Phase Assemblage Evolution Research Articles

Belite–ye’elimite–ferrite cements produced from London Clay

London Clay (LC) is generated in significant quantities from major infrastructure development projects within and around London, UK. Excavated LC spoils are typically landfilled or used for landscaping purposes with minimal value. This study investigates the feasibility of producing belite–ye’elimite–ferrite (BYF) cements using LC in combination with kaolin or low-grade bauxite. Six cement clinkers with varying compositions were synthesised at 1300°C. The phase composition of the synthesised clinkers was quantified using X-ray diffraction analysis (XRD) coupled with Rietveld refinement. The hydration behaviour, phase assemblage evolution and mechanical performance of the cements were examined using isothermal calorimetry, XRD, thermogravimetric analysis and compressive strength testing, respectively, up to 90 days of curing. It is demonstrated that LC can be used either on its own or with kaolin or low-grade bauxite to produce BYF cements with a wide range of composition: 31 to 64 wt% belite, 9.3 to 27 wt% ye’elimite and 2.1 to 31 wt% ferrite. The use of LC as the sole alumina source resulted in low compressive strength at early stages. Nevertheless, the strength developed over time, reaching 27.8 MPa at 90 days (0.5 w/b). Possible approaches to develop LC further for producing viable BYF cements are discussed.

Sulfate resistance of carbonated basic oxygen furnace slag-metakaolin-Portland cement blends

Basic oxygen furnace slag (SS) is prone to carbonation due to its alkaline properties. The combined use of carbonated basic oxygen furnace slag (CSS) with metakaolin (MK) as a composite supplementary cementitious material (CSS-MK) can improve the reactivity of CSS and significantly lower the overall embodied CO2 emission. However, the impact of CSS-MK on the resistance of external sulfate attack (ESA) of cement remains unclear. This study aims to clarify the influence of CSS-MK on the resistance of cement to ESA. Compressive strength, water absorption, and electrical flux before ESA, and macro-scale expansion, microstructure, and evolution of phase assemblage during ESA were investigated. Results indicate that CSS-MK exhibits higher strength reactivity compared to SS-MK, enabling a 30% cement replacement level without compromising compressive strength after 7 days. CSS-MK reduces water absorption by 37.9% and electrical flux by 67.6%, enhancing transport property and reducing sulfate ion intrusion into the cement paste. The carbonation treatment transforms calcium silicate in SS into CaCO3, decreasing soluble calcium content, and creating a CaCO3-based passivation layer that inhibits sulfate ion corrosion. Meanwhile, the reaction in CSS-MK blends between the CaCO3 in CSS and MK consumes Ca(OH)2, forms ettringite and thermodynamically more stable carboaluminate, and results in the absence of monosulfate prior to ESA and the presence of carboaluminate during ESA, which consequently decreases ettringite content by 40% in paste after 240 days of ESA. Furthermore, at the macroscopic level, the expansion decreased by 85.2% after 360 days of ESA, effectively mitigating specimen cracking and damage.

On the chemo-mechanical evolution process of high-volume slag cement paste

This study investigated the evolution process of high-volume slag cement (HVSC) paste from a chemo-mechanical standpoint. HVSC specimens with a 70 w.t. % slag replacement rate were studied at various ages. Evolution of phase assemblage, microstructure development, and micromechanical properties were analyzed using TGA/XRD/MIP/SEM-EDS and nano-/micro-indentation techniques. A two-scale micromechanical model was built to predict the effective elastic modulus based on the nanoindentation results. Key findings include: 1) Between 7 and 28 days, the formation of calcium silicate hydrate (C-S-H) gel phase improves the effective elastic modulus by filling capillary pores; 2) From 28 to 90 days, the phase assemblage and microstructure remain stable, with a transition from low-density to high-density C-S-H; 3) Between 90 days and 2 years, slag rims produced by slag grains result in increased elastic modulus; 4) The two-scale micromechanical model, combined with nanoindentation data, accurately predicts the effective modulus of HVSC composites, although the unhydrated slag grains-hydrated cement matrix interface may cause an overestimation at an early age. With longer curing time, this interface disappears owing to the continuous hydration of large slag particles and therefore a good match is found between the modelling and experimental results.

