AFm Phases Research Articles

The Effects of Combined Use of Sodium Citrate and PCE Plasticizer on Microstructure and Properties of Binary OPC-CAC Binder

This study examines the impact of sodium citrate and a plasticizing additive, along with their sequential introduction into a cement slurry or concrete mix, on the heat evolution of the cement slurry, the microstructure, phase composition of the cement paste, and the compressive strength of fine-grained concrete. The binder used in this research was a blended binder consisting of 90% Portland cement and 10% calcium aluminate cement. This type of binder is characterized by an increased heat evolution and accelerated setting time. The addition of sodium citrate at 5% of the binder mass alters the phase composition of newly formed compounds by increasing the quantity of AFt and AFm phases. The presence of sodium citrate significantly delays the hydration process of tricalcium silicate by a factor of 3.3. Initially, it accelerates belite hydration by 31.6%, but subsequently slows it down, with a retardation of 43.4% observed at 28 days. During the hardening process, the hydration of tricalcium aluminate and tetracalcium aluminoferrite is accelerated throughout the hardening process, with the maximum acceleration occurring within the first 24 h. During the first 24 h of hydration, the dissolution rates of tricalcium aluminate and tetracalcium aluminoferrite were 40.7% and 75% faster, respectively. Sodium citrate enhances heat evolution during the initial 24 h by up to 4.3 times and reduces the induction period by up to 5 times. Furthermore, sodium citrate promotes early strength development during the initial curing period, enhancing compressive strength by up to 6.4 times compared to the reference composition.

Improving properties of Portland cement-blast furnace slag blended paste through sodium bicarbonate-induced carbonation

The addition of NaHCO3 has the potential to enhance properties of concrete by facilitating internal carbonation. This study investigates the effects of the incorporation of NaHCO3 into ordinary Portland cement-blast furnace slag paste. The results show that the addition of NaHCO3 had an impact on the hydration process and increased the heat of hydration on all the slag samples. NaHCO3 caused internal carbonation and facilitated the formation of calcium carbonate and different carbon-based AFm phases, such as monocarbonate. The incorporation of NaHCO3 improved the pore structure of the paste and enhanced resistance against chloride penetration. The chloride migration coefficient of the Portland cement paste sample was reduced up to 96.5% with the incorporation of 2% NaHCO3 at 70% slag replacement. The compressive strength of the samples also showed an improvement with 1% NaHCO3 addition increasing the early age strength and 2% NaHCO3 addition increasing the matured age strength.

Utilizing spodumene slag as a supplementary cementitious material: A quantitative study

This study investigates the potential of spodumene slag (SPS) as a supplementary cementitious material (SCM). Firstly, the comprehensive characterization results show that SPS consists of quartz, gypsum, calcite, lazurite, and alumino-silicates, with 34 % amorphous content. Next, the hydration characterization indicates that SPS has limited self-cementing properties; however, it inhibits the initial dissolution of C3S, C3A, and C4AF due to the gypsum. This inhibition for C3S is counteracted by the dilution effect of SPS. Over time, the hydration degrees of the clinker phases improve as SPS provides additional nucleation sites and retains more free water for cement hydration. Additionally, thermodynamic modelling was established and validated by experimental data to capture the reaction process and phase changes. AFt forms in all systems, but AFm phases decrease and disappear at 40 % and 50 % SPS levels. Engineering tests reveal that 50 % SPS addition extends initial setting time by 60 %, and reduces fluidity by 20 %. Compressive strength remains acceptable up to 20 % SPS replacement, with an activity index of more than 80 %. Overall, SPS shows potential as a supplementary cementitious material, but careful adjustment of replacement levels is essential to minimize its disadvantages.

