Sorption of reactive dyes from aqueous solutions by ordered hexagonal and disordered mesoporous carbons

In the present study two synthetic mesoporous carbons, a highly ordered CMK-3 sample with hexagonal structure and a disordered mesoporous carbon (denoted DMC) were investigated for the sorption of Remazol Red 3BS (C.I. 239) dye in comparison to three commercial activated carbons and a HMS mesoporous silica with a wormhole pore structure. The structural, porosity and surface characteristics of the materials were evaluated using XRD, TEM, N₂ porosimetry, FT-IR spectroscopy and zeta-potential measurements. Optimal dye sorption occurred at pH ～2. Equilibrium sorption data followed the Langmuir model and showed that the two synthetic mesoporous carbons exhibit higher sorption capacities (q_max ～ 500–580 mg/g at 25 °C) in comparison to the commercial activated carbons which possessed either microporous (Takeda 5A and Calgon carbon) or combined micro-/mesoporous (Norit SAE-2) structures and to the HMS mesoporous silica. Thermodynamic parameters as the change in free energy, enthalpy, and entropy of sorption were also estimated. Kinetic studies were carried out and showed a rapid sorption of dye in the first ca. 30 min while equilibrium was reached after ca. 3 h. The sorption kinetics of dye was best described by a second-order kinetic model. A surfactant enhanced carbon regeneration (SECR) technique was used to regenerate the dye-loaded carbon sorbents.     

Evidence for anomalous water vapor sorption kinetics in cement based materials

We used dynamic sorption balance measurements to evaluate the diffusivity for cement pastes with three different binders (OPC, OPC + 70% slag, OPC + 10% silica fume). The diffusion of water vapor in cement based materials is normally assumed to follow Fick's law of diffusion, but our results clearly show that Fick's law cannot completely describe the sorption process in our materials. In this paper we report the evidence for this anomalous sorption behavior and discuss a possible method to evaluate diffusivities from such measurements.     

Use of glass waste as an activator in the preparation of alkali-activated slag. Mechanical strength and paste characterisation

The alkaline activation of aluminosilicates yields alkaline cements, eco-efficient alternatives to ordinary Portland cements. Alkaline cements and concretes exhibit highest strength and longest durability when activated with a solution of alkaline silicate hydrates (waterglass). To obtain these alkaline silicates, however, an aqueous solution of the proper proportion of carbonate and silica salts must be heated to temperatures of around 1300 °C. The present paper explores the feasibility of using urban and industrial glass waste as a potential alkaline activator for blast furnace slag (AAS).AAS pastes were prepared with three activators: waterglass, a NaOH/Na₂CO₃ mix and the solutions resulting from dissolving glass waste in NaOH/Na₂CO₃. Mechanical, mineralogical (XRD, FTIR) and microstructural (porosimetry, NMR and SEM/EDX) trials were conducted to characterise the pastes obtained.The findings proved the feasibility of using glass waste to alkali activate slag. Treating glass waste with NaOH/Na₂CO₃ (pH = 13.6) favours the partial dissolution of the Si in the glass into its most reactive monomeric form.The solutions resulting from the treatment of glass waste act as alkaline activators, partially dissolving vitreous blast furnace slag. The composition and microstructure of the reaction products identified in the two types of paste were similar. Strength and microstructural development in the pastes activated with glass waste were also comparable to the parameters observed in AAS pastes prepared with conventional activators.     

Influence of pozzolans and slag on the microstructure of partially carbonated cement paste by means of water vapour and nitrogen sorption experiments and BET calculations

The influence of water-to-binder ratio (0.33 to 0.50) and additions (fly ash, slag, silica fume) on the microstructure of partially carbonated cement pastes was studied by nitrogen sorption and static and dynamic water vapour sorption. The selected technique affects macropore condensation and accessibility of pores, while predrying influences removal of CSH interlayer water. BJH calculations showed the increased amount of capillary pores with higher water-to-cement ratio, and the decrease of micropores (< 2 nm), in pastes with 50% or more fly ash or slag. Paste with 10% SF showed a high amount of gel pores, related to the higher amount of CSH gel, calculated from adsorption at 23% RH. A linear relation was observed between BET specific surface and water-cement ratio. Thermogravimetric analysis illustrated the influence of water-cement ratio and pozzolanic materials on the portlandite content. Introduction of silica fume, increased the specific surface accessible to water, but not to nitrogen molecules.     

