Crystalline Alkali-silica Reaction Products Research Articles

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.

Alkali-silica reaction – a multidisciplinary approach

In the last four years, a multidisciplinary study involving several research groups in Switzerland tackled a number of unsolved, fundamental issues about the alkali-silica reaction (ASR) in concrete. The covered topics include SiO2 dissolution, the characterization of various ASR products formed at different stages of the reaction in both concrete and synthesis, crack formation and propagation. The encompassed scale ranges from nanometers to meters. Apart from conventional techniques, novel methods for the field of ASR have been used, e.g. combination of scanning electron microscopy with dissolution experiments, combination of focused ion beam with transmission electron microscopy, several synchrotron-based methods, synthesis of ASR products for in-depth characterization, time-lapse X-ray micro-tomography combined with contrast-enhancing measures and numerical models of ASR damage based on realistic crack patterns. Key achievements and findings are the quantification of the effect of aluminum on dissolution of different silicates, the variance in morphology and composition of initial ASR products, the differences and similarities between amorphous ASR products and calcium-silicate-hydrate, the link between temperature and the structure of the crystalline ASR products, the behavior of the crystalline ASR products at varying relative humidity, ASR propagation in 4D and numerical modelling based on realistic crack patterns.

Role of Aluminum and Lithium in Mitigating Alkali-Silica Reaction—A Review

Effective mitigation of alkali-silica reaction (ASR) is critical for producing durable concrete. The use of alumina-rich supplementary cementitious materials (SCMs) and chemical admixtures such as lithium salts to prevent expansion caused by ASR was first reported 70 years ago, shortly after the discovery of ASR in 1940s. Despite numerous investigations, the understanding of the mechanisms of Al and Li for mitigating ASR remain partially inexplicit in the case of Al, and hardly understood in the case of Li. This paper reviews the available information on the effect of Al and Li on ASR expansion, the influencing factors, possible mechanisms and limitations. The role of Al in mitigating ASR is likely related to the reduction of dissolution rate of reactive silica. Moreover, the presence of Al may alter the structure of crystalline ASR products to zeolite or its precursor, but such effect seems to be not that significant at ambient conditions due to the slow kinetics of zeolite formation. Several mechanisms for the lithium salts in mitigating ASR have been proposed, but most of them are not conclusive primarily due to the lack of knowledge about the formed reaction products. Combination of Al-rich SCMs and lithium salts may be used as an economic solution for ASR mitigation, although systematic studies are necessary prior to the applications.

Shear deformations in crystalline alkali-silica reaction products at the molecular scale: anisotropy and role of specific ion effects

Alkali-silica reaction (ASR) products exhibit viscoelastic behavior. Creep of nanolayered adsorbing materials is associated with shear deformations at the molecular scale. Crystalline ASR products are layered silicates with potassium and/or sodium as a charge balance ion in the interlayer space. These products are formed only in small quantities, exhibit long-range disorder, and are often mixed with other phases present in hydrated cement systems. In this context, experimental assessment of these products is a challenge and atomistic simulations arise as a valuable alternative. In this work, the shear response of crystalline ASR products is studied using classical molecular simulations. Understanding shear deformations are important to fully characterize the behavior of ASR products and may constitute a first step towards the comprehension of the viscoelastic behavior of these products. Further, the charge balance cation affects the shear modulus, von Mises yield stress, and residual strain, showing that the chemistry plays a crucial role in the mechanical response of these layered materials.

Effect of Al on the formation and structure of alkali-silica reaction products

The use of Al-rich supplementary cementitious materials can effectively reduce the expansion of concrete caused by alkali-silica reaction (ASR). However, the role of Al in mitigating the formation and structure of ASR products is only poorly understood, since direct investigation of the reaction in the micro-scale veins of aggregates is difficult. Recent successful synthesis of ASR products provides a unique opportunity to directly assess the role of Al in ASR. In this study, the effect of Al on the formation and structure of ASR products synthesized at 40 and 80 °C is investigated. It is found that the presence of Al in concentration lower than 0.1 mM neither prevents ASR nor alters the structure of the crystalline ASR products formed at 40 °C. However, formation of the crystalline ASR product at 80 °C is lowered at higher Al content due to the formation of a zeolitic precursor.

