Aluminate Hydrate Research Articles

Degradation mechanism of cement-based materials under the effects of stray current, chloride and sulfate

When infrastructure is exposed to a complicated environment containing stray current, chloride and sulfate, it may be subjected to severe degradation. In order to analyze the interaction of different factors and the degradation mechanism of cement-based materials in a complicated environment, the blended solution with four molar concentration ratios of Cl-/SO42- (0, 0.5, 1, and 2) were prepared to simulate a range of aggressive medium. Meanwhile, 40 V DC (direct current) electrical field was applied to cement-based materials to simulate stray current. After exposure, the transport properties, macro-properties, and microstructures of specimens were observed and measured. Additionally, the types and amounts of degradation products were tested to further analyze the degradation mechanism of cement-based materials. Experimental results show that chloride in the blended solution could inhibit SO42- transport into cement-based materials, causing the SO42- concentration in a specimen to decrease with the increase of Cl-/SO42- ratio. Therefore, the specimens appeared less degraded in terms of macro-properties. But the results of microstructural analysis showed that degradation products and cracks were generated in the interior of specimens. Friedel’s salt was detected in the specimens exposed to the solutions containing chloride. The formation of Friedel’s salt consumed calcium aluminate hydrate, which caused the amount of Al-phase involved in the formation of ettringite to reduce. Consequently, the amount of ettringite was reduced, and the degradation of compressive strength and appearance was also reduced. Additionally, an electrical field could induce the ion concentration in pore solution to change and even break the ion chemical equilibrium, which resulted in the decomposition of ettringite and Friedel’s salt. However, Friedel’s salt can also transform into ettringite at a specific condition. Therefore, the amounts of ettringite and Friedel’s salt in specimens were changed over the exposure time.

Wide-angle broadband antireflection coatings with nano-taper hydrated alumina film.

Wide-angle and broadband antireflection (AR) coating is of the essence in modern optical systems in many fields, which has a great influence on the stray light and imaging quality. A simple and convenient manufacturing method is proposed to address this issue based on a composite coating combining the nano-tapered Al2O3•xH2O (AH) structure and high-low index thin film stack. The optical properties of nano-tapered AH structure at various thickness are first studied and modeled in optics by several homogeneous sub-layers with the graded equivalent index. The designed composite AR coatings are manufactured by vacuum deposition and wet etching subsequently in the hot deionized water. Compared to the common dielectric multilayer antireflection stack, the composite coating presents excellent AR performance. The measured average reflectance values of the double-side coated BK7 glass are as low as 0.40%, 0.41%, 0.56% and 3.13% in 400-1100nm band at angles of incidence (AOI) of 6°, 20°, 40°, and 60°, respectively, while the measured average transmittance at normal incidence increases up to 99.3%. Finally, the process reproductivity, environmental reliability test including long term storage, high temperature annealing and 85°C-85% relative humidity storage of the composite coatings are evaluated. The proposed AR scheme provides a low-cost, efficient, wide-angle and broadband AR coating for kinds of large-curvature components and complex surfaces in fields of consumer electronics, automotive, security, etc.

Hydration of alumina (Al2O3) toward advancing aluminum particles for energy generation applications

Transforming metal particle combustion may require alteration of the metal oxide passivation shell surrounding the metal core. One approach for aluminum (Al) relies on the hydrated form of the metal oxide to incite surface reactions. This study explores the conditions required for hydrating alumina (Al2O3) particles and then extends those conditions toward hydration of the Al2O3 passivation layer surrounding an Al core particle. By raising the pH of the water slurry to 11 and controlling temperature and time in slurry (i.e., aging), aluminum hydroxide Al(OH)3 formation from Al2O3 particles was optimized. The procedure was then extended to Al nanoparticles (nAl). Even though heating and extended aging time in slurry proved advantageous for Al2O3 particles, these conditions were discarded for nAl particles because they favored formation of AlOOH, a less desirable hydrate. Through microscopy, spectroscopy, X-ray diffraction, and thermal analyses, results indicate that for a pH range of 11.26–11.56, the original Al2O3 shell on Al particles transformed into Al(OH)3. Results indicate a feasible path forward towards producing new shell chemistries that may result in more directed energy from metal particle combustion.

