Calcium Sulfoaluminate Hydrates Research Articles

Solidification/stabilization and optimization of municipal solid waste incineration fly ash with aluminosilicate solid wastes

Alkali-activation is an effective municipal solid waste incineration fly ash (MSWIFA) solidification/stabilization (S/S) technology. However, the characteristics of calcium-rich silica-poor aluminum phase in MSWIFA easily cause the structural instability and contamination of alkali activated MSWIFA S/S bodies. Therefore, the aluminosilicate solid wastes are used in this work to optimize the immobilization and structural properties. Results showed that incorporation of aluminosilicate solid wastes significantly improved the compressive strength and heavy metals pollution toxicity of MSWIFA S/S bodies. Compared to alkali activated MSWIFA, the compressive strength of S/S bodies with addition of coal fly ash, silica fume and granulated blast furnace slag improved by 31.0%, 47.6% and 50.8% when the curing time was 28 days, respectively. Leachability of Pb, Zn and Cd in these alkali activated MSWIFA S/S bodies was far below the threshold value specified in Standard GB16889. Aluminosilicate solid wastes provided abundant Si/Al structural units, and some new phases such as ettringite(AFt, 3CaO⋅Al2O3⋅3CaSO4⋅32H2O), calcium sulfoaluminate hydrate (3CaO⋅Al2O3⋅CaSO4⋅12H2O) and Friedel's salt (CaO⋅Al2O3⋅CaCl2⋅10H2O) can be detected in S/S matrix with aluminosilicate solid wastes, along comes increased the amount of the amorphous phases. Lower Ca/Si molar ratio tended to form the network structure gel similar to tobermorite with higher polymerization degree. Meanwhile, the silica tetrahedron of the gels changed from the oligomerization state like island to the hyperomerization state like chain, layer network or three-dimensional structure, and average molecular chain length increased. These findings provide theoretical basis for structural properties optimization and resource utilization of MSWIFA S/S matrices.

Insight into the role of early C3A hydration in structural build-up of cement paste

This research sheds light on the two origins of C3A hydration in structural build-up of cement paste. The largest critical strain is associated with colloidal interaction, whereas the smallest critical strain originates from the cohesion between calcium sulfoaluminate hydrate (CA$H) particles. Interestingly, we have found that the structural build-up primarily relies on the total colloidal interaction within 2 h of hydration rather than the cohesion between CA$H particles. In C3A-gypsum-CaCO3 pastes, a two-stage structural build-up was observed. The swift progression within the first tens of minutes can be traced back to the augmentation in the number of bonds (solid volume fraction), which then proceeds to the bond strength (colloidal interaction between two flocculated particles). The subsequent slow, linear development is attributed to the strengthening of bonds. As a result of this analysis, a time-dependent model for structural build-up was established. Two rates, namely Acol and ACA$H, were employed to illustrate the impact of the strength and number of bonds. This model is proven to be well-suited for analyzing cement paste within the initial tens of minutes.

Associated Effects of Sodium Chloride and Dihydrate Gypsum on the Mechanical Performance and Hydration Properties of Slag-Based Geopolymer

The associated effect of sodium chloride and dihydrate gypsum on the mechanical performance of a slag-based geopolymer activated by quicklime was investigated by compressive strength, shrinkage, and square circle anti-cracking tests of mortar with a 0.5 water–binder ratio and a 1:3 binder–sand ratio, as well as paste soundness, powder X-ray diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS), and mercury intrusion porosimetry (MIP) of the paste. The results indicate that (1) when dihydrate gypsum is used alone, it combines with calcium aluminate hydrate (C-A-H) to form calcium sulfoaluminate hydrate (AFt), which encourages the hydration process of slag. A 7.5% addition can result in an increase of 97.33% and 36.92% in 3-day and 28-day compressive strengths, respectively. When NaCl is used by itself, it facilitates the condensation of the aluminum silicate tetrahedron unit and generates zeolite. A 2% dosage can lead to a 66.67% increase in the 3-day compressive strength, while causing a 15.89% reduction in the 28-day compressive strength. (2) The combined effect of 2% NaCl and 7.5% gypsum results in the formation of needle-like and rod-shaped AFt, Friedel’s salt, and plate-like Kuzel’s salt in the geopolymer. This leads to an increase in 3-day and 28-day compressive strengths by 148% and 37.85%, respectively. Furthermore, it reduces the porosity by 18.7%. (3) Both NaCl and gypsum enhance the paste soundness of the slag-based geopolymer, and they do no harm to the crack resistance of the geopolymer. The drying shrinkage of the geopolymer at 28 days is just 0.48 × 10−3, which is only 66.7% of OPC. This slag-based geopolymer has a simple preparation process, good volume stability, low raw material cost, low energy consumption, and low carbon emissions. It can be used instead of 32.5 slag Portland cement in plain concrete applications, and has high engineering, economic, and environmental values.