Moisture stability of crystalline alkali-silica reaction products formed in concrete exposed to natural environment

Crystalline ASR products were extracted from concrete exposed to natural environment and analyzed by scanning electron microscopy, energy-dispersive X-ray spectroscopy, Raman microscopy, X-ray diffraction and dynamic water vapor sorption. All extracted samples display similar compositions and Raman spectra. They sorb a limited amount of water vapor between 70 and 97% relative humidity (RH), amounting to 13–25 mass-%. Three different phases are identified, with d-spacings at low diffraction angles of 13.4, 12.3 and 10.8 Å. However, none of the phases show a change in d-spacing going from dry (~50% RH) to moist (~100% RH) conditions, indicating that no extra water is sorbed in the interlayer. This clearly indicates that swelling of crystalline ASR products is not responsible for stress generation and concrete cracking.

Characterization of amorphous and crystalline ASR products formed in concrete aggregates

Amorphous and crystalline alkali silica reaction (ASR) products formed in aggregates of two different concrete mixtures exposed to the concrete prim test both at 38 °C and 60 °C have been analysed by scanning electron microscope with energy dispersive X-ray spectroscopy and by Raman microscopy. Additionally, amorphous ASR products were synthesized and analysed with Raman microscopy and 29Si nuclear magnetic resonance. Amorphous ASR products display a higher Na/K-ratio than crystalline ones. Both types of products display a structure dominated by Q3-sites (Si-tetrahedra with three bridging oxygen atoms typical for a layer structure) with a secondary amount of Q2-sites (Si-tetrahedra with two bridging oxygen atoms typical for a chain structure). Temperature in the CPT alters the structure of the crystalline ASR. While the Raman spectra of the product formed at 38 °C is identical to the one formed in concrete structures, the one of the 60 °C product corresponds to K-shlykovite.

Mechanical behavior and phase change of alkali-silica reaction products under hydrostatic compression.

Alkali-silica reaction (ASR) causes severe degradation of concrete. The mechanical property of the ASR product is fundamental to the multiscale modeling of concrete behavior over the long term. Despite years of study, there is a lack of consensus regarding the structure and elastic modulus of the ASR product. Here, ASR products from both degraded field infrastructures and laboratory synthesis were investigated using high-pressure X-ray diffraction. The results unveiled the multiphase and metastable nature of ASR products from the field. The dominant phase undergoes permanent phase change via collapsing of the interlayer region and in-planar glide of the main layer, under pressure >2 GPa. The bulk moduli of the low- and high-pressure polymorphs are 27±3 and 46±3 GPa, respectively. The laboratory-synthesized sample and the minor phase in the field samples undergo no changes of phase during compression. Their bulk moduli are 35±2 and 76±4 GPa, respectively. The results provide the first atomistic-scale measurement of the mechanical property of crystalline ASR products.

Synthesis of alkali-silica reaction product structurally identical to that formed in field concrete

Alkali-silica reaction (ASR) can cause expansion and cracking of concrete. Despite significant progress over the past 80 years, the molecular structures of the ASR products remain poorly understood. These reaction products are present in very small amounts within concrete aggregates, which severely limit their chemical and physical characterizations. In this study, synthesis of an ASR product structurally identical to that formed in field concrete is achieved in large quantities at 40 °C. The temperature is found to affect the formation of different types of crystalline ASR products: 12.0 Å (<40 °C), 10.8 Å (around 40 °C), and 13.1 Å (60–80 °C for K-shlykovite), which all have layered silicate-sheet structures. Both 10.8 and 13.1 Å ASR products cannot swell at high relative humidity. This suggests that swelling of the studied crystalline ASR products by uptake of water cannot be the mechanism of ASR-induced expansion at 40 to 80 °C. These findings help to better understand ASR in concrete and pave the way for developing innovative solutions for limiting ASR.

Specific ion effects control the thermoelastic behavior of nanolayered materials: the case of crystalline alkali-silica reaction products.