Strength damage mechanism and pore structure evolution of modified Pisha sandstone cement soil with metakaolin

To fully understand the solidification effect of metakaolin (MK) on Pisha sandstone cement soil, unconfined compressive strength (UCS), ultradepth 3D microscopy (U3DM), nuclear magnetic resonance (NMR), X-ray diffraction (XRD) and scanning electron microscopy (SEM) tests were carried out on the solidified soil. The solidification mechanism of MK was comprehensively analysed from the perspectives of macromechanics, mesostructure and micromechanisms. From the perspective of macromechanics, the damage variable V tended to decrease with increase in MK content, which indicates that the addition of MK helps improve the ability of the solidified soil to resist damage. From a mesoscopic perspective, the addition of MK reduced the average height difference of the surface of the solidified soil. The complexity of the size, number, distribution and shape of pores in the solidified soil was comprehensively analysed based on NMR experiments. With increase in MK content, the proportion of pores with a pore radius >1 µm decreased, whereas the fractal dimensions D T and D 2 increased. XRD and SEM experiments show that the hydration products (calcium aluminate hydrate and calcium silicate hydrate) formed after the incorporation of MK were tightly combined with the soil particles and filled the pores. A grey entropy correlation analysis carried out to analyse the correlation effect of different pore structures on the strength of the solidified soil showed that pore sizes of 0–0.1 µm had a large influence on the UCS. This is a valuable finding on the link between the microstructure and macromechanics of solidified soils.

The Elastic Modulus and Damage Stress-Strain Model of Polypropylene Fiber and Nano Clay Modified Lime Treated Soil under Axial Load.

Using polypropylene fiber (PPF) and nano clay modified lime treated soil (LS), the static and dynamic properties of fiber modified lime treated soil (FLS), nano clay modified lime treated soil (NLS), and fiber nano clay composite modified lime treated soil (NFLS) were studied. Through the unconfined compressive strength (UCS) test and dynamic triaxial test of FLS, NLS, and NFLS, the static and dynamic elastic modulus characteristics at 7 day curing age were explored, and the damage stress–strain model was established. The results show that: (1) Polypropylene fiber and nano clay can significantly enhance the mechanical properties of NFLS. Nano clay can promote the reaction between lime and soil to produce calcium silicate hydrate (C-S-H) and calcium aluminate hydrate (C-A-H), thus improving the strength of NFLS, and UCS can be increased by up to 103%. Polypropylene fiber can enhance the ductility of NFLS and increase the residual ductility strength, and the residual strength can be increased by 827%. (2) Nano clay can enhance the static and dynamic elastic modulus of modified lime treated soil. The static and dynamic elastic modulus of NLS, FLS, and NFLS are linear with the change of polypropylene fiber and nano clay content. The static and dynamic elastic modulus of NLS, FLS, and NFLS are linear, exponential, and logarithmic, respectively. (3) The mesoscopic random damage model can characterize the stress–strain relationship of NFLS. Polypropylene fiber and nano clay can improve the ductility and strength of modified LS, and the composite addition of polypropylene fiber and nano clay can improve the ability of modified LS to resist damage.

A Facile Temperature-Controlled "Green" Method to Prepare Multi-kinds of High-Quality Alumina Hydrates via a Ga-In-Sn-Alloyed Aluminum-Water Interface Reaction.

The Al sheets alloyed by Ga-In-Sn are generally utilized to react with water for H2 production, while the valuable byproducts, i.e., alumina hydrates, have not been fully studied. In this work, through controlling the reaction temperature, three types of alumina hydrates, bayerite (40 °C), pseudo-boehmite (PB) (70–120 °C), and boehmite (130–160 °C), were successfully prepared based on a series of interface reactions and structural transformations. These alumina hydrates and their calcined products (alumina) possess high purity with a total impurity element content of <450 ppm, especially an extremely low sodium content (<21 ppm) and iron content (<52 ppm). Significantly, the obtained pseudo-boehmite displays excellent surface properties (specific surface area: 332.7 m2 g–1, pore volume: 0.3 cm3 g–1, and pore diameter: 3.6 nm), competitive to the current commercial SB powder by Sasol. This work not only deepens the understanding of the byproducts in a Ga-In-Sn-alloyed Al–water reaction but also establishes a facile “green” method oriented to industrial applications, which is promising for the linkage benefits of the hydrogen production industry.