Investigating the hydration of C3A in the combined presence of anhydrite and limestone using solid-state NMR, XRD and TG techniques

Diverse AFm phases are commonly found during the hydration of calcium sulfoaluminate (CSA), Portland (OPC), and calcium aluminate (CAC) cements. Due to the flexibility in their layered double hydroxide (LDH) structure, they are observed in variable single or multi-anion bearing forms, water contents and transient states. When multiple sulfate and carbonate anion sources, in variable compositions, are available during the hydration of cements, the identity, quantity, and stability of the formed AFm phases vary considerably. They may also precipitate as microcrystalline and/or amorphous phases, rendering their detection by X-ray powder diffraction (PXRD) difficult. In this study the AFm phases have been generated through the hydration of tricalcium aluminate (C3A) in the presence of sulfate (anhydrite) and carbonate ions (limestone) in various compositions. A multi-technique approach involving solid-state nuclear magnetic resonance spectroscopy (ssNMR), thermogravimetric analysis/differential scanning calorimetry (TGA/DSC) and PXRD has been employed to govern the sequence of hydration reactions, the evolution, structure, and the stability of AFm phases at 1 and 28 days of C3A hydration. The results show that the identity and quantity of AFm phases formed vary considerably according to the nature of the anion sources available during the hydration. Hydration products such as crystalline C3AH6, ettringite and amorphous as well as crystalline/microcrystalline monoanionic AFm phases (monosulfate, monocarbonate, hydroxy-AFm) and bi-anionic phases (hemicarbonate and solid-solutions involving more anions) have been identified and quantified.

Hydration characteristics of calcium sulfoaluminate-fly ash mixed with different alkalies

Due to its notable low carbon footprint, calcium sulfoaluminate (CSA) cement has become a research hotspot for cement chemists in recent years. Relevant studies have shown that supplementary cementitious materials (SCMs) can greatly improve the properties of CSA cement pastes, but the utility of SCMs in CSA cement has not yet been fully exploited due to their low alkalinity. To stimulate the activity of pozzolanic materials, the effect of different alkalies on the hydration of CSA-fly ash (CSA-FA) was systematically investigated, and the results indicate that after calcium hydroxide (Ca(OH)2) is incorporated into the CSA-FA paste, it first forms a saturated solution and then rapidly hydrates to form ettringite. The initial pH value of the suspension is approximately 12.65, which then slumps to a level lower than that without Ca(OH)2. The initial pH value of the CSA-FA suspension mixed with 1 M sodium hydroxide (NaOH) solution is 13.0, which is then rapidly decreased to a stable value of approximately 11.3. Ca(OH)2 can promote the hydration of CSA to form ettringite and improve the compressive strength of the paste, and the larger the Ca(OH)2 content, the more obvious this effect. NaOH inhibits the hydration of CSA. With increasing NaOH concentration, the compressive strength of the paste first increases and then decreases, and the effect of inhibiting the hydration of CSA becomes stronger. The CSA-FA paste blended with 1 % Ca(OH)2 and 1 M NaOH shows collapsing and cracking behavior, for which a large number of well-developed flaky aluminum hydroxide crystals are formed, and strong diffraction peaks for unhydrated CSA are observed.

