Phase Assemblage Research Articles

Synergistic effects of supplementary cementitious materials in limestone and calcined clay-replaced slag cement

• LC 2 can enhance both compressive and flexural strength of slag cement mortars. • LC 2 can optimize the phase assemblages of slag cement. • The synergistic effect could be beneficial for the property gain at all ages. The replacement of slag cement with limestone and calcined clay (LC 2 ) is beneficial to expand the sources of SCMs and reduce CO 2 emissions in the global cement industry. In this paper, the synergistic interactions between slag cement and LC 2 are studied using multi-technique characterization including TGA, XRD and SEM. The results indicate that the addition of LC 2 enhanced both compressive and flexural strength of slag cement mortars. Characterization of the microstructures show LC 2 can optimize the phase assemblages of slag cement by stabilizing ettringite whilst promoting the formation of hemicarboaluminate and monocarboaluminate. It is concluded that faster reaction of calcined clay and nucleation effect of limestone compensate for the slow early hydration of slag cement whilst slag contributes to the later performance. The synergistic hydration effects of slag and LC 2 offers unique opportunities to reduce CO 2 footprint without compromising early and long-term performances.

Regional high‐T/low‐P metamorphism of the Kemum Basement High, Bird’s Head, West Papua, Indonesia

AbstractThe northernmost part of the Australian continent is exposed in a basement high in the Bird's Head Peninsula at the NW tip of New Guinea. This basement consists of Silurian–Devonian turbidites that were regionally metamorphosed in the lower greenschist facies. Along its eastern margin, the basement high is intruded by numerous granitoid bodies and records a second, higher‐grade metamorphic overprint. We present the first detailed study of metamorphic rocks from this eastern part of the basement high, supported by field and petrographic observations, bulk rock and mineral chemistry, and phase equilibria modelling. Lower‐grade basement rocks include chlorite‐mica schists, slates, silt‐, and sandstones, which give way to andalusite‐ and cordierite‐bearing pelites and psammites, interspersed with layers of quartzite. The dominating pelitic phase assemblage is biotite+white mica±andalusite±cordierite+plagioclase+quartz+ilmenite; incipient sillimanite (fibrolite) is present in some rocks and replaces andalusite in baked margins around the numerous dykes and sills. Other rock types include metasedimentary migmatites, metacarbonates and calcsilicate concretions, rare metavolcanics, and kyanite‐bearing segregation veins. Metamorphism is of regional extent and often unrelated to granitoid intrusions. Our observations support the previous notion of an increase in peak metamorphic temperature from west to east. Microstructural relations suggest two phases of metamorphism, which are tentatively correlated with two phases of magmatic activity in the Devonian–Carboniferous and the Permian–Triassic. Based on petrographic observations and phase equilibria modelling, we propose that during the Permian–Triassic, metamorphism proceeded under high temperature/low pressure (HT/LP) or Abukuma‐type conditions along a narrow, counter‐clockwise P–T–t path with peak metamorphic conditions of the amphibolite facies (~520–650°C and ~2–4 kbar), potentially in an active continental margin setting. While much less evidence is available, similar metamorphic conditions are proposed for the Devonian–Carboniferous metamorphic event.

Domain structure and phase evolution in quenched and furnace cooled lead-free Na1/2Bi1/2TiO3–BaTiO3 ceramics

Relaxor ferroelectric Na1/2Bi1/2TiO3-based materials have gained considerable attention as a potential lead-free alternative in recent years and can be tailored to exhibit giant strain or superior high power properties. Quenching (1-x)(Na1/2Bi1/2)TiO3-xBaTiO3 (NBT-BT) ceramics in air from the sintering temperature is beneficial in enhancing the depolarization temperature and the lattice distortion. Here, a comparative study using transmission electron microscopy (TEM) and X-ray diffraction is presented for unpoled, furnace cooled and quenched NBT-BT (3, 6, 9 and 12 ​mol. % BT) ceramics describing the domain structure and phase assemblage. In contrast to the furnace cooled sample, an enhanced lamellar domain contrast is observed for the quenched morphotropic phase boundary composition with 6 ​mol. % BT. The phase fraction obtained using high resolution X-ray diffraction changes from a near pseudocubic structure with small distortions towards a more pronounced rhombohedral and tetragonal phase assemblage. On the NBT-rich side (3 ​mol. % BT), a second rhombohedral phase emerges in addition to the R3c symmetry, exhibiting a long-range lamellar domain structure. Further, quenched and subsequently poled NBT-6BT features an increased tetragonal fraction associated with a highly lamellar domain contrast. The quenching treatment stabilizes the ferroelectric order, evidenced from the development of a long-range ferroelectric domain structure, which rationalizes the enhanced depolarization temperature.