Aqueous carbonation of steel slags: A comparative study on mechanisms

This study investigated the aqueous carbonation mechanisms of three typical steel slags: ladle metallurgy furnace (LMF) slag containing high Al content, electric arc furnace (EAF) slag featuring high Si content and relatively low Al content, and ladle-arc fusion (LAF) slag with medium-Al content. It was found that the carbonation kinetics of the three slags were similar and followed the surface coverage model within the first 6 h of carbonation. Initially, the carbonation process was primarily governed by the reaction product precipitation. After 3 h of carbonation, the process was dominated by mineral dissolution, controlled by the uncovered reactive sites. The carbonation-reactive Ca-bearing minerals in the slags, including silicates (CRSis) and aluminates (CRAls), sequestered CO2 to form calcite during carbonation, accompanied by the formation of silica gel and alumina gel, respectively. CRSi, mainly larnite, was present in EAF and LAF slags, showing high reactivity, whereas mayenite (i.e., C12A7), a CRAl mineral present across all slags, exhibited high reactivity in LMF slag but lower reactivity in EAF and LAF slags. Furthermore, AFm phases and katoite (i.e., C3AH6) were detected in LMF slag as CRAls along with mayenite, and their carbonation reactivity decreased in the order of AFm>mayenite>katoite. As a result, low-Al steel slag tends to have higher carbonation reactivity, as manifested by the high carbonation degree of EAF slag throughout the reaction period, notably achieving a 40 % carbonation degree within 30 min and 71 % after 24 h under the studied conditions.

Hourly three-minute creep testing of an LC3 paste at early ages: Advanced test evaluation and the effects of the pozzolanic reaction on shrinkage, elastic stiffness, and creep

In this study, hourly three-minute creep testing is used to elucidate the evolution of the viscoelastic behavior of cement pastes produced with ordinary Portland cement (OPC), limestone Portland cement (LPC), and limestone calcined clay cement (LC3), from 1 to 7 days after production. An innovative test evaluation protocol, accounting for shrinkage, is used to identify values of the elastic modulus, the creep modulus, and the creep exponent, without making assumptions. The S-shaped shrinkage evolution of the LC3 paste is explained by Portlandite dissolution and the associated redistribution of chemical shrinkage-induced compressive stresses to the remaining solid skeleton. The evolution of the elastic stiffness of the LC3 paste is explained by space filling by C-A-S-H phases. The small creep compliance of the LC3 paste is explained by C-A-S-H which creeps less than C-S-H, and by AFm phases which precipitate in nanoscopic slit pores between C-S-H structures, gluing viscous interfaces.

Enhanced hydration reaction of synthesized C4A0.81F1.19 with the use of different grinding agents

Tetracalcium aluminoferrite (C4AF) presents in ordinary Portland cement (OPC) as a solid solution phase, as C4AxF2−x (0.7 ≤ x ≤ 1.1). There is a limited understanding of the hydration reactions of C4AF, particularly regarding the influence of alkanolamines (AA) on C4AF, which can substantially alter OPC hydration. In this study, synthesized C4A0.81F1.19 was subjected to grinding, using three different AA (triisopropanolamine, ethanol diisopropanolamine, and diethanol isopropanolamine) at dosages of 0, 0.1, and 0.3%. It was experimentally confirmed that the crystal structure of unhydrated C4AF was partially changed during the grinding process, and the hydration properties of C4AF were modified. From these results, the compressive strength improved significantly, and the rate of strength enhancement was the highest with 0.1% of diethanol isopropanolamine. More specifically, it was revealed that both Fe and Al sources were proficiently activated, leading to the production of Al/Fe-containing AFm phases and efficiently enhanced mechanical properties.

Exploring microwave activation as a novel method for activating coal gangue: Focus on microwave activation mechanisms and hydration characteristics of cementitious supplementary materials

To support the decarbonization of the cement industry, coal gangue (CG) has potential as a supplementary cementitious material. The calcination process significantly impacts the phase transformation of clay minerals and the pozzolanic activity of CG. This study involved calcination of CG at 600 ℃ and 800 ℃ using both microwave and thermal activation methods. Employ XRD, XPS, FTIR, TG, and SEM to examine the impact of calcination mechanism on the crystal structure, element binding energy, chemical bonds, microstructure, and mineral composition of CG. The results indicate that the transformation of kaolinite in CG to metakaolin is completed around 600 °C. Microwave activation increased the reactive aluminate content in CG, enhancing pozzolanic activity and promoting secondary hydration with cement. This process generated additional C-(A)-S-H gels and AFm phases, which filled matrix pores, increased system density, and improved mechanical properties. The optimal activation method for CG was identified as microwave activation at 600 °C.