Effect of the nanosilica source on the rheology and early-age hydration of calcium sulfoaluminate cement pastes

This work investigates the role of different nanosilica (NS) sources on the rheology and early-age hydration of calcium sulfoaluminate (CSA) cement paste. Specifically, pastes were produced with three NS (two powder and one colloidal suspension) in 1 wt% replacement of cement, with and without superplasticizer. Rotational rheometry, isothermal calorimetry, and in-situ X-ray diffraction (XRD) tests were conducted. The results showed that incorporation NS increased the yield while decreased the viscosity and delayed the shear thickening onset point of CSA pastes, regardless of the NS source. Calorimetry and in-situ XRD revealed that NS incorporation enhanced the CSA cement hydration kinetics while superplasticizer delayed it. Overall, colloidal NS led to the best overall early-age performance, leading to the lowest viscosity and the highest hydration degree over time among the CSA cement pastes investigated.

Study on the activation mechanism and hydration properties of gold tailings activated by mechanical-chemical-thermal coupling

This paper mainly studies the activation mechanism and hydration properties of gold tailings. The reactivity of gold tailings under various activation methods was evaluated by the compressive strength test of cement mortar test blocks and Frattini tests. The activation mechanism of gold tailings and the hydration properties of the activated gold tailings were studied by XRD, FTIR, SEM, EDS, TG-DTG. The research results show that the 28-day compressive strength ratio of the mortar test block mixed with mechanical-chemical-thermal coupling activated gold tailings are 95.60%, and the consumption ratio of [CaO] in the slurry of coupled activated gold tailings is 60.76% at 15 days. After the gold tailings are mechanically ground, the crystallinity of the main mineral phase is reduced, but the pozzolanic reactivity is low, which mainly plays the filling effect of the micro-aggregate in the mortar test blocks. High-temperature calcination can remove the structural water in the muscovite to make it further decompose. In the presence of calcium oxide, high-temperature calcination is conducive to the depolymerization of siloxane radicals and improves the pozzolanic reactivity of the tailings. As the content of activated gold tailings increases, the content of calcium hydroxide in the hardened slurry decreases, and the content of C-S-H gel and calcium sulfoaluminate hydrate increases, proving that coupled activated gold tailings have pozzolanic reactivity. The gold tailings activated by mechano-chemical-thermal coupling can be used as supplementary cementitious materials for cement, and the practical application needs to be further studied.

Novel Bionic Soy Protein-Based Adhesive with Excellent Prepressing Adhesion, Flame Retardancy, and Mildew Resistance.

Soy protein (SP)-based adhesives can replace traditional aldehyde-based adhesives for the manufacturing of wood-based panels. However, developing a SP-based adhesive with excellent prepressing bonding strength, flame retardancy, and mildew resistance remains a challenge. Herein, an inorganic crystal cross-linked hybrid SP adhesive was developed inspired by the "secreting-hardening" process of the mussel adhesive protein and the organic-inorganic hybrid adhesive system of the oyster. Calcium sulfoaluminate (CSA) was introduced into the adhesive mixture of SP and acrylic acid to induce the in situ polymerization of acrylic acid to achieve adhesive gelation. The generation of the inorganic crystals by hydration of CSA not only contributed to the formation of a stable cross-linked hybrid adhesive system for strong cohesion but also provided strong interfacial adhesion between the adhesive layers and the plywood veneers. As anticipated, the prepared plywood sample bonded with the hybrid adhesive gel had an excellent prepressing bonding strength of 544 kPa, representing a significant increase compared to that of the pure SP adhesive (19 kPa). Moreover, the generated inorganic crystals endowed the adhesive with excellent mildew resistance and flame retardancy. This study provides a novel and effective strategy for the preparation of high-performance SP-based adhesives.

Hydration of calcium sulfoaluminate cement blended with blast-furnace slag

The present study investigates the hydration of calcium sulfoaluminate (CSA) cement blended with blast-furnace slag (BFS). BFS was added at dosages of 0, 30, 50 and 70% of the binder mass. The samples were cured for up to 360 days and characterized using various analytical techniques. The obtained results indicate that BFS-blended CSA cement incorporating 30% BFS exhibits compressive strength comparable to that of the sample solely consisting of CSA cement. BFS incorporation in CSA cement leads to the formation of more strätlingite, possibly preventing the development of porosity due to the destabilization of CAH10. The main hydration products of CSA cement blended with BFS are identified as ettringite, monosulfate, Al(OH)3 and strätlingite, similar to those forming during the hydration of CSA cement, though strätlingite increasingly forms at higher BFS contents.

