Amorphous Reaction Products Research Articles

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.

In-situ characterisation of the corrosion products formed on C-steel immersed in a soil-simulating solution

In this research C-steel corrosion products are allowed to form in a water-based solution containing 1000 ppm of SO42−, which simulates a highly corrosive soil. The electrode Fermi level was controlled with a three electrodes electrochemical set-up, scanning a potential window between the corrosion and the passivation regions. This, combined with in-situ Raman spectroscopy, allows to characterise the surface chemistry of the products formed according to the ongoing electrochemical reactions. When the potential is held in the stability region of the ionic species the surface state of the metal is characterised principally by the presence of lepidocrocite (γ-FeOOH) and green rust (GR-SO42−). The application of an anodic potential, simulating the effect of a DC anodic interference, combined with factors like moisture, presence of oxygen and sulphate ions contribute to develop a multicoloured (mix orange-like, yellow and green) mainly amorphous reaction product composed by a mix of iron oxide and oxi-hydroxide phases.

Variation in mineral composition by hydration and carbonation in calcium hydroxide matrix containing zeolite

Natural zeolite can modify the mechanical properties of concrete as a supplementary cementitious material based on pozzolanic activity. However, the pozzolanic reaction is difficult to explain because of the formation of amorphous reaction products and the complexity of concrete systems. In this study, the mineral, amorphpus phase compositions and pozzolanic reaction mechanism of zeolite were further investigated by performing X-ray diffraction (XRD) and magic angle spinning nuclear magnetic resonance (MAS NMR) analysis. Additionally, the pozzolanic activity of Ca(OH)2 (CH)/zeolite (CHZ, 0–50 wt% proportions of zeolite) specimens was investigated after 1, 3, 7, and 28 d of hydration. The pozzolanic products were identified as C-(A)-S-H gel and monocarboaluminate (AFmc) mineral. Zeolite, having a 5 wt% proportion of CHZ specimens, completely reacted after 3 d of hydration. The optimal zeolite proportion for forming AFmc minerals in CHZ specimens after hydration (HCHZ) was 5 wt%. As the zeolite proportion increased beyond 20 wt%, [Al(OH)4]- tended to react with [SiO(OH)3]- to form aluminosilicate gel instead of AFmc. After carbonation curing (CO2 concentration, 99%; gas pressure, 0.2 MPa; curing time, 2 h), the compressive strength of CHZ specimens (CCHZ) improved by 14–34% compared with that of pure CH specimens. An amorphous phase/calcite ratio of approximately 0.21 might produce an optimum strength resistance property in CCHZ. The heat released from the carbonation was beneficial for the polycondensation reaction of amorphous aluminosilicate to partially form Si[OSi]4 or Al[OSi]4 structures.

Understanding the Stability and Recrystallization Behavior of Amorphous Zinc Phosphate

Zinc phosphate, an important pigment in phosphate conversion coatings, forms protective films on rubbing surfaces. We have simulated the underlying reactions under shear by ball-milling zinc phosphate and monitored the reaction of hopeite (Zn3(PO4)2·4H2O) and the retarded recrystallization of the amorphous reaction product by powder X-ray diffraction (PXRD) and quantitative infrared (IR) spectroscopy. Abrasion of stainless steel was simulated by addition of pure 57Fe. The results provide insight into the chemistry of phosphate conversion coatings or during battery cycling of metal phosphates and give theoretical guidance for the preparation of amorphous phosphates. Thermal analysis revealed that the release of structural water is a key step during the reaction of hopeite under shear to ball-milled amorphous zinc phosphate. The back-reaction and associated recrystallization kinetics of amorphous zinc phosphate show a classical Langmuir behavior. Fe impurities inhibit the recrystallization of ball-milled amorphous zinc phosphate strongly. 57Fe Mössbauer spectroscopy and PXRD revealed that Fe is oxidized to Fe2+ and Fe3+ during ball-milling and incorporated locally at the tetrahedral and octahedral sites of the structure. Ball-milled amorphous zinc phosphate is metastable as γ-Zn3–xFex(PO4)2. EPR studies showed the incorporation of Fe3+ to be coupled with the formation of Zn2+ vacancies. The Fe3+ defect sites bind water because of their higher Pearson hardness (compared to Fe2+ and Zn2+), thereby reducing water mobility and inhibiting further reactions like the recrystallization to hopeite. Our findings reveal the amorphization mechanism of Zn3(PO4)2·4H2O in stainless steel ball mills at the atomic scale and highlight how the reactivity of amorphous products is affected by impurities associated with the processing method.