The influence of reactive MgO on the hydration and carbonation performance of slag-rich cement system

To enrich the choice of minor addition for cement production, the paper investigated the influence of reactive MgO on the long-term hydration and carbonation behavior of slag-rich cement paste. Three different mixtures (cement-slag-MgO, C3S-slag-MgO, and cement-slag) were adopted in the study, and specimens were sealed cured for 90 days, followed by a six-month accelerated carbonation test. It was found that the addition of MgO did not modify the phase assemblage evolution and microstructure development of slag-rich cement paste fundamentally during hydration and carbonation. Brucite occurred as the main hydration product of MgO. As for its carbonation product, Mg-containing carbonate phases and magnesium silicate hydrates were found to agglomerate with each other in the matrix. Compared with cement-slag mixture, cement-slag-MgO blend exhibited a slightly higher carbonation depth and a lower CO2 binding capacity. Brucite did bind some CO2; however, it cannot compensate the reduction of C–S(A)−H gel phase and portlandite due to the substitution of cement clinker with MgO. Therefore, the addition of MgO reduced the carbonation resistance of the corresponding system from a long-term perspective.

Hydration of Portland cement with seawater toward concrete sustainability: Phase evolution and thermodynamic modelling

To mitigate the shortage of freshwater resource in the island and coastal regions, using seawater (SW) for concrete mix can provide significant economic and environmental benefits. To achieve a safe and reliable application, in-depth investigation is needed on hydration of Portland cement in SW. The composition of solid and liquid phases in hydrated Portland cement was quantitively determined and analysed in this study. The use of SW not only significantly increases the hydration rate of clinker but also affects the evolution of phase assemblage. Both the thermodynamic calculations and experimental determinations indicates the formation of Friedel's salt (FS) instead of sulfo-AFm in hydrated cement by SW, implying sulfate ions cannot compete with chloride ions to combine with AFm phases. The characteristic reaction in SW leads to higher sulfate concentration, thus indirectly hindering ettringite (AFt) conversion at the late stage. Through the experimental quantification of thermogravimetric analysis and X-ray diffraction analysis, the kinetic model of clinker dissolution was modified to be more suitable for the hydration of Portland cement in SW. The calculation from coupled models exhibits a novel method to evaluate the evolution of phases in cement hydration. Through model calculations, 3.70% higher solid volume and 12.2% lower liquid volume were obtained in the cement-SW paste at the end of the hydration, which may cause the mechanical properties to be more sensitive under environmental humidity and the temperature.

Influence of sodium chloride on the hydration of C3S blended paste

Understanding the chloride influence mechanism on the hydration kinetic of cementitious materials is meaningful for the application of chloride rich raw materials. This paper explored the main hydration peak and AFm peak evolution for various chloride content in C3S systems blended with metakaolin and limestone. Calorimetry test was applied to reveal the hydration process under chloride condition. Phase assemblage evolution was analyzed through X-Ray powder diffraction (XRD)/Rietveld analysis. It was observed that the hydration rate was accelerated and enhanced for both silicate and aluminate peaks when chloride was present. Silicate peak change comes from the ions effect of the pore solution owing to chloride. AFm peak heat release presents linear relation with chloride content. In addition, MK and LS shows little effect at the beginning and then some chemical reaction effect after the main peak. Therefore, pure Friedel’s salt is formed at the beginning of hydration in all systems, while solid solution of Friedel’s salt would be produced after the silicate peak in systems with limestone, which is controlled by the carbonate in the system.