Influence of silica fume on the microstructure of cement pastes: New insights from 1H NMR relaxometry

¹H NMR has been used to characterise white Portland cement paste incorporating 10 wt.% of silica fume. Samples were measured sealed throughout the hydration without sample drying. Paste compositions and C–S–H characteristics are calculated based on ¹H NMR signal intensities and relaxation analysis. The results are compared with a similar study of plain white cement paste. While the presence of silica fume has little influence on C–S–H densities, the chemical composition is impacted. After 28 days of sealed hydration, the Ca/(Si + Al) ratio of the C–S–H is 1.33 and the H₂O/(Si + Al) ratio is 1.10 when 10% of silica fume is added to the white cement. A densification of the C–S–H with time is observed. There are no major changes in capillary, C–S–H gel and interlayer pore sizes for the paste containing silica fume compared to the plain white cement paste. However, the gel/interlayer water ratio increases in the silica fume blend.     

Investigation of the carbonation mechanism of CH and C-S-H in terms of kinetics,microstructure changes and moisture properties

The purpose of this article is to investigate the carbonation mechanism of CH and C-S-H within type-I cement-based materials in terms of kinetics, microstructure changes and water released from hydrates during carbonation. Carbonation tests were performed under accelerated conditions (10% CO₂, 25 °C and 65 ± 5% RH). Carbonation profiles were assessed by destructive and non-destructive methods such as phenolphthalein spray test, thermogravimetric analysis, and mercury intrusion porosimetry (destructive), as well as gamma-ray attenuation (non-destructive). Carbonation penetration was carried out at different ages from 1 to 16 weeks of CO₂ exposure on cement pastes of 0.45 and 0.6 w/c, as well as on mortar specimens (w/c = 0.50 and s/c = 2). Combining experimental results allowed us to improve the understanding of C-S-H and CH carbonation mechanism. The variation of molar volume of C-S-H during carbonation was identified and a quantification of the amount of water released during CH and C-S-H carbonation was performed.     

Internal curing by superabsorbent polymers in ultra-high performance concrete

To limit self-desiccation and autogenous shrinkage that may lead to early-age cracking of ultra-high performance concrete (UHPC), internal curing by means of superabsorbent polymers (SAP) may be employed. Cement pastes and UHPC with water-to-cement ratio below 0.25, with or without SAP, were studied. The absorption capacity of a solution-polymerized SAP was first determined on hardened cement pastes by SEM image analysis. It was observed that the SAP cavities become partially filled with portlandite during cement hydration. Isothermal calorimetry showed that water entrainment with SAP delays the main hydration peak, while after a couple of days it increases the degree of hydration in a manner similar to increasing the water-to-cement ratio. Internal curing by SAP is effective in reducing the internal relative humidity decrease and the autogenous shrinkage. Although the mechanical properties are affected by SAP addition, it is possible to reach compressive strengths of almost 150 MPa at 28 days.     

Chemistry of cement hydration in polymer-modified pastes containing lead compounds

The role of polymeric additives on the hydration process of cement pastes admixed with a lead compound (Pb₃O₄) was investigated. Three series of pastes were prepared: the reference series, mixing water with Ordinary Portland Cement (OPC), and two series in which whether a styrene–butadiene rubber latex or a superplasticiser based on acrylic-modified polymer was added to the pastes. For each series, 5 and 10 wt% of Pb were mixed with the pastes. Phase analysis and microstructural characterisation were carried out by means of X-ray powder diffraction and SEM–EDX. Thermogravimetric analysis was performed to monitor the hydration degree of the three pastes; indeed, quantitative determination of portlandite and calcite was performed.Dynamic leach tests were performed on solidified monoliths to evaluate the effective immobilisation of Pb₃O₄. After 384 h leaching, excellent results were obtained by pastes mixed with superplasticiser that showed a cumulative release of Pb equal to 0.62 mg/l for samples containing 5 wt% of Pb, and equal to 0.84 mg/l for samples bearing 10 wt% of Pb.     