Influence of water removal techniques on the composition and microstructure of hardened calcium sulfoaluminate cement pastes

Influence of different drying techniques, namely, vacuum drying, oven drying (40 °C) and D-drying on the composition and microstructure of the calcium sulfoaluminate (CSA) cement was investigated in this study. The vacuum drying and oven drying were performed after the samples were immersed in acetone or ethanol. The results show that D-drying decomposes ettringite (AFt) greatly. Following solvent replacement, oven drying slightly decomposes AFt when compared with the vacuum dried sample, possibly due to the carbonation of AFt in the oven drying process. The decomposition of AFt due to D-drying and oven drying affects both the compositional and microstructural analysis of CSA hydrates. In addition, the soaking time of the samples in the solvents has no effect on the thermogravic (TG) analysis of the hydration products of CSA cement, which indicates that the CSA cement hydrates have no interaction with the solvents.

Phase development and hydration kinetics of belite-calcium sulfoaluminate cements at different curing temperatures

The influence of different curing temperatures on the hydration of belite-calcium sulfoaluminate cement was investigated at 20, 40 and 60 °C. The hydration kinetics and the hydrated phase assemblages were studied by isothermal calorimetry, X-ray powder diffraction, differential thermal analysis and thermogravimetric analysis, as well as field emission scanning electron microscopy. The compressive strength development of the cement pastes was also determined. Results showed that, at early ages, hydration was faster and early compressive strength was higher at elevated temperatures than at ambient temperature. On the other hand, at late ages in cement pastes cured at 60 °C, the amount of ettringite decreased, leading to lower compressive strength, indicating that the degree of hydration was lower at higher temperatures. Moreover, at elevated temperatures prismatic ettringite crystals became smaller due to faster hydration. Other hydration products present were aluminium hydroxide, which is formed together with ettringite from the hydration of calcium sulfoaluminate and gypsum, and C–S–H which precipitates as a main hydration product of belite. Belite hydrated in a lesser amount, especially at 60 °C, when the lowest amount of C–S–H was observed.

Early hydration of CSA cement modified with styrene–butadiene copolymer dispersion

The effect of styrene–butadiene copolymer (SB) dispersion on the early hydration of calcium sulfoaluminate (CSA) cement was investigated. Pastes with five different ratios (0, 5, 10, 15 and 20%) of SB to CSA cement at a constant water to cement ratio of 0·4 were investigated by ultrasonic measurements, isothermal calorimetry, X-ray diffraction, thermogravimetry and scanning electron microscopy. It was found that SB retarded the initial hydration of the CSA cement paste within 3 h, during which time the formation of ettringite was strongly delayed. The hydration degree of the CSA cement paste was caught up by that of the SB-modified CSA cement paste at 3 h based on ultrasonic and calorimetry analysis. In addition, an increase in the addition of SB led to the generation of more ettringite and aluminium hydroxide. The most ettringite was formed and the size of ettringite crystals was increased in the pastes with 20% SB. Meanwhile, the addition of SB tended to reduce the amount of hemicarboaluminate in CSA cement pastes.

The effect of polymer dispersions on the early hydration of calcium sulfoaluminate cement

The influence of three polymer dispersions [styrene–butadiene copolymer (SB), styrene–acrylic ester copolymer (SA) and polyacrylic ester (PA)] on the hydration of calcium sulfoaluminate (CSA) cement within 72 h was investigated by using isothermal conduction calorimetry, X-ray diffraction analysis and thermal gravimetric analysis. The results indicate that these three polymer dispersions perform different influences on the hydration heat flow of CSA cement during different periods, they all postpone the occurrence time of the maxima peaks, and its extent is mainly dependent on the addition amount. Polymer dispersions manifest great retardation on the initial hydration of CSA cement, and the effect is much more significant within 1 h. In this stage, the generation of ettringite is strongly delayed; however, the formation of ettringite is accelerated by these polymer dispersions at and after 2 h. Among these three polymer dispersions, PA demonstrates the highest acceleration effect on the hydration degree.