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.

Quantitative phase analysis of slag hydrating in an alkaline environment

An X-ray diffraction (XRD)-based evaluation of the crystalline and amorphous phases in slag hydrating in an alkaline environment is presented. A method is developed for the quantification of the amorphous phases present in hydrating slag in a sodium hydroxide solution. In hydrating slag, the amorphous reaction product is identified as calcium aluminosilicate hydrate. A water-soluble sodium-based amorphous reaction product is also produced. The XRD-based quantification method relies on the direct decomposition of the XRD intensity pattern of the total amorphous phase present in partially hydrated slag into the intensity patterns of the amorphous unreacted slag, the hydrate and the sodium-based product. The unreacted slag content in partially hydrated slag is also determined from the decomposition of the intensity signature of the total amorphous phase. An independent verification of the amorphous unreacted slag content in hydrating slag is obtained from measurements of blends of unhydrated and partially hydrated slag. The XRD-based phase-quantification procedure developed here provides a basis for evaluating the extent of reaction in hydrating slag.

Effect of active components on strength development in alkali-activated low calcium fly ash cements

The compressive strength achieved in alkali-activated low-calcium fly ash depends on the total reactive oxide ratios in the activated system, the reactive alumina content in fly ash and the initial molarity of NaOH. The total reactive silica in the system is the reactive silica contributed by the fly ash and soluble silica from the alkaline solution. A minimum molarity of NaOH is required to ensure complete dissolution of the glassy phase present in fly ash. The maximum compressive strength depends on the amorphous reaction product content formed, which is determined by the reactive alumina content in the fly ash. The highest compressive strength is achieved for the ratio of the total reactive silica to sodium ratio in the activated system equal to 4.72 and lower. More Na is incorporated in the reaction product on increasing the Na2O content relative to the reactive oxides.

Influence of processing temperature on the reaction product and strength gain in alkali-activated fly ash

The roles of temperature and curing periods on product formation and strength gain in alkali activation of low-calcium siliceous fly ash are evaluated. Higher temperature favors the kinetics of amorphous reaction product formation and inclusion of Si in the reaction product, which accompanies the densification of the microstructure. Following initial curing at 60 °C, the reaction product continues to form in the activated system on decreasing the temperature to 25 °C. Initial curing at 25 °C does not lead to formation of reaction product. Reaction product forms in the system for initial curing temperature of 40 °C and higher. The rate of the solid-based reaction, after hardening, is increased by temperature. The total amorphous reaction product content formed in the system depends on the water to solids ratio of the activated mix. The amorphous product composition, however, varies with temperature. With time there is an increase in the Si/Al ratio in the reaction product formed 40 °C and higher. For initial curing at 60 °C, there is no increase in the Si/Al ratio in the reaction product when the temperature is decreased to 25 °C. Reaction product with a higher Si/Al atomic ratio achieves a higher ultimate compressive strength. On increasing the Si/Al ratio in the reaction product from 2.5 to 3.3, the ultimate compressive strength achieved increased from 60 MPa to 74 MPa. Early curing, immediately after mixing, at a higher temperature is critical in achieving a higher Si/Al atomic ratio in the reaction product. Optimum initial curing time at high temperature is based on achieving a high Si/Al atomic ratio in the reaction product.

Preparation and characterization of geopolymers based on a phosphoric-acid-activated electrolytic manganese dioxide residue

Electrolytic manganese dioxide residue (EMDR) is a type of industrial solid waste that contains many soluble heavy metal ions. To reduce the hazard to environment, EMDR was proposed as raw material to fabricate phosphoric-acid-activated geopolymers in the present work. Relevant characteristics of these materials such as the compressive strength, bulk density, crystalline components, microstructure, leaching characteristic, high temperature resistance and corrosion resistance were investigated in this work. The results showed that EMDR was highly reactive with phosphoric acid and could be cured to form geopolymer gel materials with compressive strength up to 96.3 MPa after curing for 2 days at 80 °C with river sand added as an aggregate. Based on the analysis of X-ray diffraction and microstructure, the high compressive strength was due to the amorphous reaction products of phosphoric acid and magnetite in EMDR. The results of the leaching test of the geopolymer samples from EMDR showed that the manganese stabilization efficiency was 95.4%, and other measured elemental concentrations were all within the limits of Integrated Wastewater Discharge Standard in China when the CaO to wet EMDR ratio was 0.10. In addition, the geopolymers from EMDR showed good resistance to high temperature and aggressive environment except strong acid. Therefore, the results obtained in this study reveal that this method for the fabrication of geopolymers is a promising way to recycle EMDR and other iron-rich industrial solid wastes.

