Search

This study focused on the synthesis of aragonite whiskers through a synergistic wet carbonation technology utilizing waste magnesia slag (MS) and magnesium sulfate (MgSO4), aiming to improve the microstructure and mechanical properties of ordinary Portland cement (OPC) paste. The influence of MgSO4 concentration on the wet carbonation process, phase composition, and microstructure of MS was investigated. Furthermore, the effect of incorporating carbonated MS (C-MS) on the mechanical properties and microstructure of Portland cement paste was evaluated. Results showed that appropriate MgSO4 concentrations favored aragonite whisker formation. A concentration of 0.075 M MgSO4 yielded 86.6% aragonite with high aspect ratio nanofibers. Incorporating 5% of this C-MS into OPC increased the seven-day compressive strength by 37.5% compared to the control OPC paste. The improvement was attributed to accelerated hydration and reduced porosity by the filling effect and microfiber reinforcement of aragonite whiskers. MS demonstrated good CO2 sequestration capacity during carbonation. This study provides an effective method to synthesize aragonite whiskers from waste MS and use it to enhance cementitious materials while reducing CO2 emissions, which is valuable for the development of a sustainable cement industry. [ABSTRACT FROM AUTHOR]

Synthesized nanocalcium-silicate-hydrate (C─ S─ H) was successfully employed as an accelerator in portland cement. C─ S─ H nanomaterials were produced in this study utilizing a process that used separate nucleation and growth steps with varied solvent compositions. The effects of the water/alcohol volume ratio (W/A) in the combined solvent on the calcium silicon ratio, particle size, morphology of C─ S─ H nanomaterials, and compressive strength of portland cement were studied. The results reveal that when the W/A ratio decreased, the calcium to silicon (Ca/Si) molar ratio reduced, and the morphology of the C─ S─ H nanomaterials turned from micalike to mixed lamellar and granular, then to granular with a wrinkled surface, and finally to foillike. When the W/A was 3∶1 , the small particles (100 nm) of C─ S─ H nanomaterials had the highest W/A, whereas the large particles (>200 nm) had the highest W/A. C─ S─ H nanomaterials offered calcium silicate hydrate nucleation sites, altered C─ S─ H gel development modes, and enhanced the number of hydration products. The composition, particle size, and shape of C─ S─ H nanomaterials all influenced the compressive strength of the cement paste. [ABSTRACT FROM AUTHOR]

The use of industrial solid waste to produce a cement clinker can not only effectively reduce the consumption of natural resources, but also reduce the pollution of waste. In this paper, the solidification trend of Cr ions in the clinker phases of ordinary Portland cement (OPC) was studied by combining experiments with theoretical simulations. The results from energy dispersive spectrometry, X-ray diffraction, and density functional theory (DFT) simulations suggests that Cr ions will preferentially enter the mineral C4AF by displacing Fe ions. Further analysis of the bond length and bond order, electron density differences and partial density of states suggests that this phenomenon is mainly due to the similar electronic contributions of the host and guest ions. These insights will provide important fundamental guidance for understanding the doping behaviour of chromium ions in an OPC clinker and the utilization of Cr-containing industrial solid waste, thus paving an efficient way for the treatment of Cr-bearing industrial solid waste. [ABSTRACT FROM AUTHOR]

Ordinary Portland cement (OPC) clinker mainly consist four minerals, tricalcium silicate (C 3 S), dicalcium silicate (C 2 S,), tricalcium aluminate (C 3 A), and tetracalcium aluminoferrite (C 4 AF). To learn the doping behaviors of Zn in OPC clinker, a series of samples were prepared by calcinating the mixtures of CaCO 3 , SiO 2 , Al 2 O 3 , Fe 2 O 3 , and ZnO. Our results from energy-dispersive spectroscopy, X-ray diffraction and density functional theoretical simulations show that a small amount of ZnO enter C 3 S and C 2 S by replacing Ca ions while most incorporate into C 4 AF by substituting Fe atoms, resulting in a decrease of C 3 A in OPC as dosage increases. Further analyses from partial density of states and distributions of bond order-bond length indicate that the doping preference can be ascribed to the similar electron contributions and small structure distortions between host and guest ions. Unlike the strong Fe‒O bond, the newly formed Zn‒O is much weaker. The weak Zn‒O may be responsible for the limited solubility of Zn in C 4 AF. These results provide a possibility of increasing solubility of Zn in OPC clinker by increasing the contents of C 3 A and C 4 AF, thus will be very meaningful in the synthesis of OPC clinker by utilizing Zn-bearing alternative raw materials. ga1 • This work systematically studied the doping behaviors of Zn in OPC clinker phases. • Most Zn incorporate into C 4 AF by substituting Fe while few enter silicate phases. • The similar electron contributions are responsible for doping behaviors of Zn. • The small distortions between Fe-O and Zn-O also favor the doping behaviors of Zn. [ABSTRACT FROM AUTHOR]