Investigation of sulfate attack on aluminum phases in cement-metakaolin paste

Al-containing phases such as stratlingite, C-A-S-H, and TAH play an essential role in studying the sulfate attack of MK blended cement paste. This paper investigates the sulfate attack of various aluminate hydrate products commonly formed in metakaolin (MK) blended cement paste and the evolution of aluminate hydrate products of the cement-MK paste under sulfate attack. The results show that both the synthesized stratlingite and the stratlingite produced by the cement-MK paste have excellent sulfate attack resistance. Additions of 30% and 50% MK consume all portlandite produced by the cement hydration and result in the formations of C-A-S-H, stratlingite, TAH, and C4AH13. For the 30% MK blended cement paste under sulfate attack, C4AH13 reacts with SO42− to form ettringite at the beginning because of its poor sulfate resistance. Once C4AH13 is consumed, SO42− directly targets the stratlingite, C-A-S-H, and TAH due to no portlandite in the system; C-A-S-H and TAH are the two primary contributors of Ca2+ ions and Al3+ ions to the ettringite formation. The ettringite formation is suppressed due to the absence of CH, and the existence of stratlingite, C-A-S-H, and TAH reduces the content of reactive Al that can react with sulfate.

Influence of the availability of calcium on the hydration of tricalcium aluminate (C3A) in seawater‐mixed C3A–gypsum system

AbstractThis work examined the effects of seawater (SW) on the hydration of tricalcium aluminate (C3A) in C3A–gypsum and C3A–gypsum–Ca(OH)2 systems through the characterization of hydration heat release, the evolution of aqueous phase composition and hydration products with the hydration time. It was found that SW increased the dissolution driving force of C3A and solubility of gypsum, which accelerated the early hydration of C3A and the formation of ettringite (AFt), leading to a higher hydration degree of C3A at an early age compared with the deionized (DI) water–mixed pastes. After gypsum depletion to form AFt, and in the absence of Ca(OH)2, the formation of chloroaluminate hydrates was slower due to the insufficient Ca resulted in an accumulation of Al in solution. This would delay the subsequent transformation of AFt to monosulfate (SO4–AFm) and the formation of hydrogarnet (C3AH6), which would further reduce the hydration degree of the C3A at the later ages. However, in the presence of Ca(OH)2, the hydration degree of C3A–gypsum–Ca(OH)2 at later ages was increased, which was similar to that of the corresponding DI pastes. This can be inferred that the amount of Ca available in SW‐mixed cement concrete can affect the hydration degree of C3A in cement.

Mineralogy of tricalcium aluminate hydration with silica nanoparticles

Formation of siliceous-hydrogarnet (Si-hydrogarnet) and stability of katoite (C3AH6) in tricalcium aluminate (C3A) system in presence of silica nanoparticles (n-SiO2) have been examined in the present study. C3A hydration in presence of n-SiO2 upto 24 h show retardation in hydration due to the surface coverage of C3A grain through calcium-silicate-hydrate (C-S-H)/calcium-alumino-silicate-hydrate (C-A-S-H) (previous study). However, in the present investigation, hydration was studied upto 10 days using QXRD, TGA, FTIR, FESEM, EDS and Mapping and 29Si NMR techniques. QXRD results show that in case of control samples, the katoite content increases from 61 to 73%, while in n-SiO2 incorporated C3A samples, it decreases from 28 to 15% as the hydration proceeds showing katoite phase destabilization in presence of n-SiO2. Further, FTIR results show symmetric stretching vibration of Si-O-Si at ∼784 cm−1 and ∼818 cm−1, while asymmetric stretching vibration of Si–O–Si/Si-O-Al at ∼1028 cm−1 indicating the formation of C-S-H/C-A-S-H and Si-hydrogarnet in presence of n-SiO2. In 29Si NMR results, Q2u peak at −89 ppm confirm the formation of Si-hydrogarnet at 10 days of hydration. The present study is important from performance and durability point of view as formation of C-A-S-H and Si-hydrogarnet can act as an aluminum reservoir, which may prevent severe chloride and sulphate attack.

The impact of carbonation at different CO2 concentrations on the microstructure of phosphogypsum-based supersulfated cement paste

This study investigates mechanisms of carbonation on the microstructure of phosphogypsum-based supersulfated cement (PSSC) paste at different CO2 concentrations (atmospheric, 5% and 20%). The products was characterized using XRD, TGA, FTIR and 27Al MAS NMR. It is observed that the main hydration products of PSSC are ettringite, C-A-S-H gels and third aluminate hydrate (TAH), and the carbonation products are calcium carbonate (mainly vaterite and calcite), secondary gypsum, and C-A-S-H gels and separate TAH with varying degrees of decalcification. The quantitative analysis of BSEM images shows that the volume change of products occurring on carbonation significantly increases porosity and pore size of the PSSC paste. Results indicate that the carbonation of PSSC at different CO2 concentrations results in a progressive carbonation degree, 5% CO2 supports the accelerated carbonation experiment in PSSC to simulate the natural carbonation process due to the same phase composition and similar microstructure with atmospheric CO2 and 20% CO2 further loosens the microstructure of PSSC for the complete decomposition of ettringite and the severe decalcification of gel products.

