Immobilization of heavy metals by calcium sulfoaluminate cement

Two types of calcium sulfoaluminate cement containing 20% and 30% phosphogypsum, respectively, were investigated for their ability in hazardous waste stabilization. Fourteen series of pastes were prepared, each containing the following soluble salt: Na₂CrO₄·4H₂O; Na₂Cr₂O₇·2H₂O; CrCl₃·6H₂O; Pb(NO₃)₂; Zn(NO₃)₂·6H₂O; ZnSO₄·7H₂O; and CdCl₂·5H2O. The level of pollution was 0.069 mol of heavy metal per Kg of cement.

The study has been carried out by means of X-ray diffraction, thermal analysis, scanning electron microscopy, mercury intrusion porosimetry, electrical conductivity, and leaching tests. Very good retention of lead, cadmium, zinc and trivalent chromium is observed. The retention of hexavalent chromium depends upon the nature of the binder: the cement containing 20% gypsum develops the best behaviour. This is explained by the microstructure of the hydrated paste: in the paste containing 30% gypsum, delayed ettringite precipitates and damages the hardened paste.     

Microstructure of amorphous aluminum hydroxide in belite-calcium sulfoaluminate cement

Belite-calcium sulfoaluminate (BCSA) cement is a promising low-CO₂ alternative to ordinary Portland cement. Herein, aluminum hydroxide (AH₃), the main amorphous hydration product of BCSA cement, was investigated in detail. The microstructure of AH₃ with various quantities of gypsum was investigated via scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The AH₃ with various morphologies were observed and confirmed in the resulting pastes. Particular attention was paid to the fact that AH₃ always contained a small amount of Ca according to the results of EDS analysis. The AH₃ was then characterized via high resolution transmission electron microscopy (HRTEM). The results of HRTEM indicated that Ca arose from nanosized tricalcium aluminate hexahydrate which existed in the AH₃.     

Behaviour of calcium sulfoaluminate cement in presence of high concentrations of chromium salts

This paper deals with the behaviour of calcium sulfoaluminate cement hydrated in presence of high concentration of soluble chromium salts (65 g/kg): chromium nitrate and sodium chromate. Chromium nitrate was chosen to verify the good retention of Cr³⁺ by calcium sulfoaluminate cement. Sodium chromate was employed to assess the ability of calcium sulfoaluminate to immobilize Cr⁶⁺.Leaching tests were carried out on two types of calcium sulfoaluminate cement containing 20% and 30% phosphogypsum, respectively. The microstructure was also investigated at 28 and 90 days.The results obtained point two main findings:

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the retention of Cr³⁺ is excellent (> 99%), regardless of the type of cement,

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the retention of Cr⁶⁺ depends on the type of cement and, in this case, the U phase is detected and promotes some expansion leading to cracking of the matrix.

     

Leaching of calcium sulfoaluminate cement pastes by water at regulated pH and temperature: Experimental investigation and modeling

Two cement pastes, prepared with a ye'elimite-rich calcium sulfoaluminate binder containing 0% or 20% gypsum, were leached by water at regulated pH (7) and temperature (20 °C) for three months with the aim of understanding and modeling the degradation processes. The cumulative quantities of Ca^2 +, OH⁻ and SO₄^2 − ions in the leachates increased linearly versus the square root of time, showing that leaching was controlled by diffusion. The gypsum-free material exhibited the highest decalcification rate. Examination of the solid samples after three months revealed three zones:

• -the surface layer, highly porous and composed of AH₃, perovskite and possibly C-(A)-S-H,
• -an intermediate zone, less porous, in which several precipitation and dissolution fronts occurred, and
• -the sound core.

The hydrated phase evolution along depth was accurately determined and qualitatively reproduced with a model coupling transport by diffusion and chemical reactions.     