Kinetics of Silver Photodiffusion Into Amorphous Ge20S80 Films: Case of Pre‐Reaction

Silver photodiffusion into amorphous chalcogenide has attracted much attention because of its potential applications for example in memory devices. For its development, it is important to know how Ag ions diffuse in chalcogenide layers upon light exposure. In this paper, the photo‐induced effect on “pre‐reacted” films before light exposure, originating from Ag/Ge20S80/Si substrate and Ge20S80/Ag/Si substrate is investigated, using neutron reflectivity, X‐ray reflectivity, and X‐ray diffraction. Two types of “pre‐reacted” films according to the original stacking order are obtained. In both cases, a pure Ag layer almost disappeared, and there is a small amount of monoclinic Ag2S. The reaction time for the photodiffusion is shorter, in both cases, than that in Ag/Ge20S80 with a pure Ag layer, indicating a different reaction process. After prolonged light exposure, a uniform amorphous reaction layer is produced in the films originating from the Ag/Ge20S80/Si substrate, while both an amorphous reaction product and the Ag2S fragments exist in the films originating from Ge20S80/Ag/Si substrate. The mechanism of the specific photo‐reaction is discussed in terms of the role of the Ag2S fragments.

Influence of Temperature and Added Lime on the Glassy Phase Dissolution in Low-calcium Fly ash Binary Blend

The influence of temperature and added lime on the dissolution of the glassy phase in a binary blend of low-calcium siliceous fly ash and cement. An XRD-based technique is used for quantification of the glassy phase content and the amorphous reaction products in the binary blend. The experimental technique allows for tracking the formation of amorphous reaction products, the availability of lime and the unreacted glassy content. The rate limiting step in the pozzolanic reaction of low calcium fly ash is established to be the dissolution of its glassy phase. Results indicate that increasing the temperature from 25℃ to 40℃ produces a significant increase in the rate of dissolution of the glassy phase of fly ash and an increase in the rate of formation of reaction products. Addition of quicklime is very effective in producing reactive lime in the solution. The rate of dissolution of the glassy phase of fly ash and the rate of pozzolanic reaction are not significantly influenced by the lime content in the system. The proportion of fly ash which remains unreacted which is indicated by the undissolved glassy phase depends on the availability of lime in the system.

Method for Direct Determination of Glassy Phase Dissolution in Hydrating Fly Ash‐Cement System Using X‐ray Diffraction

A method for quantitative X‐ray diffraction analysis is developed for direct determination of the glassy phase content of fly ash in a hydrating binary blend of cement and low‐calcium, siliceous fly ash. The intensity contributions of the unhydrated glassy phase of fly ash and the amorphous reaction products to the intensity pattern of the total amorphous phase in the hydrating binary blend are obtained by decomposition of the total intensity signature as a sum of component pseudo Voigt (PV) peaks. An experimental program involving binary blends with three different low calcium siliceous fly ashes and two different curing temperatures is reported. The centers of the component PV peaks of the fly ash glassy phase fitted to the overall intensity pattern are found to be invariant. The glassy phase content of hydrating binary blends determined from the XRD method were found to agree well with the values obtained from a selective dissolution method.