Manufacturing non-sintered ceramsite from incinerated municipal solid waste ash (IMSWA): Production and performance

The resource utilization of solid wastes has become a critical problem in resource recovery. In this study, IMSWA (Incinerated municipal solid waste ash) was washed to remove chloride and then mixed with OPC (Ordinary Portland Cement) and SS (Silica Sol) to prepare a non-sintered ceramsite. Through single factor experiments, the quality scores of water-washed IMSWA: OPC: SS is initially determined to be 6:3:1, and the curing time is 14 d. Then the uniform test design was used to optimize the ceramsite formula and the influence of different raw material ratios on the performance of the non-sintered ceramsite. Finally, use DPS (Data Processing System) optimization to determine the best raw material quality scores: 59% for IMSWA, 30% for OPC, and 11% for SS. The performance follows (1) Ceramsite's compact surface structure, ensuring high strength and low water absorption. (2) Ceramsite section forms a complex calcium aluminate hydrate gel (C-A-H) structure (3) A profit of $60.97/ton of ceramsite has good economic benefits. These findings demonstrate that preparing a non-sintered ceramsite with IMSWA is a promising artificial lightweight aggregate.

Waste glass as a precursor in alkali‐activated materials: Mechanical, durability, and microstructural properties

AbstractThe long‐term characteristics and durability of alkali‐activated mortars based on waste glass are crucial to better understand their performance in aggressive environments. In this regard, the performance of waste glass as a building material and its characteristics such as mechanical (compressive, flexural, and tensile strengths), durability (chloride migration coefficient, sulfate resistance, and drying shrinkage), and microstructural properties [x‐ray diffraction (XRD), Fourier transform infrared (FTIR), and scanning electron microscopy (SEM) with energy dispersive x‐ray analysis (SEM/EDX)] were evaluated. For this purpose, the specimens with different amounts of silica modulus and Na2O content were prepared. The specimens were cured at 95°C for 20 h and then kept at a relative humidity of 50% at room temperature until testing. The specimen with 10% Na2O and a silica modulus of 1.5 demonstrated the highest compressive, flexural, and tensile strengths. The specimens illustrated a lower chloride migration coefficient and lower expansion than the Portland cement one. By increasing both silica modulus and Na2O contents, the drying shrinkage of the specimens increased due to the presence of more free water. The microstructural results indicated that amorphous gels such as sodium aluminum silicate hydrate (N‐A‐S‐H) and sodium (calcium) silicate aluminum hydrate [N‐(C)‐A‐S‐H] were formed. The results of this study signify the utilization possibility of waste glass as an eco‐friendly material with desirable characteristics.

Microscopic phase identification of cement-stabilized clay by nanoindentation and statistical analytics

The mechanical properties of cement-stabilized clay (CSC), are significantly inferior to that of concrete and mortar due to the presence of clay minerals. To reveal the interaction between clinker's functional constituent and clay minerals within the CSC matrix to promote the performance of stabilized clay, large data sets cross-scale nanoindentation coupled with thermogravimetric analyses were involved. Two commercially available clays (i.e., kaolin and bentonite, whose dominant clay minerals are kaolinite and montmorillonite, respectively) stabilized by two key functional constituents of cement clinker, i.e., tricalcium silicate (C3S) and tricalcium aluminate (C3A), were investigated to unravel the primary hydration of the cement clinker, pozzolanic reactions with clay minerals, and the microstructure's evolution. Large data sets of elastic modulus and hardness derived from the nanoindentation tests were statistically investigated by two deconvolution methods: probability density function and cumulative distribution function. The results indicated that calcium silicate hydrates (C-S-H) and calcium aluminate hydrates (C-A-H) were the major reaction products for C3S-stabilized and C3A-stabilized clays. However, during the secondary pozzolanic reaction process, the dissolved colloids from the clay minerals and the hydrated products would react and accumulate at the interface between the primary cementitious paste and clay particles to form a new soft, porous phase. The deconvolution and thermogravimetric analysis also indicate that more pozzolanic products and high-density (HD) porous phases were detected in bentonite than in kaolin, suggesting that the pozzolanic reaction in montmorillonite is much more pronounced than that in kaolinite. This understanding can clarify the interaction between clay minerals, hydration products, and distinct CSC for traditional cementitious materials.

