Showing papers on "Ettringite published in 2023"

Recent Advances in C-S-H Nucleation Seeding for Improving Cement Performances

Abstract: Reducing cement CO2 footprint is a societal need. This is being achieved mainly by replacing an increasing amount of Portland clinker by supplementary cementitious materials. However, this comes at a price: lower mechanical strengths at early ages due to slow pozzolanic reaction(s). This is being addressed by using accelerator admixtures. In this context, calcium silicate hydrate nucleation seeding seems to have a promising future, as it can accelerate cement and pozzolanic reactions at early ages, optimising their microstructures, without compromising late strength and durability performances. In fact, these features could even be improved. Moreover, other uses are low temperature concreting, precasting, shotconcrete, etc. Here, we focus on reviewing recent reports on calcium silicate hydrate seeding using commercially available admixtures. Current knowledge on the consequences of nucleation seeding on hydration reactions and on early and late mechanical strengths is discussed. It is noted that other features, in addition to the classic alite hydration acceleration, are covered here including the enhanced ettringite precipitation and the very efficient porosity refinement, which take place in the seeded binders. Finally, because the seeded binders seem to be denser, durability properties could also be enhanced although this remains to be properly established.

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

Abstract: 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.

Hydration and compressive strength of supersulfated cement with low-activity high alumina ferronickel slag

Abstract: This research aims to explore the feasibility of using low-activity high alumina ferronickel slag (FS), carbide slag and hemihydrate phosphogypsum (HG) as main ingredients to fabricate supersulfated cement (SSC). The effect of HG dosage and FS fineness on hydration and compressive strength of SSC-FS was systematically investigated. Experimental results indicated that high mechanical strength can be achieved at ambient temperature by adjusting HG dosage and FS fineness. Increase in HG dosage postpones the initial formation of hydrates to some extent, but the amount of the final hydration products is promoted to generate higher mechanical strength. FS with smaller particle size has higher reaction activity that significantly accelerates the hydration process of SSC, leading to higher compressive strength and smaller volume expansion of SSC, and more ettringite crystals with smallish morphology formed in the pastes. If the particle size of FS is too small, however, ettringite crystals tend to precipitate at the surface of FS particles, which is unfavorable for the development of microstructure and will restrict the generation of C–S–H, thus resulting in a decrease in compressive strength and an increase in volume expansion.

Understanding the role of carbon nanotubes in low-carbon concrete: From experiment to molecular dynamics

Abstract: This research aligns with green and sustainable development principles, aiming to augment concrete's mechanical properties and durability. The low-carbon reactive powder concrete was developed by substituting ordinary Portland cement with sulfoaluminate cement. Additionally, an effective method was proposed to enhance the interfacial transition zone (ITZ), involving the pre-saturation of sand particles in a dispersion of carbon nanotubes (CNTs). The role of CNTs on the concrete's mechanical property, hydration process, and microstructure was extensively explored using experiments and MD (molecular dynamics) simulations. The results indicated a 27.8% and 10.1% increase in 28-day compressive strength, along with a 16.7% and 6.3% rise in 28-day flexural strength for the sample with pre-saturated sand dispersed in CNTs (CS2), in comparison to samples without CNTs (C0) and those with dispersed CNTs (C2). The primary enhancement mechanism is ascribed to the CNTs' ability to improve the ITZ through adsorption and nucleation. The CNTs facilitated the generation of AFt (ettringite) and AH3 (gibbsite), enhancing the hydration degree. This mechanism was corroborated by MD simulations that analyzed the effect of CNTs' on the mobility of calcium ions (Ca2+). When dispersed at the ITZ, CNTs had a seeding effect, leading to the generation of AFt and AH3 around the CNTs. This, in turn, reduced the ITZ's width and dramatically improved the bonding between the CNTs and the ITZ, thus effectively enhancing the ITZ's microscopic structure.

Degradation Mechanism of Coal Gangue Concrete Suffering from Sulfate Attack in the Mine Environment

Abstract: Recycling coal gangue as aggregate to produce concrete in situ is the most effective way to solve the problem of deposited coal gangue in mines. Nevertheless, the mine environment underground is rich in sulfate ions, posing a threat to the durability of coal gangue concrete (CGC). Hence, the degradation process of sulfate-attacked CGC is investigated. A series of tests is performed to evaluate its variation law of mass, dynamic elastic modulus, compressive strength and sulfate ion distribution. Meanwhile, the microstructure and phases of sulfate-attacked CGC are identified by scanning electron microscopy, X-ray diffraction and thermogravimetric analysis. The results indicate that the residual compressive strength ratio of CGC is higher than that of normal concrete after a 240 d sulfate attack, implying a superior sulfate resistance for CGC. Additionally, the higher the sulfate concentration, the more severe the degradation. Except for the secondary hydration of CGC itself, the diffused sulfate ions also react with Ca(OH)2, forming gypsum and ettringite; this plays a positive role in filling the pores at the early stage, whereas, at the later stage, the generated micro-cracks are detrimental to the performance of CGC. In particular, the proposed sulfate corrosion model elucidates the degradation mechanism of CGC exposed to a sulfate-rich environment.