Exploring the electronic, magnetic and thermoelectric properties of TbPtBi half-Heusler: DFT study

Abstract In this investigation, we employed density functional theory to scrutinize the structural, electronic, magnetic, thermoelectric, and phonon properties of the topological half-Heusler (HH) TbPtBi compound. The stable phonon dispersion spectrum affirms the dynamical stability of the compound. The inclusion of spin-orbit coupling (SOC) significantly influenced the compound’s electronic and thermoelectric properties. The density of state (DOS) confirmed the impact of SOC on the topologically non-trivial metallic behavior of TbPtBi under the equilibrium lattice constant. The SOC altered the DOS at the Fermi level, leading to band splitting and a notable 70% reduction in state density. The Tb-4f electrons in the compound induce total magnetization in AFM (−5.93 µB/cell) and FM (5.94 µB/cell) phases, while SOC eliminates this magnetization. The thermoelectric performance of TbPtBi under compressive and tensile strain has been systematically studied. The result indicate that compressive strain causes a notable increment in Seebeck coefficient and Power factor (20.4 × 1011 W K−2 m−1) of this compound at room temperature. High thermoelectric performance under compressive strain in the HH compound TbPtBi might open new avenues for investigating other topological thermoelectric materials.

Sorption of Se(VI) and Se(IV) on AFm phases

Selenate and selenite can sorb on AFm phases such as monosulfate, monocarbonate or hemicarbonate. Sorption and co-precipitation experiments were performed to determine their potential for the immobilization of two of the main aqueous species of selenium: SeVIO42− and SeIVO32−. A preferential uptake by hemicarbonate over monosulfate, and a much weaker sorption on monocarbonate was observed for both Se(IV) and Se(VI). Increased sulfate and carbonate concentrations as present at higher pH lowered the uptake of Se(VI) and Se(IV).Experimentally obtained sorption isotherms on hemicarbonate and monosulfate were reproduced with thermodynamic solid solution models, considering both surface sorption and interlayer bonding. For monocarbonate interlayer sorption was suppressed by the rigid structure and narrow interlayer distance. The observed limited uptake could be described by an anion exchange model on the outer surfaces only. This study shows that both anion exchange in the interlayer as well as sorption on outer surface sites can determine Se uptake by AFm phases.

Multi-objective optimization of seawater mixing in cement-based materials with supplementary cementitious materials using response surface methodology

Seawater instead of freshwater to produce concrete is an effective measure to save freshwater resources. However, the presence of harmful ions in seawater restricts its broader application. The incorporation of supplementary cementitious materials (SCMs) has the potential to significantly mitigate these adverse effects. Consequently, there is a growing interest in understanding the mechanisms by which SCMs function in seawater-mixed cementitious systems. In this study, a quadratic polynomial prediction model with the mechanical properties and dry shrinkage of the composite matrix as the response target was analyzed by the response surface method (RSM). Tests and analytical techniques elucidated the effects of metakaolin (MK) and ground granulated blast-furnace slag (GGBS) on the hydration process of the pastes after seawater mixing. The results revealed that the compressive strength and dry shrinkage were influenced by single-factor or multi-factor interactions. The optimal mix design (MK=20.2 % and GGBS=10.0 %) was experimentally validated, showing an absolute error of 1.5 %. The microstructural analysis indicated that MK and GGBS expedited essential pozzolanic reactions. Enhanced strength in mixtures before 3 d was attributed to cation exchange and initial flocculation. In the 28 d specimens, the increase in strength was related to the growth of Friedel’s salt crystals and C-(A)-S-H gels. Due to the presence of the C-(A)-S-H gels and AFm phases, chloride ions were efficiently immobilized, and the shrinkage of the matrix was reduced.

Gluconate and formate uptake by hydrated cement phases

The uptake of formate and gluconate by C–S–H, AFm phases, ettringite and hydrated Portland cement (PC) was studied at pH 13 by batch sorption experiments. The formation of gluconate and formate-AFm phases was observed in pure systems, but not in hydrated cement. Gluconate sorbs more strongly on AFm phases and ettringite than formate. Higher calcium concentrations increase the gluconate sorption on C–S–H and hydrated Portland cements due the formation of Ca-gluconate surface complexes on C–S–H. Measured Rd values for gluconate sorption on C–S–H increase from 2.0 dm3‧kg−1 for C–S–H with Ca/Si = 0.8 to 34 dm3‧kg−1 for Ca/Si = 1.4 at pH 13. They are a factor 5 to 10 higher at lower pH values, and higher Ca-concentrations. Calcium concentration does not significantly affect the uptake of formate by C–S–H. Formate sorbs on hydrated PC with Rd values in the range of 3–33 dm3‧kg−1.