Effect of Ca/Si and Al/Si on micromechanical properties of C(-A)-S-H

Calcium silicate aluminate hydrate (C(-A)-S-H) is the main hydration product used in cement-based materials. It plays a specific role in both the mechanical properties and volumetric deformation of the cement matrix. This study aims to clarify the effect of porosity and chemical composition of C(-A)-S-H on its micromechanical properties using nanoindentation. The results showed that porosity significantly affected the indentation, elastic, and creep moduli of the porous C(-A)-S-H compacts. Micromechanical properties of the “pure” C(-A)-S-H were improved by decreasing the Ca/Si ratio. The incorporation of Al slightly altered the creep modulus by affecting the volume of the large gel pores, basal spacing, and mean chain length (if no cross-linking occurred in C(-A)-S-H). Finally, the nanoindentation results of the mixed C(-A)-S-H and Al(OH)3 confirmed the distinct contribution of the individual component characteristics to the overall properties of porous mixtures.

Effect of reverse-osmosis brine and sodium aluminate on the hydration properties and strength of alkali-activated slag cement

The effects of sodium aluminate (NA) and reverse-osmosis brine (RW) on alkali-activated slag cement (AASC) using sodium hydroxide and sodium silicate as activators were investigated. NA was mixed at 0%, 2.5%, and 5.0% by binder weight, and the mix waters tested were tap water (TW) and RW. In the experiment NA showed the effect of improving the compressive strength, with the highest strength value obtained at 2.5%. However, the strength improvement effect of NA was more clearly demonstrated in RW samples than in TW samples. Changes in the C4AH13 (Tetracalcium aluminate hydrate; Ca4Al2(OH)14.6 H2O), C2ASH8 (stratlingite; Ca2Al2SiO7.8 H2O), and C3AH6 (katoite; Ca3Al2(OH)12) phases were observed according to the increase in NA through hydration reaction analysis. Friedel’s salt (Ca4Al2O6Cl2.10 H2O) was additionally observed in the RW sample. In the TW samples, more of the C3AH6 phase was formed than in the RW samples, which affected the strength reduction. In the RW samples, increases in the chloride ions and calcite phase in the pore solution due to carbonation of Friedel’s salt were confirmed. This phenomenon of the RW samples became more pronounced as the NA concentration increased. The technique of utilizing RW in AASC showed the possibility of being used as a novel stabilization/solidification method for reverse osmosis brine.

Study on the Strength and Hydration Behavior of Sulfate-Resistant Cement in High Geothermal Environment.

The hydration process and compressive strength and flexural strength development of sulphate-resistant Portland cement (SRPC) curing at 20 °C, 40 °C, 50 °C, and 60 °C were studied. In addition, MIP, XRD, SEM, and a thermodynamic simulation (using Gibbs Energy Minimization Software (GEMS)) were used to study the pore structure, the types, contents, and transformations of hydration products, and the changes in the internal micro-morphology. The results indicate that, compared with normal-temperature curing (20 °C), the early compressive strength (1, 3, and 7 d) of SRPC cured at 40~60 °C increased by 10.1~57.4%, and the flexural strength increased by 1.8~21.3%. However, high-temperature curing was unfavorable for the development of compressive strength and flexural strength in the later period (28~90 d), as they were reduced by 1.5~14.6% and 1.1~25.5%, respectively. With the increase in the curing temperature and curing age, the internal pores of the SRPC changed from small pores to large pores, and the number of harmful pores (>50 nm) increased significantly. In addition, the pore structure was further coarsened after curing at 60 °C for 90 d, and the number of multiple harmful pores (>200 nm) increased by 17.9%. High-temperature curing had no effect on the types of hydration products of the SRPC but accelerated the formation rate of hydration products. The production of the hydration products C-S-H increased by 13.5%, 18.6%, and 22.8% after curing at 40, 50, and 60 °C for 3 d, respectively. The stability of ettringite (AFt) reduced under high-temperature curing, and its diffraction peak was not observed in the XRD patterns. When the curing temperature was higher than 50 °C, AFt began to transform into monosulfate, which consumed more tricalcium aluminate hydrate and inhibited the formation of “delayed ettringite”. Under high-temperature curing, the compactness of the internal microstructure of the SRPC decreased, and the distribution of hydration products was not uniform, which affected the growth in its strength during the later period.