Phase and structural evolution of zirconolite ceramics prepared by solid-state reaction sintering

The evolution of phase assemblage and microstructure of stoichiometric zirconolite (CaZrTi 2 O 7 ) ceramics, prepared by a solid-state reaction sintering route, was systematically investigated as a function of sintering temperature. Using powder XRD and quantitative phase analysis data, it was determined that the formation of zirconolite was a one-step reaction, without formation of intermediate phases. The accompanying fractions of secondary CaTiO 3 and ZrO 2 phases were reduced to approximately 2 wt % each after sintering at 1200 °C, with zirconolite formed as the major phase (> 99 wt%) after reaction at 1300 °C. Notable product densification only occurred at T ≥ 1400 °C, at which it was possible to achieve a relative density of 96.97% which is highly desirable for applications as a nuclear wasteform. The zirconolite-2M polytype structure (space group: C2/c) was formed in all products as expected, confirmed by combined high resolution TEM-ED analyses.

Influence of limestone addition on sodium sulphate activated blast furnace slag cements

The effect of limestone replacement in the reaction and phase assemblage evolution of two sodium sulfate activated slag cements was investigated. The slag composition and its reactivity influenced the reaction kinetics of these materials. Paste with limestone addition exhibited an acceleration in reaction kinetics, particularly for slowly reacting slags. Ettringite and an aluminium substituted calcium silicate hydrate were identified as main reaction products in these cements, independently of the slag type or limestone replacement level. No significant changes in phase assemblages were observed with limestone addition for over 365 days of testing; however, these composite cements exhibited an increased compressive strength, consistent with a refined pore structure. These results indicate that it is possible to partially replace slag by limestone in sulfate activated slag cements without changing the type of reaction products forming in these systems, while also increasing compressive strength and achieving a similar refined pore structure.

Effect of Boron and Water-to-Cement Ratio on the Performances of Laboratory Prepared Belite-Ye'elimite-Ferrite (BYF) Cements.

The effect of superplasticiser, borax and the water-to-cement ratio on BYF hydration and mechanical strengths has been studied. Two laboratory-scale BYF cements—st-BYF (with β-C2S and orthorhombic C4A3 ) and borax-activated B-BYF (with α’H-C2S and pseudo-cubic C4A3 )—have been used, and both show similar particle size distribution. The addition of superplasticiser and externally added borax to BYF pastes has been optimised through rheological measurements. Optimised superplasticiser contents (0.3, 0.4 and 0.1 wt % for st-BYF, B-BYF and st-BYF with externally added 0.25 wt % B2O3, respectively) result in low viscosities yielding homogeneous mortars. The calorimetric study revealed that st-BYF is more reactive than B-BYF, as the values of heat released are 300–370 J/g and 190–210 J/g, respectively, after 7 days of hydration; this fact is independent of the water-to-cement ratio. These findings agree with the higher degree of hydration at 28 days of β-C2S in st-BYF (from 45 to 60%) than α’H-C2S in B-BYF (~20 to 30%). The phase assemblage evolution has been determined by LXRPD coupled with the Rietveld method and MAS-NMR. The formation of stratlingite is favoured by increasing the w/c ratio in both systems. Finally, the optimisation of fresh BYF pastes jointly with the reduction of water-to-cement ratio to 0.40 have allowed the achieving of mortars with compressive strengths over 40 MPa at 7 days in all systems. Moreover, the st-BYF mortar, where borax was externally added, achieved more than 70 MPa after 28 days. The main conclusion of this work does not support Lafarge’s approach of adding boron/borax to the raw meal of BYF cements. This procedure stabilises the alpha belite polymorph, but its reactivity, in these systems, is lower and the associated mechanical strengths poorer.

Properties of calcium aluminate blended cement incorporating iron-rich slag: Evolution over a curing period of 1 year

