magnesium nitrate on the evolutions of phase, molecular structure, hydroscopic and mechanical properties of ASR gels with varying Mg/Si ratios from 0.1 to 1.1 was investigated. The results indicate that the primary phases of ASR products, tobermorite-type calcium silicate hydrate (C–S–H) and alkali kanemites, can be suppressed into brucite and eventually converted into magnesium-silicate-hydrate (M-S-H) in the presence of increasing Mg/Si ratios and the consequent decreasing pH. The Si–O–Si bridging bonds and Si–O symmetric stretching in the Q3 sites of ASR products can be suppressed. The phase and structure modifications resulted in a 93.5% reduction in hydroscopic swelling, a 94.7% decrease in strength, and a 152.3% drop in modulus of elasticity rendering the ASR products less destructive.

Characteristics of the alkali-silica reaction in seawater and sea sand concrete with different water-cement ratios

Research of seawater and sea sand concrete (SWSSC) is the current hot topic, but effects of water-cement ratios (W/C) on alkali-silica reaction (ASR) of SWSSC haven't been investigated. To solve this issue, the ASR products, pore structure and expansion rate of specimens with different W/C (0.35 ∼ 0.55) were measured. The study shows that pore structure and Na+ consumption are two key factors influencing ASR of SWSSC. When the W/C increases from 0.45 to 0.55, Na+ in specimen replaces K+ in ASR-P1(K0.52Ca1.16Si4O8(OH)2.84·1.5H2O), and ASR-P1 is converted into Na-shlykovite(NaCaSi4O8(OH)3·2.3H2O); the porosity is the dominant factor for the expansion rate of SWSSC. When the W/C increases from 0.35 to 0.45, the Na+ consumption is the dominant factor for the expansion rate of SWSSC. Magnesium element mainly coexists in Na-shlykovite and transition zone in SWSSC with W/C = 0.55. The research results are beneficial to inhibit ASR of SWSSC with different W/C and improve their durability.

Effect of type and quantity of inherent alkali cations on alkali-silica reaction

In this study, the macroscopical expansion induced by alkali-silica reaction (ASR) and its corresponding ASR products are investigated using ordinary Portland cement (OPC) mortar specimens with a gradient of boosted alkalis. Experimental results show that the expansion increases with the concentration of inherent alkalis. Sodium-boosted samples expand approximately three times as much as potassium-boosted samples. ASR gels that are present in aggregate veins are calcium-free and amorphous; the atomic ratios of ASR gels are nearly independent of the type and quantity of alkali cations. Aggregate ASR gel exudation occurs in high (≥2.5 %) sodium cases and produces potential Na-shlykovite. Crystalline and amorphous calcium-containing ASR products are present in aggregate vicinities in either Na- or K-boosted samples. The higher hydrophilicity of Na-gel in aggregate veins accounts for the larger expansion. Boosted alkali cations are more effective in ASR products formation than in exposing solution. A new observation that NaOH exposure inhibits ASR in K-boosted samples (zero expansion) is reported.

Assessment of the thermodynamics of Na,K-shlykovite as potential alkali-silica reaction products in the (Na,K)2O-CaO–SiO2-H2O system

The thermodynamic description of the alkali-silica reaction is incomplete, due to the lack of thermodynamic data for the alkali-silica reaction products. Here, we estimate the temperature-dependent thermodynamic properties of Na- or K-shlykovite phases which are found to form during alkali-silica reaction in concrete. The thermodynamic properties are then used to produce a series of binary phase diagrams considering different concentrations of alkalis. The results show that these two products start to form at 0.01 mol/kg of alkalis in the CaO–SiO2 binary phase diagrams. Both Na- and K-shlykovite can co-exist with C-S-H at an increased concentration of CaO, consistent with findings that these alkali-silica reaction products may form in real cement systems. However, there is a maximum alkali concentration in the binary phase diagram beyond which shlykovite cannot form, due to limited calcium solubility. This work can help understand and improve alkali-silica testing protocols proposed in standards.