Performance of High-Strength Concrete with the Effects of Seashell Powder as Binder Replacement and Waste Glass Powder as Fine Aggregate

Abstract: Seashell powder (SSP) is a waste from aquatic life that is generally available near the coastal region. Due to its high calcium content, SSP can be utilized as a supplementary cementitious binder. SSP can be used as a sustainable binder to replace ordinary Portland cement (OPC) and significantly reduce the carbon footprint. The present study investigates the effects of SSP and waste glass powder (WGP) on the fresh, mechanical, and microstructure properties of high-strength concrete (HSC). The SSP utilized in this research was varied, with 5%, 10%, and 15% cement replacement levels. The impact of WGP was also observed with two replacement levels, 5% and 10%, replacing natural sand. The slump flow of all the HSC mixes varied between 700 and 785 mm. A maximum compressive strength of 112.91 MPa was found for the C75SSP5 mix at 56 days. The split tensile strength values of all the HSC mixes were found in a range from 5.45 to 10.56 MPa. The modulus of elasticity values of all the HSC mixes were found to lie between 40.2 and 46.8 GPa. The lowest water absorption was observed in the mix containing 5% SSP. The SEM image of the HSC with increased SSP showed that it was denser and had fewer unreacted particles. XRD and EDS showed the presence of various gels, such as calcium silicate hydrates (CSHs), ettringite, calcium hydroxide (CH), and calcium carbonate (CC). The predicted equations for its split tensile strength, flexural strength, modulus of elasticity, and water absorption were also carried out in the present research.

Properties and microstructure of total tailings cemented paste backfill material containing mining and metallurgical solid waste

Abstract: Introduction: In order to improve the utilization efficiency of industrial waste discharged in the process of iron and steel metallurgy, a kind of material that can replace cement for mine filling is sought. In the test, steel slag (SS) and vanadium-titanium slag (VTS) were used as the primary raw materials to prepare cementing agents (CA). Then, combine it with vanadium-titanium iron ore tailings (VTIOTs) to make mine cemented paste backfill material (CPBM). Methods: The composition, properties and hydration mechanism of CPBM are studied through various tests, in-cluding mechanical property test, hydration heat test, X-ray diffraction (XRD), scanning electronic mi-croscopy (SEM), and fourier transform-infrared spectroscopy (FT-IR). Results: The results show that when the following conditions are met, the slump is 216 mm, and the 28-day flexural strength and compressive strength of CPBM reach 4.25 and 9.41 MPa, respectively, which meets the requirements of Chinese National Standard GB/T 39489-2020 Technical specification for the total tailings paste backfill: the mass percentage SS: VTS: phosphogypsum (PG): dicyandiamide waste slag (DWS) = 31:59:6:4; the content of compound phosphoric acid (CPA) accounts for 4% of the dry material; the cement sand ratio of CPBM is 1:4; the paste mass concentration (PMC) is 80%; and the content of water reducing agent (WRA) is 0.18%. Discussion: Mechanism studies show that the hydration product of the CA is mainly ettringite (AFt) and C-S-H gel. The addition of CPA promotes the hydration of active minerals in SS and VTS. The existence of PG pro-motes the formation of AFt, and the formation of AFt further promotes the fracture of [AlO4]5− and [SiO4]4− along the bridge oxygen in VTS and SS.

Portland/Sulfoaluminate Cement Blends for the Control of Early Age Hydration and Yield Stress

Abstract: Early age hydration and the rheological behavior of cement pastes are related to their reactivity. The reactivity of Ordinary Portland Cement (OPC) and calcium sulfoaluminate cement (CSA) blends, with a CSA percentage of less than 10%, are investigated in this paper. Different percentages of CSA replacement are studied. First, an isothermal calorimetry study of the different cement pastes is performed to understand the effect of CSA on the evolution of the heat of hydration. Then, in order to understand the reactional mechanism of these cement pastes, X-ray diffraction (XRD) and thermogravimetric analysis (TGA/DTG) studies were conducted on one blended mix made out of 7% CSA and compared to the pure cement pastes of OPC and CSA. Later, the evolution of the yield stress as a function of the CSA percentage was studied. A synergy is shown between both OPC and CSA once mixed together, leading to a 57% and 45% higher ettringite formation for the blend compared to 100% OPC and 100% CSA cement pastes, respectively. This implies that the CSA amount affects the reactivity of OPC/CSA blends. This was confirmed by the variation in both the heat of hydration and yield stress evolution as a function of the percentage of CSA.

Up to 100% Replacement of Natural Materials from Residues: Recycling Blast Furnace Slag and Fly Ash as Self-Leveling Cementitious Building Materials

Abstract: The objective of this research is to study the use in the construction industry of recycled slag (SL) and fly ash (FA) using from 0.1 to 5% calcium sulfate (wCS¯). These wastes have been used to make ternary mixture systems and evaluated in terms of technological properties as cementitious materials for building applications. Studying their micro-structure as hydration products, setting times and mechanical properties shows a way to develop new mixtures from high proportion of waste, which are alternatives to the traditional nature ternary systems: Portland cement (PC), calcium aluminate cement (CAC) and calcium sulphate (CS¯). Based on previous work with natural products, the selected SL/FA ratios were 9 and 2.3 and the sulphate contents were 0, 1 and 5%. The water/binder ratio used for these cementitious mixes was 0.4. The specimens prepared for strength determination were prisms of 10 × 10 × 60 mm. The pastes were prepared and cured at 20 °C and 98% relative humidity for 1 day and then either stored at 20 °C at 98% humidity (dry) or immersed in distilled water (wet) for 14 and 28 days. The results showed that both FA and SL mixed with CS¯ produce ettringite after 28 days of setting, and this phase was the main crystalline hydrated product in all mixes. Calcium sulphate stimulates the hydration reactions of the mixes and the strength increases when the CS¯ content is higher due to ettringite formation, while the setting time decreases, as happened in the systems prepared with natural materials.