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.

Effects of sulfates on the hydration of Portland cement – A review

Presented within is a critical overview of the effect of sulfates on cement hydration, properties, and optimum sulfate content in Portland cement. Calcium sulfate is used in Portland cement to control the C3A reaction to induce the optimum hydration of C3S to occur. The amount of calcium sulfate in the Portland cement influences the hydration, rheology, setting, phase assemblage, porosity distribution, and strength in cementitious materials. If added in excess, it can also lead to durability problems, thus a better understanding is needed about the mechanisms of sulfate on C3A and C3S hydration. The optimum sulfate content is well known as a key pathway to produce workable, good strength, and durable concretes. Despite many years of research, questions regarding sulfate optimization remain. Further investigation on the influence of clinker and calcium sulfate characteristics, the use of different supplementary cementitious materials and chemical admixtures, and the curing conditions on the sulfate demand of Portland cement are needed. The main methods used to determine the optimum SO3 content are reported, and the advantages and disadvantages are examined. Finally, recommendation for future research is also discussed.

Early age hydration and application of blended magnesium potassium phosphate cements for reduced corrosion of reactive metals

Magnesium potassium phosphate cements (MKPC) were investigated to determine their efficacy towards retardation of reactive uranium metal corrosion. Optimised low-water content, fly ash (FA) and blast furnace slag (BFS) blended MKPC formulations were developed and their fluidity, hydration behaviour, strength and phase assemblage investigated. In-situ time resolved synchrotron powder X-ray diffraction was used to detail the early age (~60 h) phase assemblage development and hydration kinetics, where the inclusion of BFS was observed to delay the formation of struvite-K by ~14 h compared to FA addition (~2 h). All samples set within this period, suggesting the possible formation of a poorly crystalline binding phase prior to struvite-K crystallisation. Long-term corrosion trials using metallic uranium indicated that MKPC systems are capable of limiting uranium corrosion rates (reduced by half), when compared to a UK nuclear industry grout, which highlights their potential application radioactive waste immobilisation.

Experimental and theoretical studies of hydration of ultra-high performance concrete cured under various curing conditions

The hydration of UHPC from 1 to 180 days was experimentally and theoretically studied. Three different curing regimes were employed during the sample preparation process in order to observe their effects on the hydration and reaction products. The samples were experimentally characterized, and the experimentally obtained degrees of the reaction of the cement and amorphous silica in the UHPC samples were used for thermodynamic calculations to predict the phase assemblages of UHPC. The decomposition of AFt phases occurred in the sample that underwent a heat treatment, which also resulted in Al-substitution for Si in C-S-H gels. The degree of the reaction of the cement and amorphous silica varied greatly depending on the curing condition applied during the first few days, and higher reaction degree was observed in the heat-treated sample, which showed a significant amount of C-S-H present at an early age, making its microstructure denser than those of the other samples.

A Reappraisal of Peridotite Solidus Phase Equilibria from 6 to 14 GPa in the System CaO–MgO–Al2O3–SiO2

AbstractExperimentally determined isobaric invariant melting phase relations from 6 to 14 GPa in the system CaO–MgO–Al2O3–SiO2 (CMAS), involving the crystalline phases forsterite + orthopyroxene + clinopyroxene + garnet and liquid, are reported. Experiments were conducted using a multianvil device with stepped lanthanum chromite heaters in the pressure cells. At a fixed pressure, the five-phase assemblage identified above can exist only at a single temperature. As such, these isobaric invariant points correspond to the solidus of model garnet peridotite in this part of the composition space in the studied system, as is the case at lower pressures in some previous studies. The solidus of model peridotite is univariant in pressure–temperature space, has a positive Clapeyron slope, and the isobaric invariant solidus temperatures, at 6, 8, 10, 12, and 14 GPa, are 1965 °C, 2090 °C, 2200 °C, 2280 °C, and 2320 °C, respectively. Over the investigated pressure range, orthopyroxene is in reaction relation with the liquid, with the fusion reaction taking the form forsterite + clinopyroxene + garnet = orthopyroxene + liquid. The compositions of liquids reported here do not seem to depend on orthopyroxene being present in the experiments. Compositionally, liquids here are fairly magnesian and siliceous, and have lower alumina and lime concentrations than at lower pressures with the identical crystalline phase assemblage in the system CMAS. In contrast to some previous studies, in this study, there is no evidence of maximum and minimum normative forsterite concentration of the isobaric invariant liquid at around 8 and 12 GPa, respectively, nor of a substantial curvature in the track of liquid compositions, when such liquids coexist with the above-mentioned four-phase crystalline phase assemblage. Instead, here, with increasing pressure from 6 to 14 GPa, liquids at the isobaric invariant points (defining the univariant solidus) become progressively (quasi-linearly) enstatite-normative. This experimental observation on liquid compositions from the present study might be important for future work aimed at investigating the chemistry of liquids derived from partial fusion of anhydrous peridotite at pressures, and corresponding depths in Earth, greater than investigated here.