The role of alumina on performance of alkali-activated slag paste exposed to 50 °C

The strength and microstructural evolution of two alkali-activated slags, with distinct alumina content, exposed to 50 °C have been investigated. These two slags are ground-granulated blast furnace slag (containing 13% (wt.) alumina) and phosphorous slag (containing 3% (wt.) alumina). They were hydrated in the presence of a combination of sodium hydroxide and sodium silicate solution at different ratios. The microstructure of the resultant slag pastes was assessed by X-ray diffraction, differential thermogravimetric analysis, and scanning electron microscopy. The results obtained from these techniques reveal the presence of hexagonal hydrates: CAH₁₀ and C₄AH₁₃ in all alkali-activated ground-granulated blast-furnace slag pastes (AAGBS). These hydrates are not observed in pastes formed by alkali-activated ground phosphorous slag (AAGPS). Upon exposure to 50 °C, the aforementioned hydration products of AAGBS pastes convert to C₃AH₆, leading to a rapid deterioration in the strength of the paste. In contrast, no strength loss was detected in AAGPS pastes following exposure to 50 °C.     

Mechanical performance and microstructure of the calcium carbonate binders produced by carbonating steel slag paste under CO2 curing

Calcium carbonate binders were prepared via carbonating the paste specimens cast with steel slag alone or the steel slag blends incorporating 20% of Portland cement (PC) under CO₂ curing (0.1 MPa gas pressure) for up to 14 d. The carbonate products, mechanical strengths, and microstructures were quantitatively investigated. Results showed that, after accelerated carbonation, the compressive strengths of both steel slag pastes and slag-PC pastes were increased remarkably, being 44.1 and 72.0 MPa respectively after 14 d of CO₂ curing. The longer carbonation duration, the greater quantity of calcium carbonates formed and hence the higher compressive strength gained. The mechanical strength augments were mainly attributed to the formation of calcium carbonate, which caused microstructure densification associated with reducing pore size and pore volume in the carbonated pastes. In addition, the aggregated calcium carbonates exhibited good micromechanical properties with a mean nanoindentation modulus of 38.9 GPa and a mean hardness of 1.79 GPa.     

Novel high-resistance clinkers with 1.10 < CaO/SiO2 < 1.25: production route and preliminary hydration characterization

New amorphous calcium silicate binders, hydraulically active, were produced by a process consisting in fully melting and rapid cooling of a mixture of typical raw materials (limestone, sand, fly-ash and electric furnace slag) with overall CaO/SiO₂ molar ratios (C/S) comprised between 1.1 and 1.25. Pastes were produced from these materials by mixing them with water in a water/binder ratio of 0.375. Compressive strength was determined at the ages of 7, 28 and 90 days and the hydration of these pastes was followed during this period by XRD, FTIR and ²⁹Si MAS-NMR. Tobermorite-like structures with low C/S and semi-crystalline character were observed to develop upon hydration of these new amorphous calcium silicate hydraulic binders. Moreover, no Portlandite was formed during hydration of these materials. The maximum compressive strength after 90 days is above 40 MPa. TGA was performed in order to determine the amount of structural water present in the pastes and their content related to the amount of hydrated products obtained. The relation between compressive strength and the amount of hydration products was investigated and some considerations about the mechanical properties of the hydration products and paste microstructure were inferred.     

Effect of FeAl3 on properties of (W,Ti)C–FeAl3 hard materials consolidated by a pulsed current activated sintering method

Using a pulsed current activated sintering (PCAS) method, the densification of (W,Ti)C and (W,Ti)C–FeAl₃ hard materials was accomplished within 3 min. The advantage of this process is not only rapid densification to near theoretical density, but also prevention of grain growth in nano-structured materials. Highly dense (W,Ti)C and (W,Ti)C–FeAl₃ with a relative density up to 99% were obtained within 3 min by PCAS under a pressure of 80 MPa. The average grain size of the (W,Ti)C was less than 100 nm. Hardness and fracture toughness of the dense (W,Ti)C and (W,Ti)C–FeAl₃ produced by PCAS were also investigated. The fracture toughness and hardness values of (W,Ti)C, (W,Ti)C–5 vol.% FeAl₃, and (W,Ti)C–10 vol.% FeAl₃ consolidated by PCAS were 7.5 MPa m^1/2 and 2650 kg/mm², 10.5 MPa m^1/2 and 2480 kg/mm², 11 MPa m^1/2 and 2300 kg/mm², respectively.     