Carbonated calcium silicates as pozzolanic supplementary cementitious materials

This study examined the carbonation mechanisms and products of various precursors, including hydrated cement pastes and calcium silicate clinkers. Regardless of the materials investigated, all exhibited rapid carbonation reactions and achieved high degrees of carbonation. Calcium carbonate and siliceous gel were consistently formed as carbonation products. The starting materials significantly influenced the characteristics of the siliceous gel. Carbonated concrete pastes displayed a higher presence of alumina in the gel, resulting in greater surface area and nano-porosity compared to gels formed from calcium silicate clinkers. The pozzolanic reaction products also differed depending on the precursor, with carbonated concrete pastes forming C-S-H and AFm phases, while carbonated calcium silicate clinkers produced only C-S-H phase. Such variations impacted the evolution of mechanical performance in composite cements. Notably, carbonated composite cements containing recycled concrete pastes exhibited higher early compressive strength compared to carbonated calcium silicate clinkers. However, the final compressive strength is similar.

Enhancing fluidity and mechanical properties in Limestone Calcined Clay cements with one-third Portland clinker content

Limestone Calcined Clay Cements (LC3) are attracting considerable attention due to their low CO2 footprints and relatively fast hydration kinetics. LC3-50 formulations (approximately 50 % Portland clinker, 30 % kaolinitic calcined clay, 15 % limestone and 5 % gypsum) are being extensively investigated and the hydration chemistry, rheology, mechanical strength developments and durability performances are starting to be well established. LC3 binders with lower clinker factors are possible, offering an opportunity for further CO2 footprint reduction. However, these blends have been much less studied. This work contributes to fill this gap. Here, a LC3-35 binder (i.e. 35 % Portland clinker content) is investigated and its performances are compared to those of the original Portland cement (PC) and a standard LC3-50 binder. The effects of two superplasticizers and a strength-enhancing admixture are thoroughly investigated in mortars and pastes. Larger calcined clay contents result in an exacerbation of the slump retention problem, but this issue is solved by the use of a new superplasticizer. Moreover, the low mechanical strengths at early ages can be partly mitigated with a C-S-H nucleation seeding admixture. Employing the right admixtures, LC3-35 binders can develop strength values similar to those of neat PC and LC3-50 at 7 and 28 days. The laboratory X-ray powder diffraction results for LC3-35 pastes, when using the strength-enhancing admixture, show that C4AF exhibits an accelerated reaction rate, leading to the formation of larger amounts of AFt and AFm phases. Approximate mass balance calculations are carried out to estimate the metakaolin reaction degree. Finally, an environmental performance indicator is used to place the footprints of the reported binders within the LC3 landscape.

Sorption of 32Si and 45Ca by isotopic exchange during recrystallisation of cement phases

The uptake kinetics of 32Si and 45Ca on cement minerals including C-S-H phases, portlandite, AFm phases, ettringite, and aged hardened cement paste were determined through batch sorption experiments. A two-step uptake kinetics was observed, with a fast initial step during ∼1 day followed by a much slower second step, not yet completed after one year. The working hypothesis that the fast uptake is caused by exchange of the radioisotopes with stable isotopes adsorbed on the mineral surface, whereas the slow uptake step is due to uptake in the crystal lattice during recrystallisation, was tested with the help of phenomenological models that combine surface adsorption and homogeneous recrystallisation. The experimental data could be reproduced satisfactorily using a refined version of the formerly published continuous homogeneous recrystallisation (CHOR) model, supporting the working hypothesis and allowing equilibrium sorption coefficients (Rd; L kg−1) for these radionuclides to be calculated on a mechanistic basis. This provides insight into the intrinsic rates and mechanisms of interaction between cementitious materials and their pore fluids which contain dissolved calcium and silicon.