Effects of activator dosage and silica fume on the properties of Na2SO4-activated high-volume fly ash

The Na2SO4-activated high-volume fly ash (Ns-activated HVFA) is a promising alternative to ordinary portland cement (OPC) due to its ecological and technical benefits. In this work, the effects of alkali dosage and silica fume incorporation on the flowability, setting time, compressive strength, porosity, composition and phase assemblage, and sulfate and acid resistance of Ns-activated HVFA are studied. The results show that an intermediate alkali dose at 2.5% by binder mass largely accelerates and enhances the strength development of HVFA. The synergic effect of Ns and silica fume further improves the strength of HVFA by about 20%. Exposure to the sodium sulfate solution can potentially promote the later activation of HVFA; while being exposed to the sulfuric acid solution, HVFA exhibits better resistance and less strength loss than the OPC-counterparts. The incorporation of silica fume negatively affects the acid resistance of Ns-activated HVFA.

Hydrothermal Synthesis of Nd–Fe–B Nanoparticles with Enhanced Magnetic Properties by Nd Content Tunning and Dy Doping

This paper reports a novel, facile and low-energy consumption chemical method to synthesise Nd–Fe–B and Dy-substituted Nd–Fe–B magnetic nanoparticles with enhanced magnetic properties. NdxFe95-xB5 (15≤ x≤ 40) and Nd35-yDyyFe60B5 (0≤ y ≤8) magnetic nanoparticles were synthesised using a hydrothermal method, followed by a reduction-diffusion annealing process. We carried out a detailed experimental study of the effects of Nd content and Dy substitution on the phase assemblage and magnetic properties in the two systems. The results indicated that the coercivity of the Nd–Fe–B powders increased with increasing Nd content, and the maximum coercivity measured was 262.5 kA/m for Nd40Fe45B5 (by weight percentage). Conversely, the saturation magnetisation decreased with increasing Nd content. When the addition of Dy was less than 8 wt%, both the coercivity and the saturation magnetisation varied monotonically with the amount of Dy added, and a coercivity of 548.9 kA/m was measured for Nd27Dy8Fe60B5. The results show that hydrothermal method followed by a reduction-diffusion process is simple and economical in preparing Dy-substituted Nd2Fe14B magnetic nanoparticles, and the controllable and enhanced magnetic properties can be realised by simply adjusting the ratio of raw materials. This is also a good complement to the preparation of Nd–Fe–B nanoparticles by chemical methods.

Low-Temperature Nitridation of Fe3O4 by Reaction with NaNH2

Low-temperature soft chemical synthesis routes to transition-metal nitrides are of interest as an alternative to conventional high-temperature ammonolysis reactions involving large volumes of chemotoxic NH3 gas. One such method is the reaction between metal oxides and NaNH2 at ca. 200 °C to yield the counterpart nitrides; however, there remains uncertainty regarding the reaction mechanism and product phase assemblage (in particular, noncrystalline components). Here, we extend the chemical tool box and mechanistic understanding of such reactions, demonstrating the nitridation of Fe3O4 by reaction with NaNH2 at 170–190 °C, via a pseudomorphic reaction. The more reduced Fe3O4 precursor enabled nitride formation at lower temperatures than the previously reported equivalent reaction with Fe2O3. The product phase assemblage, characterized by X-ray diffraction, thermogravimetric analysis, and 57Fe Mössbauer spectroscopy, comprised 49–59 mol % ε-Fe2+xN, accompanied by 29–39 mol % FeO1–xNx and 8–14 mol % γ″-FeN. The oxynitride phase was apparently noncrystalline in the recovered product but could be crystallized by heating at 180 °C. Although synthesis of transition-metal nitrides is achieved by reaction of the counterpart oxide with NaNH2, it is evident from this investigation that the product phase assemblage may be complex, which could prove a limitation if the objective is to produce a single-phase product with well-defined electrical, magnetic, or other physical properties for applications. However, the significant yield of the FeO1–xNx oxynitride phase identified in this study opens the possibility for the synthesis of metastable oxynitride phases in high yield, by reaction of a metal oxide substrate with NaNH2, with either careful control of H2O concentration in the system or postsynthetic hydrolysis and crystallization.