With an outlook directed towards circular economy, the valorization of municipal waste streams and industrial residues has recently conveyed a variety of novel industrial by-products to various applications such as supplementary cementitious materials (SCM) in cement blends or as precursors to inorganic polymers. Despite the synergistic benefits on recycling, reducing the cost and the carbon footprint of cements, the use of such non-conventional SCM is often limited to ordinary Portland cement (OPC)-based formulations. This study investigates the properties of a calcium aluminate cement (CAC)-based blend incorporating 30 wt% of iron (Fe)-rich slag produced at pilot scale from an industrial lead–zinc production. Compressive and flexural strength (EN 196-1 standard mortars), setting time, dimensional stability (Walter + Bai shrinkage measuring test), early hydration reactions (isothermal calorimetry), and phase assemblage evolution (XRD and TGA) were followed from 1 day to 1 year of curing period for both the slag-containing- and a corresponding reference cement formulation. Additionally, a fast non-destructive technique for quantifying the degree of slag hydration is demonstrated using a combination of X-ray computed tomography (XCT) and volume analysis based on grayscale threshold segmentation. The results from the different techniques suggest the long term (>28 days) reactivity of the slag with the slag-containing mortars attaining a comparable compressive strength of 39 MPa versus 41 MPa of the corresponding reference mortars at 90 days. Likewise, the prominent contribution of the slag to the hydration reactions is manifested by the differences in the evolution of the phase assemblage of the two formulations. The degree of slag hydration was calculated to progressively increase over time reaching up to 49% after 1 year of hydration based on the XCT scans. These results along with the long term dimensional stability of the slag-containing formulation support the potential for valorization of the Fe-rich slag as SCM in the CAC-based blend.

Phase assemblage of a 5 year-old cement paste after submission to various high temperature and cooling regime

In case of fire in a structure, concrete undergoes internal transformations that can alter its durability. It is also important to take into account the effect of the cooling regime after temperature exposure on the phase assemblage of the cementitious matrix. An experimental investigation was carried out to investigate the changes of a cement paste, heated up to various temperature up to 1000 °C then cooled in water or cooled in air for comparison. Quantitative evolution of phase assemblage is obtained by various techniques (TGA, XRD, NMR and ICP-AES).For both cooling regimes, the hydrated crystallised phases tend to become disordered phases for lower exposition temperature. These phases coexist with anhydrous products for intermediate temperature and these new anhydrous phases become the majority for higher temperature. However, the two cooling regimes show differences. Equilibrium of hydrated aluminate phases is modified, (e.g. AFm/AFt ratio). There are structural modifications of the phases in particular for C-S-H. For water cooling regime, beyond 600 °C, a leaching is observed in particular for calcium.

A systematic investigation of the phase assemblage and microstructure of the zirconolite CaZr1-xCexTi2O7 system

A series of zirconolite ceramics with nominal stoichiometry CaZr1-xCexTi2O7 (0.0 ≤ x ≤ 0.4) were synthesised under oxidizing, inert and reducing conditions, by a conventional solid-state cold press and sinter method. The evolution of phase assemblage and microstructure were characterized by XRD, SEM-EDS, Ce-L3 XANES and Raman spectroscopy techniques. It was determined that the relative yield of zirconolite-2M and zirconolite-4M polytypes, alongside secondary perovskite phases, was influenced by processing environment via the control of Ce3+/4+ speciation. Oxidizing conditions produced a mixture of zirconolite-2M and zirconolite-4M polytypes due to the formation of Ce4+. Inert and reducing conditions yielded predominantly zirconolite-2M and a Ce-bearing perovskite phase through favouring Ce3+ speciation.

On the prediction of low-cost high entropy alloys using new thermodynamic multi-objective criteria

In an attempt to identify new low-cost metallic materials with interesting thermo-physical properties from the Fe-Cr-Mn-Ni-V-Ti-Al- (Co-Mo) system, we present here an original tool for the design of first-generation High Entropy Alloys (HEAs). The composition of potential HEAs is calculated under a set of non-smooth and non-linear constraints and multi-objective functions linked to the single-phase start temperature, the room-temperature driving force for phase assemblage evolution and the solidification range. These are all new thermodynamic criteria for the design of HEAs. This tool links a constrained Gibbs energy minimization algorithm that uses accurate thermodynamic databases to an optimization algorithm implemented for solving “blackbox” objective functions and constraints. As a result of this work, we have identified entire sets of new FCC and BCC first-generation HEAs potentially suitable for future industrial applications. The proposed methodology is also successfully applied to identify two-phase heavily alloyed materials.