Observation of pressure-induced variation in magnetization and magnetic anisotropy for half-doped La0.5Ca0.5MnO3 nanoparticles

The effects of pressure on the magnetic properties of half-doped La0.5Ca0.5MnO3(LCMO) were investigated. Pressure was found to decrease the transition temperature (Tc) from the paramagnetic (PM) phase to the ferromagnetic (FM) phase, from 226 K to 223 K. Moreover, the pressure was also involved in decreasing the transition temperature (TN) for the FM-antiferromagnetic (AFM) phase from 180 K to 175 K. Below TN, the inhomogeneous AFM phase, where both the FM and AFM phases coexist, is observed in LCMO. Pressure induces changes in the zero-field-cooled magnetization (ΔMZFC) exhibiting ΔMZFC ≈ 0 in the PM region above 285 K.

Properties of saline soil stabilized with fly ash and modified aeolian sand

Against the backdrop of saline soil solidification and the resource utilization of solid waste and aeolian sand in cold and arid regions, this study employs locally accessible fly ash and aeolian sand to solidify saline soil. By combining unconfined compressive strength tests, X-ray diffraction analysis, scanning electron microscopy, orthogonal experiments, and single-factor analysis, the strength characteristics, mineral composition, and interfacial structure changes of saline soil solidified with different freeze-thaw cycles and varying amounts of fly ash, aeolian sand, and alkali activators were investigated. The effects of each factor were analyzed to determine the optimal mixture ratio and to explore the solidification mechanism.The results indicate that the unconfined compressive strength of saline soil is most significantly enhanced when solidified with a combination of fly ash, aeolian sand, and alkali activators. The optimal mixture ratio was found to be 24 % fly ash, 7 % aeolian sand, and 4.5 mol/L alkali activator. With the incorporation of these solidifying materials, the failure mode of saline soil transitions from plastic to brittle, and the stress-strain curve exhibited a strain-softening behavior. The combined solidification method demonstrated the most pronounced effect in mitigating freeze-thaw damage, with the unconfined compressive strength of the solidified soil reaching 7.01 MPa after seven freeze-thaw cycles, compared to 0.03 MPa for the untreated soil, an increase by a factor of 234.This significant enhancement is attributed to the formation of substantial gel substances, which mitigate the strength loss caused by freeze-thaw cycles. The gel locking mechanism between particles in the solidified soil far exceeds the detrimental effects of freeze-thaw cycles, effectively inhibiting freeze-thaw deterioration. Additionally, the reaction pathways involving AFt and AFm phases reduce the content of SO42- and Cl- in the solidified soil, effectively suppressing salt expansion and significantly improving the soil's strength.

Molecular dynamics simulation of hydrocalumite as adsorbent for anionic radionuclides

Hydrocalumite, is a hydration product of aluminum-rich cements, and is known in cement chemistry as an AFm phase. Structurally, it belongs to the family of layered double hydroxides, or “anionic clays”, where positively charged crystal layers require the presence of negatively charged ions in the interlayer space. Therefore, AFm phases can serve as potential adsorbents for anionic radionuclides (e.g., 35Cl−, 125I−, 129I−, 131I−) from aqueous solutions. Here we use classical molecular dynamic simulations to analyze the structure and properties of AFm phases containing Cl− and I−. The classical ClayFF force field is used to quantitatively study the structure, energetics and mobility of anions and H2O molecules in the interlayers of these phases and at their interfaces with CsCl and CsI aqueous solutions. In this study we report that the basal (001) surfaces of AFm phases can strongly adsorb hydrated Cl− and I− anions due to the donated hydrogen bonds from the interfacial hydroxyls, but primarily due to their strong attraction to the structural Ca cations exposed at the surface. However, our simulations show that the adsorption of I− is weaker than that of Cl−, leading to the higher surface mobility of I− due to its stronger chaotropic effect. The interlayer diffusional mobility of the Cl− and I− anions in the AFm phases is also investigated by using the Eyring-Vineyard approach and is shown to be significantly lower than in larger nanopores. Hence, the most likely transport of such anionic radionuclides takes place through the nano- and micro-pores of hardened cement.