Resistance of High-Volume Fly Ash Self-Compacting Concrete to Internal Sulfate Attack

This paper investigates the durability of high-volume Fly ash self-compacting concrete, HVFASCC that exposed to internal sulfate attack. At the present work, HVFASCC was produced with two Fly ash replacements: 50 and 60% by weight of Portland cement. The internal sulfate attack was simulated by adding natural gypsum (CaSO4) that contain ion (SO3-2) to fine aggregate by two weight percentages: 1 and 2%. Limestone dust was used as filler with a content of 100 kg/m3. The cementitious materials, cement and Fly ash, content was 400 kg/m3 and the water to powder ratio for the studied mixes was 0.34 by weight. To ensure the self-compact ability of the mixes, slump flow, T500, V-funnel and L-box tests were done. The Compressive, Splitting and Flexural strength Tests were extended to the age of 240 days. The results showed that there is no significant difference between 1 and 2% of SO3 content on the behavior of all mixes. The presence of limestone powder in the paste solution could have a role in stabilizing ettringite and reducing paste porosity at early ages. At later age, 240 days, the harmful effect of SO3 is diminished and that may be caused by the depletion of gypsum and the dominant product will be calcium monosulfoluminate hydrates instead of calcium sulfoaluminate hydrates.

Microstructural evolution of aluminum hydroxide gel during the hydration of calcium sulfoaluminate under different alkali concentrations

The present paper discusses the microstructural evolution of aluminum hydroxide (AH3) formed during the hydration of calcium sulfoaluminate in neutral and alkaline solutions (0-M NaOH, 0.01-M NaOH, 0.1-M NaOH, 1-M NaOH, 5-M NaOH and 8-M NaOH). Rietveld method and field emission scanning electron microscopy measurements were used to observe the evolution of crystallite size and particle size distribution, respectively. Furthermore, various characterization methods were used to complement the microstructure of AH3 phase. The result of 27Al MAS-NMR spectroscopy indicated the absence of the amorphous AH3 phase in all hydrated pastes. The degree of crystallinity, with respect to the AH3 phase, improved with increasing alkali concentration. However, AH3 phase can also react with OH− ion and even disappear in high alkaline conditions. In low alkaline conditions (i.e., 0-M NaOH, 0.01-M NaOH, 0.1-M NaOH), the AH3 phase had a nanocrystalline structure with an average crystallite size of about 8–9 nm and particle size distribution ranging from 110 nm to 210 nm. In high alkaline conditions (i.e., 5-M NaOH and 8-M NaOH), the AH3 phase presented a well-crystalline nature. The average crystallite size increased up to about 30–40 nm and the particle size distribution was mostly 800–1400 nm (5-M NaOH) or 1000–1500 nm (8-M NaOH).

Retardation effect of elevated temperature on the setting of calcium sulfoaluminate cement clinker

The hydration of calcium sulfoaluminate (CSA) clinker at 5, 20 and 40 °C was characterized by a combination of XRD, DTA-TG and ESEM analysis (also with backscattered electron imaging, BEI). Meanwhile, the setting times and compressive strength development were studied. It was interesting to notice that elevated temperature retards the setting of CSA clinker and decreases compressive strength prominently, while it promotes the hydration heat releasement and hydration degree. With the temperature rising from 5 °C to 40 °C, the amount of ettringite formed after 1 d decreases by half, but alumina gel increases by more than 4 times, accompanying with a more inhomogeneous microstructure and higher porosity. ESEM observations indicate that such massive alumina gel covers on the unreacted particles and other hydrates, which could account for the retardation of elevated temperature on the setting of CSA clinker.