Fly ash-slag interaction during alkaline activation: Influence of activators on phase assemblage and microstructure formation

In this work, the influence of activator on the interaction mechanisms between slag and low-calcium fly ash (FA-F) during alkaline activation was investigated. In particular, the evolution of phase assemblage and microstructure of alkali-activated binary slag and FA-F paste was monitored by means of X-ray diffraction with Rietveld analysis, scanning electron microscopy, and energy-dispersive spectroscopy (SEM/EDS). The results show that the nature of amorphous gels and type of crystalline phases formed are dependent on sample age, initial slag to FA-F ratio, and activator type and dosage. The interaction between FA-F and slag during alkaline activation or geopolymerization is primary demonstrated as: (1) modified nanostructure (e.g. crystallology and lattice parameters) and chemical composition of precipitates, (2) depressing or triggering the formation of some crystalline phases, and (3) spatial heterogeneity of composition in microstructure. The activator is chemically incorporated into interactions between slag and FA-F, and likely affects the dissolution and reaction kinetics.

Mineral assemblage transformation of a metakaolin-based waste form after geopolymer encapsulation

Mitigation of hazardous and radioactive waste can be improved through conversion of existing waste to a more chemically stable and physically robust waste form. One option for waste conversion is the fluidized bed steam reforming (FBSR) process. The resulting FBSR granular material was encapsulated in a geopolymer matrix referred to here as Geo-7. This provides mechanical strength for ease in transport and disposal. However, it is necessary to understand the phase assemblage evolution as a result of geopolymer encapsulation. In this study, we examine the mineral assemblages formed during the synthesis of the multiphase ceramic waste form. The FBSR granular samples were created from waste simulant that was chemically adjusted to resemble Hanford tank waste. Another set of samples was created using Savannah River Site Tank 50 waste simulant in order to mimic a blend of waste collected from 68 Hanford tank. Waste form performance tests were conducted using the product consistency test (PCT), the Toxicity Characteristic Leaching Procedure (TCLP), and the single-pass flow-through (SPFT) test. X-ray diffraction analyses revealed the structure of a previously unreported NAS phase and indicate that monolith creation may lead to a reduction in crystallinity as compared to the primary FBSR granular product.

Evolution of phase assemblage of blended magnesium potassium phosphate cement binders at 200° and 1000°C

The fire performance of magnesium potassium phosphate cement (MKPC) binders blended with fly ash (FA) and ground granulated blast furnace slag (GBFS) was investigated up to 1000°C using X-ray diffraction, thermogravimetric analysis and SEM techniques. The FA/MKPC and GBFS/MKPC binders dehydrate above 200°C to form amorphous KMgPO4, concurrent with volumetric and mass changes. Above 1000°C, additional crystalline phases were formed and microstructural changes occurred, although no cracking or spalling of the samples was observed. These results indicate that FA/MKPC and GBFS/MKPC binders are expected to have satisfactory fire performance under the fire scenario conditions relevant to the operation of a UK or other geological disposal facility

Long length scales of element transport during reaction texture development in orthoamphibole-cordierite gneiss: Thor-Odin dome, British Columbia, Canada

First-order factors controlling the textural and chemical evolution of metamorphic rocks are bulk composition and pressure–temperature–time (P–T–t) path. Although it is common to assume that major element bulk composition does not change during regional metamorphism, rocks with reaction textures such as corona structures record evidence for major changes in effective bulk composition (EBC) and therefore provide significant insight into the scale, pathways, and mechanisms of element transport during metamorphism. Quantifying changes in EBC is essential for petrologic applications such as calculation of phase diagrams (pseudosections). The progressive growth of complex corona structures on garnet and Al2SiO5 porphyroblasts in orthoamphibole-cordierite gneiss Thor-Odin dome (British Columbia, Canada) reduced the EBC volume of the rock during metamorphism and therefore had a dramatic effect on the evolution of the stable mineral assemblage. These rocks contain a chemical and textural record of metamorphic reactions and preserve 3D networks (reaction pathways) connecting corona structures. These coronal networks record long (>cm) length scales of localized element transport during metamorphism. P–T, T–X, and P–X pseudosections are used to investigate the control of effective bulk composition on phase assemblage evolution. Despite textural complexity and evidence for disequilibrium, mineral assemblages and compositions were successfully modeled and peak metamorphic conditions estimated at 750°C and 9 kbar. These results illustrate how textural and chemical changes during metamorphism can be evaluated using an integrated petrographic and pseudosection approach, highlight the importance of effective bulk composition choice for application of phase equilibria methods in metamorphic rocks, and show how corona structures can be used to understand the scale of compositional change and element transport during metamorphism.