We perform molecular simulations to characterize the structure and the thermo-mechanical behavior of crystalline alkali-silica reaction (ASR) products, which are layered silicate analogous to shlykovite. As charge-balancing cations, we study Na and K (the most relevant alkali involved in ASR) as well as Li (relevant for ASR therapy) and Cs (relevant for nuclear applications). For the first time, the thermal and elastic properties of these crystalline products are computed using molecular simulations. The simulations reveal that the charge-balancing cations control thermo-elastic properties of shlykovite, following the example of other phyllosilicates. Change balance ion affects the number of H-bonds in the order Cs > K > Na > Li, and that observation can be associated with the variations in the computed thermo-mechanical properties. Using as input the elastic properties obtained from the molecular scale and assuming long-range disorder at the mesoscale (through a simplified representation of the gel scale), we provide mean-field homogenization estimates of the elastic constants at the gel scale that are consistent with indentation data. This result suggests a nanogranular texture of ASR gels.

Synthesis, characterization, and water uptake property of alkali-silica reaction products

The reaction mechanism of alkali-silica reaction (ASR) is poorly understood due to the difficulties to directly characterize ASR products in concrete. In this study, ASR products with initial Ca/Si of 0.25 and (K + Na)/Si ratio 0.5 with different K/Na ratios are synthesized at 80 °C. The synthesized ASR products are characterized by different techniques, also the solution chemistry is analyzed and saturation indices are calculated. The results show that crystalline and nano-crystalline phases are formed in the presence of both alkalis and calcium. No ASR product is present in the absence of calcium. All synthesized crystalline ASR products highly resembles the crystal structure of shlykovite, indicating that a substitution of K in shlykovite by Na can occur. Its silicate sheet structure has strong similarity to the ASR products formed in concrete according to Raman spectra, while some differences are observed in their morphologies and basal peak of the XRD patterns.

Raman microscopy of alkali-silica reaction (ASR) products formed in concrete

Recently, the structure of the crystalline alkali-silica reaction (ASR) product formed in affected concrete has been identified based on μ-XRD measurements. However, the data were obtained from a single aggregate. In this study, Raman microscopy is applied on crystalline ASR products formed in several aggregates and concrete mixtures, enabling a comparison of their spectra and with it their structure. In a first step, samples from the same concrete used for μ-XRD measurements are analyzed and compared. In a second step, samples from a second structure and from a concrete prism test are measured. In addition to Raman microscopy, SEM with EDX is used to characterize the microstructure. The Raman spectra of the crystalline ASR product are practically identical in all studied aggregates and concrete mixtures, showing it is the same phase. This conclusion is further supported by the microstructural data.

Stress-relaxation of crystalline alkali-silica reaction products: Characterization by micro- and nanoindentation and simplified modeling

Alkali-silica reaction (ASR) is a major issue for the durability of concrete structures worldwide. Fostering recent microindentation works on the elastic property of ASR products, this work aims at further characterizing their viscous property. In particular, this study focuses on the relaxation behaviour of ASR crystalline products which are preferentially formed within cracks at the center of the reactive aggregates in real field concrete structures. First, the surface roughness and the chemical composition of ASR products were examined by Scanning Electron Scanning (SEM) and Energy Dispersive X-ray spectroscopy (EDS). Finally, the viscous behaviour of crystalline ASR products was characterized by performing load-relaxation tests by microindentation, providing new knowledge on the relaxation behaviour of the ASR crystalline products which was about 40%. Furthermore, nanoindentation test were also carried out to better understand the possible length-scale effect. In spite of the large scatter, the characterization at lower scale by nanoindentation showed that the ASR rosette crystals have a greater modulus and lower relaxation rate. This is likely due to the fact that the volume probed by nanoindentation is close to the size of ASR crystals and related interfaces. Finally, a simplified rheological model was employed to estimate the viscosity and characteristic time of the ASR crystalline products. The present result provide new knowledge on the viscous behaviour ASR products which can be helpful for better understanding ASR products and improve existing multiscale models.

Crystallized alkali-silica gel in concrete from the late 1890s

The Elon Farnsworth Battery, a concrete structure completed in 1898, is in an advanced state of disrepair. To investigate the potential for rehabilitation, cores were extracted from the battery. Petrographic examination revealed abundant deposits of alkali silica reaction products in cracks associated with the quartz rich metasedimentary coarse aggregate. The products of the alkali silica reaction are variable in composition and morphology, including both amorphous and crystalline phases. The crystalline alkali silica reaction products are characterized by quantitative X-ray energy dispersive spectrometry (EDX) and X-ray diffraction (XRD). The broad extent of the reactivity is likely due to elevated alkali levels in the cements used.