Availability of in-situ reinforced active-transient liquid phase bond with good wettability for 70 vol.% SiCp/A356 composite using Al-Mg-Ga-Ti interlayer

For transient liquid phase bonding of as-cast 70vol.% SiCp/A356 composite, Al-24Mg-16Ga-1Ti (wt.%) active braze was designed by adding Ti as melting point increaser (MPI) for Al into Al-Mg-Ga active alloy near eutectic composition to produce in-situ reinforcement by precipitating a fine and dispersive Ti-containing intermetallic compound as reinforcement within the solid-solution bond seam after isothermal solidification. Bonding was performed at three temperature levels of quasi-melting (450°C), just fully melting (500°C), and over melting (550°C) for the braze under 0.5–1.75MPa in argon. The braze was able to produce fine (less than 1μm) and dispersive Ti-containing precipitated phase (Al-2Mg-1Ga-0.5Ti, at.%) as in-situ reinforcement only at 500°C under medium pressure (1–1.5MPa), increasing bond seam microhardness by 30Hv over the matrix. The braze robustly showed good wettability for each temperature due to the matrix dissolution and detectable amorphous reaction products containing Mg and Ga on SiC surface. Higher temperature caused coarsening of needle-like Si-containing Al-Si-Ti precipitated phase while higher pressure produced blocky Al3Ti with a shell of Mg and Ga. Sound joint produced at 500°C under 1.5MPa with 118.6MPa shear strength (98.8% parent composite) fractured completely within the composite.

Quantitative XRD study of amorphous phase in alkali activated low calcium siliceous fly ash

A technique based on direct decomposition of the XRD signature of the amorphous phase in alkali activated siliceous fly ash for determining the unreacted glassy phase content in fly ash and the amorphous reaction product content is presented. For the purpose of evaluation, a low-calcium siliceous fly ash, alkaline working solutions with different oxide ratios and two different curing temperatures were used. The intensity profile for the amorphous phase obtained from a Pawley intensity refinement of the XRD signature is expressed using pseudo-Voigt peaks for the individual intensity profiles of the glassy phase of fly ash and the amorphous reaction products. The peak characteristics of the amorphous reaction products identified with the aluminosilicate gel are found to be constant for the different curing temperatures and alkaline solutions. The percentage of reaction product and unreacted glassy content in fly ash were determined using selective dissolution combined with Rietveld refinement. The reaction product content and unreacted glassy phase content of fly ash determined from the decomposition of the XRD signature compare favourably with values obtained from selective acid dissolution. The amorphous products of reaction identified from the XRD analysis are shown to correlate well with the compressive strength. XRD is shown to be a powerful technique which can be used to study the influence of process variables on changes in the glassy phase content of fly ash and the evolution of products of reaction in alkali activated siliceous fly ash.

Types of alkali–aggregate reactions and the products formed

The alkali–aggregate reaction comprises the alkali–silica reaction (ASR) and the alkali–carbonate reaction (ACR). Reaction kinetics of the ASR depends on the grain size and crystalline structure of the reactive silicon dioxide. The reaction starts in the aggregates along the particle periphery and progresses inward. After cracking of the particle, larger amounts of reaction products are formed and are eventually extruded in the cement paste, where they fill cracks and voids. ASR products within aggregates predominantly consist of (hydrated) silicon, alkalis and calcium with characteristic atomic ratios of (Na + K)/Si ∼ 0·25 and Ca/Si ∼ 0·25. They take up additional calcium while releasing alkalis when extruded. Amorphous and crystalline reaction products occur and coexist. Expansion is the result of water adsorption by the reaction products. In ACR, harmless dedolomitisation is distinguished from deleterious reaction of fine-grained silica disseminated throughout the carbonate matrix. Further research is needed to gain more in-depth knowledge about the thermodynamics and kinetics of ASR products and the mechanisms of expansion. This should allow the establishment of a better link between concrete structures and both accelerated testing and models.

Direct decomposition X-ray diffraction method for amorphous phase quantification and glassy phase determination in binary blends of siliceous fly ash and hydrated cement

A direct decomposition method of X-ray diffraction pattern associated with amorphous phases in a binary blend containing hydrated cement and siliceous fly ash is developed. The method relies on identifying patterns associated with individual components within the overall intensity pattern produced by the total amorphous phase. The intensity pattern associated with the total amorphous phase in a binary blend of siliceous fly ash and hydrated cement is obtained using the Pawley intensity refinement method. The features of the intensity patterns associated with the amorphous reaction products in hydrated cement and the glassy content of siliceous fly ash extracted by decomposition of the overall intensity pattern produced by the combined amorphous phase are shown to match the features obtained directly from the pure phases. The direct decomposition method allows for quantification of amorphous components, which can be applied to study the evolving phases in a hydrating binary system.