The doping behaviors of Pb in sulfoaluminate cement (SAC) clinker phases were systematically studied combined with density functional theoretical simulations and experiments. The results present that, in the three composed minerals of C4A3 S , C2S, and C4AF, Pb ions prefer to incorporate into C4A3 S by substituting Ca ions. Further analyses from partial density of states, electron density difference, and local distortions show that such doping preference can be attributed to the small distortions as Pb introduced at Ca sites of C4A3 S. The results and clear understandings on the doping behaviors of Pb ions may provide valuable information in guiding the synthesis of Pb-bearing SAC clinker, thus should draw broad interests in fields from sustainable production of cement and environmental protection. [ABSTRACT FROM AUTHOR]

Sulfoaluminate cement-based materials (SCBMs) are widely used for grout reinforcement. It is common to use a large water-to-binder ratio in the construction of sulfoaluminate cement-based materials (SAGMs) to obtain good workability and permeability, which results in poor early strength. Nanotechnology has been increasingly employed to enhance the mechanical properties of cementitious materials. Nano Zn-Mg-Al hydrotalcite-like (LDHs) and nano Mg-Al LDHs increased the early compressive strength, while nano Zn-Al LDHs had the opposite effect. However, the reason for this observation remains unclear. In this study, the influence of LDHs structures with different laminar element compositions on the hydration and hardening processes of SCBMs was analysed. The results showed that nano-ZnAl-LDHs, ZnAl-LDHs, and MgAl-LDHs could provide nucleation sites for ettringites. The three hydrotalcite-like materials slowly released different concentrations of zinc and/or magnesium ions and carbonate ions in the cement paste. Zinc ions entered the hydration product of ettringite and aluminum gel and promoted the transformation of amorphous aluminum gel to bayerite, which inhibited the hydration reaction of SCBMs. Magnesium ions could also enter the ettringite, and carbonate ions promoted the formation of calcium carbonate, which facilitated the hydration reaction of SCBMs. The nucleation effect and the release of zinc and/or magnesium ions and carbonate ions acted synergically to influence the hydration rate, pore size distribution, and compressive strength of SCBMs. • Nano MgAl-LDHs, ZnMgAl-LDHs and ZnAl-LDHs could provide nucleation sites for AFt. • The three LDHs released different concentrations of Zn2+ and/ or Mg2+ and CO 3 2-. • Cement hydration was affected by both nano crystal nucleus and the released ions. [ABSTRACT FROM AUTHOR]