Raman and glow discharge optical emission spectroscopy studies on structure and anion incorporation properties of a hydrated alumina film on aluminum

Hydration treatment of aluminum in hot water is important for its corrosion protection, and also this treatment is a crucial pretreatment in forming dielectric crystalline alumina films on the aluminum anode in aluminum electrolytic capacitors. In this study, a two-layer hydrated film formed on high purity aluminum by hot water treatment was examined using a combination of glow discharge (GD) sputtering and Raman spectroscopy. In addition, the anion penetration behavior through the hydrated alumina layer during anodizing aluminum in several electrolytes was investigated. The results indicate that the outer layer, with a nanosheet-like morphology, consists of a crystalline pseudo-boehmite phase, whereas the relatively compact inner layer is amorphous or poorly crystalline. The inner amorphous layer plays a crucial role in the incorporation of the anions, which is found for the first time. When hydrated aluminum is anodized in phosphate, silicate, and tungstate electrolytes, the inner hydrated alumina layer impedes the penetration of these electrolyte anion species. In contrast, boron species readily penetrate the hydrated alumina layer into the barrier layer formed by anodization in boric acid. This anion-specific incorporation behavior is discussed in terms of ion-selective permeation in the amorphous hydrated alumina layer.

Preparation and characterization of HA bonded Al2O3–MgO based castables with superior slag resistance for the working lining of Si-killed stainless steel ladles

The study prepared Al2O3–MgO based castables bonded by hydratable alumina (HA) instead of calcium aluminate cement (CAC) for the working lining of Si-killed stainless steel ladles. The microstructure, phase composition, mechanical properties, and slag resistance of castables were investigated by SEM, XRD, and thermodynamic software FactSage®. The results indicated that the HA bonded castables showed superior hot flexural strength, thermal shock resistance and slag resistance than the CAC bonded castables, due to the optimized pore characteristics, less liquid content, and higher liquid viscosity of the castable matrix and the formation of a continuous insulating layer.

Characterization of Monochromate and Hemichromate AFm Phases and Chromate-Containing Ettringite by 1H, 27Al, and 53Cr MAS NMR Spectroscopy

The calcium aluminate hydrate AFm and AFt phases formed upon hydration of Portland cement have an important role in the stabilization and solidification of hazardous chromate ions in hardened cement. AFm monochromate (Ca4[Al(OH)6]2(CrO4)·12H2O), AFm hemichromate (Ca4[Al(OH)6]2(CrO4)0.5(OH)·12H2O) and the chromate-containing AFt phase, Ca6[Al(OH)6]2-(CrO4)3·24H2O, were synthesized and investigated by 1H, 27Al, and 53Cr MAS NMR spectroscopy. 27Al quadrupolar coupling parameters (CQ, ηQ) and isotropic chemical shifts (δiso) were determined for the three phases, including two distinct Al sites in chromate-AFt, as observed by 27Al MAS and MQMAS NMR. Two dominant peaks are apparent in the 1H MAS NMR spectra of each of the phases. For the AFm phases, these resonances are assigned to framework hydroxyl groups (1.7–2.0 ppm) and water molecules/hydroxyls (5.0–5.5 ppm) in the interlayer. For chromate-AFt, the peaks are ascribed to framework hydroxyl groups in the [Ca6Al2(OH)12]6+ columns (~1.4 ppm) and water molecules (~4.8 ppm) associated with the Ca ions. 53Cr MAS NMR spectra acquired at 22.3 T for the samples show a narrow resonance for both chromate AFm phases, whereas indications of three distinct Cr resonances are apparent for the chromate AFt. The absence of any second-order quadrupolar effects in the 53Cr NMR spectra strongly suggests that the chromate ions are highly mobile in the anionic sites of the AFm and AFt structures. The NMR data reported in this work are in agreement with the reported crystal structures for the chromate AFm and AFt phases and may be useful for identification and characterization of chromate fixation in cementitious systems, complementing information gained from conventional powder X-ray diffraction studies.

Effects of limestone powder on the early hydration of tricalcium aluminate

Effects of limestone powder on the early hydration of tricalcium aluminate