Composite geopolymers of metakaolin and geothermal nanosilica waste

This paper investigated the effect of a nanometric silica from a geothermal waste on the compressive strength, microstructure, and reaction products formed on geopolymers cured at room temperature; the thermal stability was evaluated after exposure up to 800°C. The geothermal silica waste replaced 0–20% of the metakaolin and was suspended in the alkaline solution, to avoid agglomeration. Sodium silicate and sodium hydroxide were used as alkaline solutions and added to adjust the molar ratios of SiO2/Al2O3 at 2.8, 3.0 and 3.2, Na2O/SiO2=0.32 and H2O/Na2O=10. The compressive strength of the geopolymers was evaluated up to 60days of curing. The pastes were characterized by X-ray diffraction, Fourier transformed infrared spectroscopy, and scanning electron microscopy. X-ray diffraction showed the formation of amorphous reaction products, with a shifting in the amorphous halo related to the raw materials; on the other hand, a shift in the characteristic band of aluminosilicates in the infrared spectra also evidenced the geopolymerization process. While the addition of the geothermal silica waste reduced slightly the strength at room and high temperature due to the formation of porosity, its use diminished the demand of waterglass and therefore obtaining low-cost and more sustainable binders.

Preparation of magnesium phosphate cement by recycling the product of thermal transformation of asbestos containing wastes

Asbestos containing wastes have been employed for the first time in the formulation of magnesium phosphate cements. Two samples were mixed with magnesium carbonate and calcined at 1100 and 1300°C. Under these conditions, complete destruction of asbestos minerals is known to occur. The product, containing MgO, after reaction with water-soluble potassium di-hydrogen phosphate, led to the formation of hydrated phases at room temperature. Crystalline and amorphous reaction products were detected, with the latter being likely the metastable precursor of the former. Measured strengths were found to be in line with data from the literature, suggesting that this material may be used as cement. The process here described represents a viable recycling opportunity for this class of hazardous wastes. Simultaneous destruction of asbestos minerals and formation of reactive MgO during thermal treatment, bring benefits in terms of energy requirements and preservation of natural resources in cement manufacturing.

Interplay between the ionic and electronic transport and its effects on the reaction pattern during the electrochemical conversion in an FeF2 nanoparticle.

Using a charge dependent embedded atom method potential in conjunction with a dynamically adaptive multibody force field, the conversion reaction in an iron difluoride nanoparticle exposed to lithium ions is investigated. The reactions take advantage of the multiple valence states of the cations. A subtle interplay between the ionic and electronic transport, which is not accessible in conventional fixed-charge simulations, has been revealed. The simulated reaction pattern is in close agreement with that observed experimentally at the nanoscale, while providing detailed atomistic mechanisms. Due to difference in the ionic and electronic transport, different stages of reaction are observed and the corresponding phase growth mechanisms have been identified. Initially local Li concentration plays a key role in driving the reaction through amorphous reaction products to the crystalline phases that inhibit Li transport. However, electronic transport and interfacial ion diffusion are shown to be important in creating further transport pathways that allow continued conversion reactions, providing the mechanism that enables the use of these materials in advanced high capacity lithium ion batteries. Such interplay between the ionic and electronic transport will also be important in other materials and devices for energy conversion and storage.

Phosphate modified calcium aluminate cement for radioactive waste encapsulation

AbrstractThe present study is part of a wider investigation to develop an alternative cementing system for the encapsulation of problematic low and intermediate level radioactive waste. It has been suggested that alternative cementing systems, with lower internal pH than conventional Portland cement based composite cements, may reduce the corrosion of some reactive metals and may be beneficial for the long term durability of wasteforms. A potential alternative is an acid–base cementing system, based on mixing calcium aluminate cement (CAC) with acidic phosphate solutions. Although these systems have been studied previously, there has been no systematic investigation to identify phosphates for producing suitable matrixes for application in radioactive waste encapsulation. In the current study, monophosphate modified CAC formulations did not set or develop significant strength, whereas polyphosphate modified CAC formulations exhibited rapid setting and strength development. It is proposed that polyphosphate modified systems form amorphous reaction products, which act as binders between the partially and unreacted CAC particles, and were responsible for high strength development. Thermogravimetric analysis and scanning electron microscopy results suggest that this binding matrix consists of amorphous calcium phosphate and alumina gel. The results presented in this investigation suggest that polyphosphate modified CAC has potential as an alternative cementing system for radioactive waste encapsulation.