Preparation, phase assemblage and microwave dielectric properties of AlPO4-BPO4-SiO2 ternaries

Microwave dielectric ceramics with low dielectric permittivities (Ɛr<6), high quality values and temperature sable resonator frequencies are strongly desired with the development of millimeter wave communication. In this paper, a compositional design for low-k materials was proposed from the point view of crystal chemistry. AlPO4-BPO4-SiO2 glass-ceramics were prepared by traditional solid state and sol-gel processes, respectively. The sintering behaviors, phase assemblages, microstructures and microwave dielectric properties of AlPO4-BPO4-SiO2 ternaries have been studied. The solid solubility, glass forming ability and crystallization of the AlPO4–BPO4–SiO2 ternaries can be understood by considering the structural flexibility via the degree of ionicity i of the bonds in the ternaries. All compositions demonstrate low dielectric permittivity less than five and negative temperature coefficient of resonant frequency. Maximum Q × f value could be obtained for the 0.45AlPO4–0.45BPO4–0.10SiO2 composition prepared by sol-gel process after sintering at 1175 °C/2 h: Ɛr∼4.16, Q × f∼59,519.

Sulfate resistance testing of shotcrete – Sample preparation in the field and under laboratory conditions

When shotcrete is in contact with sulfate containing ground water, there is a risk of a drastically reduced service life. In this case, the use of sulfate-resisting cement is recommended. Since physical characteristics of the cement matrix of shotcrete differ from conventional concrete, sulfate-resisting cements do not always guarantee sufficient shotcrete durability. Thus, testing of the sulfate resistance in accelerated tests is necessary. Providing test samples from on-site (real) shotcrete is time consuming and demanding. In this study, the sulfate resistance and physical properties such as compressive strength and transport properties of samples produced in the laboratory without spraying are compared to the properties of field samples. A new compaction method is proposed, which is apt to allow high accelerator dosages. For different alkali-free set-accelerators and cement types, the laboratory-produced samples not only reveal similar sulfate resistance test results as on-site shotcrete, but also exhibit comparable porosity, transport properties and chemical phase assemblage.

Modelling the effect of the cement components fineness on performance and environmental impact of composite cements

A modeling tool for optimizing composite cements composition and production is presented. The tool is based on three models allowing the calculation of 1) the kinetics of the hydration, 2) evolution of the phase assemblage including apparent porosity, 3) the evolution of the compressive strength and the environmental impact of cement production as function of cement composition and fineness of its constituents. Results show that cements with performance exceeding that of mostly sold cements in Europe but with clinker factor of 60% or less and contain 20% of limestone are feasible. Finer grinding of clinker and slag reduces the overall CO2 footprint as the clinker replaced offsets the extra emissions associated in grinding.

Revisiting the iron-rich “ordinary Portland cement” towards valorisation of wastes: study of Fe-to-Al ratio on the clinker production and the hydration reaction

Ordinary Portland cement (OPC) is a strong and durable material with a well-established composition defined by national and regional standards. The required chemistry limits the amount of iron-rich materials that can be used as a feedstock. The work described in this paper investigates how substitution of iron for aluminium affects OPC clinker production. Herein, it was studied how this substitution affected clinker phase assemblage during the firing process, the paste hydration kinetics and products as well as the mechanical properties of the mortars. Results suggested that Fe to Al replacement did not affect substantially the melt formation at usual production temperatures. The formation of alite and ferrite were promoted, diminishing the formation of aluminate and belite. Regarding the hydration process it was obtained portlandite, ettringite, AFm and hydrogarnet as well as amorphous phase. Additionally, the Fe-rich cements presented longer setting times and slightly lower mechanical strength. These results are a first approach in the study of OPC clinkers with Fe-rich content and additional research provided in the future can enable the use of larger amounts of Fe-rich materials and residues in the OPC raw meal.

Atomic structure and phase assemblages in novel M-(N)-A-S-H materials

This paper investigates the atomic structure and phase assemblages in new sodium-stabilized magnesium aluminosilicate hydrate (M-(N)-A-S-H) cementitious binders. Results indicate that in the absence of Ca2+, Mg2+ promotes a binder atomic structure of Si-Al tetrahedral sheets and octahedral Mg sheets with hydrated Na+ cations likely in the interlayer sites similar to trioctahedral micas (phyllosilicates). NMR studies verify the incorporation of Al in tetrahedral silicate sheets. XRD demonstrates the ability of these regions to nucleate and form zeolites (i.e., sodalite) as well as the formation of Mg-Al layered double hydroxide (LDH) phases (i.e., meixnerite), which is expected due to high concentrations of Mg and Al. TGA results indicate that M-(N)-A-S-H possesses chemically bound water and hydroxyl units similar to other Mg binders. These results evince the critical role of Mg to form unique atomic structures and durability-linked phases in low-calcium alkali-activated materials.