Pozzolanic activity of FCC catalyst waste slag (CWS) for cement and geopolymer production

Catalyst waste slag (CWS) is generated in large amounts when fabricating the catalyst required in the fluid cracking catalyst (FCC) process used for oil refining. Currently, CWS is landfilled. This paper characterizes the CWS and measures its pozzolanic activity. It determines whether the CWS can be used as partial Portland cement (PC) replacement for the first time in the literature. CWS is primarily composed of Al2O3 and CaO. It contains a negligible quantity of heavy metals that can be immobilised in a matrix. The raw CWS is totally amorphous and extremely reactive which results in flash set. Calcination is an effective method to control the reactivity of CWS. Reactivity increased when CWS was calcined between 300 and 600 °C and it peaked at 500 °C, but this caused flash set. Calcination at 800 °C lowers reactivity (as the highly disordered aluminium phases achieve a greater order) which allows for proper handling. Temperatures over 800 °C caused partial crystallization significantly lowering reactivity.The 800°C-calcined CWS reached high mechanical index (6.25), comparable to other pozzolans such as fly ash and red mud, and greater than alum sludge. CWS combines lime profusely and develops more abundant cementing hydrates than similar pozzolans such as calcined alum sludge. The pozzolanic reaction of the 800°C-calcined CWS provides abundant cementing minerals including AFt and AFm phases, calcium aluminium carbonate hydrates and calcium aluminium hydrates.The high reactivity of CWS and its prolific production of cementing phases in pozzolanic reactions indicate that it can be used as a supplementary cementitious material in PC and lime systems; and that it can be used as a precursor to produce low-carbon and geopolymer cements. CWS constitutes a reactive aluminium source which, in a PC system, participates in hydration reactions and can enhance the properties of the resultant materials.

Chloride binding by layered double hydroxides (LDH/AFm phases) and alkali-activated slag pastes: an experimental study by RILEM TC 283-CAM

Chloride binding by the hydrate phases of cementitious materials influences the rate of chloride ingress into these materials and, thus, the time at which chloride reaches the steel reinforcement in concrete structures. Chloride binding isotherms of individual hydrate phases would be required to model chloride ingress but are only scarcely available and partly conflicting. The present study by RILEM TC 283-CAM ‘Chloride transport in alkali-activated materials’ significantly extends the available database and resolves some of the apparent contradictions by determining the chloride binding isotherms of layered double hydroxides (LDH), including AFm phases (monosulfate, strätlingite, hydrotalcite, and meixnerite), and of alkali-activated slags (AAS) produced with four different activators (Na2SiO3, Na2O·1.87SiO2, Na2CO3, and Na2SO4), in NaOH/NaCl solutions at various liquid/solid ratios. Selected solids after chloride binding were analysed by X-ray diffraction, and thermodynamic modelling was applied to simulate the phase changes occurring during chloride binding by the AFm phases. The results of the present study show that the chloride binding isotherms of LDH/AFm phases depend strongly on the liquid/solid ratio during the experiments. This is attributed to kinetic restrictions, which are, however, currently poorly understood. Chloride binding by AAS pastes is only moderately influenced by the employed activator. A steep increase of the chloride binding by AAS occurs at free chloride concentrations above approx. 1.0 M, which is possibly related to chloride binding by the C–(N–)A–S–H gel in the AAS.

Solid solutions among cement AFm phases containing nitrate and nitrite ions

The AFm phase found in hydrated Portland cements refers to a family of calcium aluminate phases. Their layer structure is derived from that of portlandite (calcium hydroxide (Ca(OH)2)), but with one third of the Ca2+ ions replaced by a trivalent ion, nominally Al3+ or Fe3+. The resulting charge imbalance gives the layers a positive charge, which is compensated by intercalated anions; the remaining interlayer space is filled with water (H2O). Hydrated cements will contain mixtures of AFm phases and their solid solutions. Portland cement is often modified by addition of soluble nitrate or nitrite salts to protect embedded steel or to shorten the setting time. These nitrate and nitrite ions are capable of entering and occupying AFm anion sites, hence impacting overall phase balances between cement hydrates. In this study AFm phases and solid solution formations were investigated between AFm phases containing SO42−, CO32−, OH−, Cl− and NO2− NO3− ions. Samples were synthesised and subsequently characterised using X-ray diffraction and through the measurement of aqueous solution compositions after 180 days of equilibration in water.