The low strength of hemihydrate phosphogypsum (HPG) limits its wider applications in construction, and the use of alkali-activated mineral admixtures to improve HPG's mechanical properties has enormous environmental benefits. In this paper, slag and fly ash were added to HPG, which was then activated by NaOH. The effects of alkali activation on the mechanical properties of HPG were investigated, and the hydration mechanism of modified and enhanced HPG was investigated by X-ray diffractometer (XRD), Thermogravimetric analysis (TGA), Fourier-transform infrared (FTIR), and Low-field nuclear magnetic resonance (LNMR). The results showed that alkali-activated mineral admixtures can improve the compressive strength of HPG up to 40 MPa with the optimal mix ratio: HPG:(GGBS + FA) = 60:40 by weight, where the ratio of GGBS to FA is 4:1 by weight, and NaOH accounts for 1% by mass of the binder. When NaOH was added at 1 wt% of binder, it could effectively activate slag and fly ash to enhance the mechanical properties of HPG. With the increase of the dosage of NaOH, settings of HPG were prolonged and its strength was reduced. The hydration products such as ettringite (AFt) and C–S–H gel generated by NaOH-activated slag and fly ash can effectively fill the pores of HPG paste and thus improve its mechanical properties. When hydrated for 28d, the decrease in strength due to the increase in NaOH dosage can be attributed to the decrease in AFt generated from hydration. The findings of this study pave the theoretical and technical foundations for greater and wider utilization of phosphogypsum for a sustainable future. [Display omitted] • It is feasible to use NaOH as an activator to enhance hemihydrate phosphogypsum modified with GGBS and FA. • Mechanism of improving mechanical strength of HPG revealed by XRD, TGA, FTIR, LNMR analysis. • Decrease in ettringite is the main reason for the decrease in strength of HPG when hydrate for 28d. [ABSTRACT FROM AUTHOR]

This study focuses on industrial waste magnesium slag (MS) with a high MgO content. It simulates different temperatures of cement kiln exhaust gas during wet carbonation. The temperatures studied were 40 °C, 60 °C, and 80 °C. The aim was to investigate the influence of temperature on the growth and morphology of carbonation products. The results indicate that increasing the temperature leads to the formation of different types and quantities of carbonation products in MS. Additionally, carbonation reactions at higher temperatures result in smaller particle size, larger specific surface area, and more gel pores within the MS structure. As the temperature increases, silica gel generation is promoted, and aragonite gradually becomes the dominant crystal phase, with increased crystallites size and crystallinity. At 80 °C, the length of aragonite ranges from 3 to 5 µm, with a diameter smaller than 0.5 µm. Then, the carbonated MS prepared at 40 °C was added to ordinary Portland cement with 2% replacement amount, and it was found that the compressive strength of the cement paste could be increased to 50 MPa after 28 days. This study provides a new approach to the high-value recovery and utilization of MS. • Wet carbonation of magnesium slag produced aragonite whiskers by tuning temperature (80°C optimal). • 40°C carbonation achieved highest carbonation degree and CaCO 3 content in magnesium slag. • 2% addition of 40°C carbonated slag enhanced cement paste strength to 50 MPa at 28 days. [ABSTRACT FROM AUTHOR]

Desulfurization gypsum, the byproduct from wet flue gasdesulfurization,and red mud, from the production of aluminum oxide, are two bulk industrialsolid wastes that trigger many local environmental problems in China.This study aims to jointly utilize them. Through experimentation andmodeling using FactSage, it has been found to be feasible to preparesulfoaluminate cement using these wastes. The calcination temperaturein the preparation was as low as 1250–1300 °C, and themain mineral phases of the cement clinker were 3CaO·3Al2O3, CaSO4, β-2CaO·SiO2, and 2CaO·Fe2O3. The cement clinkerstested showed excellent mechanical strength performances. This processwas found to be an efficient way to consume industrial solid wastes,with the total proportion of desulfurization gypsum and red mud over70–90% by mass in the raw materials. The sulfoaluminate cementproducts have outstanding cost superiority over Portland cement becauseof their low material costs, low material pretreatment costs, andlow calcination temperature. Moreover, this technology could bringabout immense environmental and social benefits in terms of wasteconsumption, energy conservation, and CO2reductions. Thistechnology has considerable prospects, and it is worth undertakingfurther research into its potential applications. [ABSTRACT FROM AUTHOR]