Solubility and diffusion coefficient of supercritical-CO2 in polycarbonate and CO2 induced crystallization of polycarbonate

The solubility and diffusion coefficient of supercritical CO₂ in polycarbonate (PC) were measured using a magnetic suspension balance at sorption temperatures that ranged from 75 to 175 °C and at sorption pressures as high as 20 MPa. Above certain threshold pressures, the solubility of CO₂ decreased with time after showing a maximum value at a constant sorption temperature and pressure. This phenomenon indicated the crystallization of PC due to the plasticization effect of dissolved CO₂. A thorough investigation into the dependence of sorption temperature and pressure on the crystallinity of PC showed that the crystallization of PC occurred when the difference between the sorption temperature and the depressed glass transition temperature exceeded 40 °C (T − T_g ≥ 40 °C). Furthermore, the crystallization rate of PC was determined according to Avrami's equation. The crystallization rate increased with the sorption pressure and was at its maximum at a certain temperature under a constant pressure.     

Portlandite content and ionic transport properties of hydrated C3S pastes

This paper presents the results of a C₃S paste characterization study. The objective was to determine the parameters needed to model the process of degradation. The experimental study focused on determining the portlandite content and the ionic diffusion coefficients of C₃S paste. The molar C/S ratio of C–S–H in hydrated C₃S pastes was also investigated. The portlandite content was determined with an experimental method based on an electron microprobe analysis.This method leads to a portlandite mass content of 24.4 ± 2.3%. The diffusion coefficient of each ionic species was determined by inverse analysis of diffusion test data performed on hydrated C₃S samples using a multiionic transport model.     

Capillary rise properties of porous geopolymers prepared by an extrusion method using polylactic acid (PLA) fibers as the pore formers

The geopolymers were prepared from sodium silicate, metakaolinite, NaOH and H₂O at SiO₂:Al₂O₃:Na₂O:H₂O of 3.66:1:1:x, where x = 8–17, and curing temperatures of 70–110 °C. Since the bending strength of the geopolymers was highest (36 MPa) where H₂O/Al₂O₃ = 9 and the curing temperature = 90 °C, these conditions were adopted. The porous geopolymers were prepared by kneading PLA fibers of 12, 20 and 29 μm diameter into the geopolymer paste, at fiber volumes of 13–28 vol%. The resulting paste was extruded using a domestic extruder, cured at 90 °C for 2 days then dried at the same temperature. The PLA fibers in the composites were removed by alkali treatment and/or heating. The highest capillary rise was achieved in the porous geopolymers containing 28 vol% of 29 μm fibers. The capillary rise of this sample, estimated by the equation of Fries and Dryer¹ was 1125 mm.     

Mechanisms of NOx entrapment into hydrated cement paste containing activated carbon — Influences of the temperature and carbonation

The NO_x pollution produced by road traffic in confined volumes, such as tunnels, is an issue for public health. This work focuses on the mechanisms of NO_x removal by modified cement pastes. The samples (made of pure synthetic powders or cement paste cylinders) are continuously exposed to 220 ppbv of NO and/or 110 ppbv of NO₂ gas. The ability for the main hydrates (such as Ca(OH)₂ and C–S–H) to trap NO₂ is then quantified. After the leaching of samples, the chromatography experiments show that the adsorbed NO₂ is transformed in nitrate and nitrite ions by a disproportionate mechanism. The addition of activated carbon into cement paste enhances the NO₂ abatement, which is not influenced by carbonation. The NO₂ abatement by the activated carbon materials is also stable between 20 and 50 °C, revealing a competition between the disproportionate reaction and the gas adsorption.     