Aluminum-rich belite sulfoaluminate cements: Clinkering and early age hydration

Belite sulfoaluminate (BSA) cements have been proposed as environmentally friendly building materials, as their production may release up to 35% less CO{sub 2} into the atmosphere when compared to ordinary Portland cements. Here, we discuss the laboratory production of three aluminum-rich BSA clinkers with nominal mineralogical compositions in the range C{sub 2}S (50-60%), C{sub 4}A{sub 3}$$ (20-30%), CA (10%) and C{sub 12}A{sub 7} (10%). Using thermogravimetry, differential thermal analysis, high temperature microscopy, and X-ray powder diffraction with Rietveld quantitative phase analysis, we found that burning for 15 min at 1350 deg. C was the optimal procedure, in these experimental conditions, for obtaining the highest amount of C{sub 4}A{sub 3}$$, i.e. a value as close as possible to the nominal composition. Under these experimental conditions, three different BSA clinkers, nominally with 20, 30 and 30 wt.% of C{sub 4}A{sub 3}$$, had 19.6, 27.1 and 27.7 wt.%, C{sub 4}A{sub 3}$$ respectively, as determined by Rietveld analysis. We also studied the complex hydration process of BSA cements prepared by mixing BSA clinkers and gypsum. We present a methodology to establish the phase assemblage evolution of BSA cement pastes with time, including amorphous phases and free water. The methodology is based on Rietveld quantitative phase analysis of synchrotron and laboratory X-ray powder diffraction data coupled with chemical constraints. A parallel calorimetric study is also reported. It is shown that the beta-C{sub 2}S phase is more reactive in aluminum-rich BSA cements than in standard belite cements. On the other hand, C{sub 4}A{sub 3}$ reacts faster than the belite phases. The gypsum ratio in the cement is also shown to be an important factor in the phase evolution.

ChemInform Abstract: Progress in SiAlON Ceramics

Abstract In view of the considerable progress that has been made over the last several years on the fundamental understanding of phase relationships, microstructural design, and tailoring of properties for specific applications of rare-earth doped SiAlONs, a clear review of current understanding of the basic regularities lying behind the processes that take place during sintering of SiAlONs is timely. Alternative secondary phase development, mechanism and full reversibility of the α′ to β′ transformation in relation with the phase assemblage evolution are elucidated. Reaction sintering of multicomponent SiAlONs is considered with regard of wetting behavior of silicate liquid phases formed on heating. Regularities of SiAlON's behavior and stability are tentatively explained in terms of RE element ionic radii and acid/base reaction principle.

Review article: RE-SiAlONa¤b- composites. Formation, thermal stability, phase relationships, reaction densification

In view of the considerable progress that has been made over the last several years on the fundamental understanding of phase relationships, microstructural design, and tailoring of properties for specific applications of rare-earth doped SiAlONs, a clear review of current understanding of the basic regularities lying behind the processes that take place during sintering of SiAlONs is timely. Alternative secondary phase development, mechanism and full reversibility of thea«btransformation in relation with the phase assemblage evolution are elucidated. Reaction sintering of multicomponent SiAlONs is considered with regard of wetting behavior of silicate liquid phases formed on heating. Regularities of SiAlON’s behavior and stability are tentatively explained in terms of RE-element ionic radii and acid/base reaction principle.