With the growing awareness of CO 2 emissions reduction in the cement industry, minerals like γ-C 2 S with lower calcium content and formation temperatures have shown potential as sustainable alternatives to ordinary Portland cement. Although γ-C 2 S possesses considerable carbonation reactivity, incorporating additives can further optimize its carbonation hardening performance. This study investigated the effect of different dosages of internally incorporated MgCl 2 on the carbonation behavior of γ-C 2 S. Test results demonstrated that incorporating 1.87 % MgCl 2 led to the maximum compressive strength and CO 2 uptake. The compressive strength and CO 2 uptake following 24 h of carbonation increased from 63.2 MPa and 18.23 % to 84.2 MPa and 23.95 %, respectively. XRD and TG analyses indicated that Mg2+ promoted the transformation of amorphous calcium carbonate into more stable crystalline calcite and the formation of magnesium calcite and nesquehonite. SEM imaging revealed that the generated carbonation products tightly filled the pores, enhancing the compactness and strength. However, excessive MgCl 2 coating on particle surfaces hindered further carbonation. These findings reveal opportunities to further optimize the carbonation hardening properties of calcium silicate minerals and precisely tailor the morphology of calcium carbonate products for sustainable construction. • MgCl 2 incorporation accelerates early carbonation stages of γ-C 2 S, enhancing CO 2 uptake and strength. • Trace MgCl 2 transforms ACC into calcite, increasing carbonation product crystallinity. • MgCl 2 induces the formation of Mg-calcite and nesquehonite, improving microstructure. [ABSTRACT FROM AUTHOR]

• A novel self-pulverized low-calcium clinker primarily mineral of γ-C 2 S was designed. • Self-pulverized low-calcium clinker exhibits excellent carbonation activity. • Self-pulverized low-calcium clinker reduces CO 2 emissions and energy consumption. To reduce energy consumption and greenhouse gas emissions in the cement industry, this study develops a novel low-calcium clinker with self-pulverization properties. The clinker is synthesized from limestone and sandstone raw materials and comprises dicalcium silicate (2CaO·SiO 2 (C 2 S)) as the primary mineral and rankinite (3CaO·2SiO 2 (C 3 S 2)) as the auxiliary mineral. The influence of calcination temperature, cooling rate, and holding time on clinker pulverization and mineral formation is investigated through a series of experiments. The optimal calcination condition involves heating at 1240 °C for 1 h, followed by natural cooling, resulting in a self-pulverized low-calcium clinker (SPLCC) with main mineral is γ-C 2 S and contains β-C 2 S, C 3 S 2 , and C 2 AS (Calcium alumina feldspar (2CaO·Al 2 O 3 ·SiO 2)). The self-pulverized clinker exhibits a pulverization ratio of 93.0 %, a specific surface area of 666.4 m2/kg, and an average particle size of 8.2 μm. Additionally, the clinker displays excellent carbonation reactivity, with the compressive strength reaching 69.3 MPa and 82.2 MPa after 4 h and 24 h of CO 2 curing, respectively. The carbonation products are identified as calcite and silica gel. SPLCC can reduce and sequester approximately 277.3 kg of CO 2 emissions compared to the hydration of Portland cement clinker when considering the preparation and curing stages. The results highlight the potential of SPLCC as a sustainable cementitious material. [ABSTRACT FROM AUTHOR]

A systematic study was performed to learn the exact existence form and doping behaviors of magnesium in ordinary Portland cement (OPC) clinker. Our results show that about 1.3 wt% MgO incorporates into OPC clinker phases by forming substitutional solid solutions while excess Mg accumulate and exist as periclase. Results from Rietveld refinement present a substitution preference in C 3 S and C 3 A over C 2 S. More in-depth analyses from density function theoretical simulations show that in C 3 S, C 2 S and C 3 A, the substitution occurs by replacing Ca while in C 4 AF, it probably occurs by substituting Fe and Ca ions. The large structure distortions and sharp increase in formation energies with increasing MgO determines its low solubility. This work provides a clear understanding of the existence states and the intrinsic mechanism governing doping behaviors of Mg, thus should be very important in guiding the synthesis of OPC clinker by utilizing high-Mg limestone. [Display omitted] • The doping behaviors of Mg in OPC clinker phases were systematically studied. • About 1.3 wt% MgO enter OPC clinker phases by forming substitutional solid solutions. • In the 4 composed minerals, C 3 S, C 3 A and C 4 AF can host more Mg ions than C 2 S. • Mg ions prefer to replace Ca in C 3 S, C 2 S and C 3 A while replace Fe and Ca in C 4 AF. • Both the large structure distortions and formation energies governs the low solubility. [ABSTRACT FROM AUTHOR]