Effect of substitution of granulated slag by air-cooled slag on the properties of alkali activated slag

This article assesses the mechanical and durability performance of replacement of GBFS by ACS activated by 3:3 NaOH:Na₂SiO₃ (3:3 SH:SSL) wt% (at optimum value 6 wt%) mixed with sea water (SW) and cured at 100% R.H. at room temperature. The kinetic behavior of activated GBFS-ACS mixes was measured by determination of setting time, combined water, bulk density and compressive strength up to 90 days. The rate of activation of the AAS has been studied from some selected samples by FT-IR, TGA, DTG analysis and SEM techniques. The compressive strength of dried activated GBFS-ACS pastes in comparison with saturated GBFS-ACS pastes up to 90 days was determined. The results revealed that the blended pastes of 80% GBFS+20% ACS gives the higher combined water, bulk density and compressive strength than those of 40/60 and 60/40% GBFS/ACS and lower than the 100% GBFS up to 90 days. Also, the compressive strength of dried samples at 105 °C for 24 h activated by (3:3 SH:SSL) mixed with SW and cured in 100% R.H. at room temperature up to 90 days is greater than saturated samples cured at the same conditions. On increasing the amount of ACS up to 40%, the setting time decreases then increases at 60% but still shorter than 100% GBFS. Finally, ACS can be used as partial substitution of GBFS in AAS.     

Mechanisms and modelling of antimonate leaching in hydrated cement paste suspensions

Antimonate (Sb(OH)₆^?) leaching in hydrated cement pastes (HCP) was investigated by adsorption isotherms with cement minerals, EXAFS, geochemical modelling and by synthesising the antimonate AFm end-member, Ca₄Al₂[Sb(OH)₆]₂(OH)₁₂·3H₂O. Antimonate forms inner-sphere complexes with portlandite and ettringite surfaces. Much stronger interaction of antimonate with monosulphate was most likely AFm solid solution formation, whereas the mechanism of strong sorption by C–S–H was unclear. Modelling Sb leaching from HCP containing ca. 1000 mg kg^? 1 Sb and EXAFS analysis suggested the variable structure of calcium antimonate may explain the Sb leaching increase at decreasing pH or extended carbonation times. In HCP with lower Sb concentrations (ca. 300 mg kg^? 1) leaching is lower probably because it is controlled by a combination of AFm solid solution and/or C–S–H adsorption that can ensure long-term stabilisation, whereas calcium antimonate precipitation appears sensitive to weathering reactions such as carbonation.     

Development of a new rapid,relevant and reliable (R3) test method to evaluate the pozzolanic reactivity of calcined kaolinitic clays

The present paper introduces a new rapid, relevant and reliable (R³) test to predict the pozzolanic activity of calcined clays with kaolinite contents ranging from 0 to 95%. The test is based on the correlation between the chemical reactivity of calcined clays in a simplified system and the compressive strength of blends in standard mortar. The simplified system consists of calcined clay portlandite and limestone pastes with sulfate and alkali levels adjusted to reproduce the reaction environment of hydrating blended cements. The pastes were hydrated for 6 days at 20 °C or for 1 day at 40 °C. The chemical reactivity of the calcined clay can be obtained first by measurement of the heat release during reaction using isothermal calorimetry and second by bound water determination in a heating step between 110 °C and 400 °C.Very good correlations were found between the mortar compressive strength and both measures of chemical reactivity.     

Oxidation behaviour of a (Mo,W)Si2-based composite in dry and wet oxygen atmospheres in the temperature range 350–950 °C

The oxidation of a (Mo, W)Si₂-based composite was investigated in the temperature range (350–950 °C). The influence of temperature and water vapour on the oxidation was examined. The kinetics was studied using a thermobalance whereas the morphology and composition of the oxides were examined using X-ray diffractometer (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and energy dispersive X-ray (EDX). Focused ion beam (FIB) milling was performed on some of the oxide scales which allowed us to look at a non-mechanically disturbed scale/oxide in cross-section. Rapid oxidation was found to occur in the 550–750 °C temperature range. The mass gains were significantly larger in O₂ than in O₂ + 10%H₂O. The different mass changes in the two exposure atmospheres were attributed to the higher vapour pressure of the volatile MoO₂(OH)₂ and WO₂(OH)₂ species in O₂ + 10%H₂O than that of (MoO₃)₃ and (WO₃)₃ in dry O₂. The peak mass gain was found to occur at a temperature of about 750 °C in O₂ and 650 °C in O₂ + 10%H₂O. At temperatures above 850 °C, especially when water vapour is present, the removal of Mo and W from the oxide scales is rapid enough to allow partial healing of the silica, causing the oxidation rate to drop. At 950 °C in O₂ + 10%H₂O, a protective SiO₂ scale could be re-established quickly and maintained, causing the oxidation to essentially cease.