Identification of hydration stage of calcium sulfoaluminate cement at an early age with helium pycnometry

This work is to study the mechanism of the hydration of calcium sulfoaluminate (CSA) cement at an early age by identifying the hydration stage with helium pycnometry, heat evolution test, X-ray diffraction, thermo-gravimetric/differential scanning calorimetry and scanning electron microscope. The results show that the hydration process of CSA cement has four stages: rapid dissolution stage, dissolution-crystallization stage, crystal growth stage and stable stage. In the rapid dissolution stage, exothermic wetting and fast hydration result in the shrinkage occurring from 0 to 15 min. A significant volume decrease happens in the dissolution-crystallization stage. In the crystal growth stage, ettringite prefers to crystalize at the neighboring places of pores at the nano scale due to the combined action of crystallization pressure and size effect. The volume increase considerably slows down throughout the stable stage indicating the deceleration of CSA cement hydration. In addition, the influence of the proportions of clinker and gypsum in the CSA cement on its hydration is studied as well.

A reaction range for hydration of calcium sulfoaluminate with calcium sulfate and calcium hydroxide: theory and experimental validation

To better investigate and understand the hydration of calcium sulfoaluminate (CSA) cement, a theory reaction range for the hydration of CSA with calcium sulfate and calcium hydroxide was established by theoretical calculation with a hypothetical hydration reaction system. With different amounts of calcium sulfate and calcium hydroxide in the reaction systems, their hydration processes, hydration products and stoichiometric water/solid ratios showed regular changes. A preliminary experimental validation was made by analysing the hydration products (curing time 28 d and water/solid ratio of 10) of several composition points in the theory reaction range by means of x-ray diffraction and thermogravimetric analysis. The process for calculation of the theory reaction range and some of the experimental validation results are presented in this paper.

Early hydration characteristics of oil well cement pastes admixed with newly prepared organic admixture

A number of chemical and mineral admixtures are usually used in the oil-well cement (OWC) pastes to modify and control their fluidity to resist the higher temperatures and pressures during the drilling process of the well. In this study a newly prepared aliphatic organic compound namely cyclohexanone glyoxylate condensate (CG) was synthesized. The prepared compound (CG) was characterized using Fourier Transition Infrared spectroscopy (FT-IR) and microanalysis of carbon, hydrogen, oxygen and sulfur elemental analysis techniques. The effect of additions of 0.25, 0.50, 0.75 and 1 (mass%) of this admixture on the mechanical and early hydration characteristics of OWC pastes was studied. The phase composition for some selected hardened specimens was investigated using X-ray diffraction (XRD) and thermogravemtic analysis (DTGA) techniques. The results indicated that, addition of cyclohexanone glyoxylate condensate (CG) admixture to OWC pastes causes a slight retardation for the early rate of hydration of OWC. Addition of 0.25% of CG to OWC causes a slight improvement in the compressive strength values during nearly all ages of hydrations. XRD and DTGA results for the neat and CG admixed OWC pastes, indicate that the main hydration products are nearly amorphous calcium silicate hydrates (mainly as CSH-I and CSH-II), calcium sulfoaluminate hydrates (ettringite and monosulfate hydrate) as well as portlandite (CH).

Hydrothermal synthesis of calcium sulfoaluminate–belite cement from industrial waste materials

This study investigated the use of industrial waste materials viz., fly ash, Al-rich sludge and flue gas desulfurization gypsum in the production of calcium sulfoaluminate–belite cement using hydrothermal–calcination method. The lime deficit in the waste starting materials was recompensed by the addition of hydrated lime. Various factors viz., type of hydrothermal solvent, time of hydrothermal process and calcinations temperature of the hydrothermal products were studied. Phase developments during various synthesized conditions were studied using X-Ray Diffraction. The cement products were mixed with 20 wt% flue gas desulfurization to allow ettringite formation for solidification. The hydration development was investigated in terms of mineralogical composition, microstructures and compressive strength of the hydration products. The largest numbers of calcium sulfoaluminate–belite cement could be obtained by calcination at temperature of 1050 °C. The synthesized cement developed an acceptable 28-day compressive strength of 30.0 MPa. The main hydration products were ettringite, portlandite, calcium silicate hydrate and calcium aluminosilicate hydrate resulting from the hydration of calcium sulfoaluminate and belite phases.