Effect of supplementary cementitious materials on carbonation of cement pastes

Supplementary cementitious materials (SCM) are increasingly used in concrete for economical and environmental reasons. However, the durability of reinforced concretes against, for example, corrosion induced by carbonation varies. Here, the phase assemblage of various cement pastes with/without SCM (slag, fly ash and metakaolin), carbonated in accelerated conditions (1.5% CO2 and 65% RH) or not, has been investigated by various technics (XRD, TGA/DTA and 29Si as well as 27Al nuclear magnetic resonance spectroscopy) and compared.Results show that, after carbonation, anhydrous phases are less decalcified than hydrated phases. In cement pastes with slag, most of the calcium remains in the non-hydrated part of the slag. In contrast, the C-A,S-H phase is deeply modified and results show a coupling between C-A,S-H and hydrated aluminate phases during carbonation. In all carbonated materials, these phases tend to become an aluminosilicate gel, a very amorphous/disordered phase, containing less water than the original hydrates.

Hydration implanted reactive transport modelling in saturated cement-based materials

The hydration processes of binders determine two principal factors that controls the ions transport during external corrosion: porosity and chemical composition (solids and pore solutions) of the cementitious system. In this study, hydration modelling is introduced to reactive transporting model, from which a self-sufficient framework for reactive transport in saturated cement-based materials from hydration to corrosion is successfully developed. The proposed model gives information on transformation of both phase assemblage and pore solutions compositions. The numerical implementation implies that the developed framework can be applied to successfully predict reactive transport in saturated cement-based materials. Furthermore, mechanism and controlling factors of sulfate attack are clarified based on simulation on sulfate ingress.

Influence of exposure conditions on expansion characteristics of lime-rich calcium sulfoaluminate-belite blended cement

Expansive characteristics of calcium sulfoaluminate-belite (CSAB) cement can be harnessed for making shrinkage-compensating concrete. The level of expansion is dependent on several factors, such as the amounts of ye'elimite and calcium sulfate available in CSAB cement. Moreover, calcium sulfate from an external source can also influence the expansion characteristics by increasing the supersaturation with respect to ettringite. In this study, the influence of externally available calcium sulfate was investigated by curing a blend of Portland cement (PC) and CSAB cement to different concentrations (1 g/l and 5 g/l) of gypsum solutions. Expansion of the PC-CSAB blended cement was found to be directly affected by the ye'elimite content (CSAB dosage) and the gypsum concentration in exposure solution. The presence of external calcium sulfate during the hydration of ye'elimite was found to affect the hydrate phase assemblage, the pore structure, and the morphology of ettringite. Furthermore, the level of expansion depended on the age at which the specimens were exposed to the gypsum solution, indicating the influence of matrix stiffness on resultant expansion.

Cementless controlled low-strength material (CLSM) based on waste glass powder and hydrated lime: Synthesis, characterization and thermodynamic simulation

Sustainable infrastructure materials are always correlated with the innovative and value-added disposal of solid waste. This study investigates the production of cement-free controlled low strength material (CLSM) depending on the pozzolanic reaction between waste glass powder (GP) and hydrated lime (CH). CLSM mixtures with varying GP/CH ratios were characterized by flow consistency, mechanical properties and durability. The microstructure and phase assemblages of GP-CH binders were analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD) and thermodynamic simulation. The results showed that a high GP/CH ratio could enhance the workability and self-compacting behavior of the mixtures. Different GP/CH ratios changed the chemical composition of starting system and hence the phase assemblages of the reaction products. It was found that a higher amount of precipitated calcium-silicate-hydrate (C-S-H) gels significantly improved the mechanical properties and duability of CLSM. The GP/CH ratio of 8:2 contributed to the most C-S-H formations and best material properties. On the other hand, the portlandite could impair the water stability and frost durability due to its relatively high solubility in water. Both experimental and modelling results indicated that Ca-rich jennite-type C-S-H tended to react with SiO2 to form the low-Ca tobermorite-type C-S-H at high Si/Ca conditions, which resulted in the structure evolution of C-S-H gels as GP/CH increased. Finally, the overall phase assemblages of GP-CH binders based on Gibbs free energy minimization (GEM) were proposed, considering different reaction extents of GP. This study paves the path for the large-scale utilization of waste glass in both cement and construction industry.