The role of the interface in glass fibre reinforced cement

Glass fibre reinforced cement (GRC) provides an interesting example of interaction between a brittle fibre and a porous brittle matrix which is reactive towards the reinforcement. It is also a case in which the composite fails by multiple fracture. The durability of GRC produced from ordinary Portland cement and an alkali-resistant glass fibre recently developed in the U.K., has been studied over a period of three years under different environmental conditions by measuring the variations in the mechanical properties of these composites with age. The experimental results are interpreted in terms of the micromechanics of failure for these composites and an assessment is made of the role of the interface in controlling the behaviour of the composite at various stages of its life. It is concluded that the properties of the interface in GRC change with time, partly due to chemical attack on the fibre which weakens the reinforcement but also due to changes in the physical properties of the fibre bundle and porosity and volume changes in the matrix as it sets and hardens. It has, however, not yet been possible to characterise the materials nature of the interface in GRC composites.     

Hydration of calcium sulfoaluminate cement by a ZnCl2 solution: Investigation at early age

Hydration of calcium sulfoaluminate cement at early age was investigated as a function of the gypsum content of the binder, the thermal history of the material, and the ZnCl₂ concentration in the mixing solution. Early hydration was strongly accelerated by the presence of gypsum, but lower percentages of reaction were reached after 24 h. The slowing down effect induced by ZnCl₂, even at a concentration as high as 0.5 mol/L, was moderated compared to OPC but had a greater intensity in the absence of gypsum. Unlike what would have been expected for Portland cement, it was shown that the delay of a gypsum-free calcium sulfoaluminate cement resulted from the strong retardation caused by chloride anions, which was partly compensated by the accelerating effect of Zn²⁺ cations. The mineralogical observations revealed the precipitation of chloro–AFm phases such as Friedel's and Kuzel's salts, but no crystallized zinc-containing phases could be identified by XRD. The thermal history of the samples proved to be a key parameter. Applying a thermal cycle which reproduced the temperature rise and decrease occurring in a massive mortar block accelerated the rate of hydration and mainly modified the proportion of AFt versus AFm hydrates, especially when the binder had a gypsum content below 20%.     

Fatigue-induced deterioration of the interface between micro-polyvinyl alcohol (PVA) fiber and cement matrix

Quality of interfacial bond between fibers and matrix determines the post-cracking behavior of fiber-reinforced composites. Fatigue-induced interface deterioration between fibers and matrix has not been investigated systematically which prevents understanding of premature failure of fiber-reinforced composites subject to fatigue. This study experimentally investigated the deterioration mechanism of flexible fibers in brittle matrix subject to fatigue load. Specifically, the effect of fatigue-induced deterioration of interface between micro-PVA fiber and cement matrix was studied through the single fiber fatigue pullout tests and the micro-structural deterioration mechanism of the fiber-matrix interface under fatigue load was unveiled. It was found that fatigue load leads to fiber debonding which can be described by an empirical relation similar to the Paris' law. Fatigue-induced interface hardening can occur during fiber debonding stage as well as fiber slippage stage. Oil-treatment on surface of micro-PVA fiber was demonstrated as a mean to mitigate such fatigue-induced interface hardening.     

Synthesis and properties of an amphoteric polycarboxylic acid-based superplasticizer used in sulfoaluminate cement

A novel amphoteric polycarboxylic (APC) acid-based superplasticizer was prepared by solution copolymerization from maleic anhydride, allyl polyethylene glycol, and methacryloxyethyltrimethyl ammonium chloride (DMC). The cationic monomer DMC was introduced for improving the performances of sulfoaluminate cement (SAC) with APC. The condition on preparation of APC was determined by the orthogonal experiment, and the performances of fresh and hardened superplasticized mortars were assessed. APC was characterized by FTIR spectra. Adsorption amount and dispersing effectiveness of APC were also investigated. Results indicated that APC could improve the fluidity and enhance the mechanical performances of SAC mortars. Adsorption amount of APC in SAC is large. Compared with an anionic PC superplasticizer and a commercial superplasticizer Visco Crete 3390, APC has the better performance. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

改性纳米水化硅酸钙的制备及在深水固井水泥中的应用

纳米水化硅酸钙粒子（nano-C-S-H）表面能高,易团聚,在水泥浆中的分散程度差,削弱了其在油井水泥中的低温早强性能,基于此,本文制备了一种改性nano-C-S-H,并研究其在深水固井水泥中的低温早强性能。首先,本文以硅烷偶联剂KH570对nano-C-S-H进行表面接枝改性,并利用红外、接触角、热重分析等分析手段表征了KH570对nano-C-S-H表面疏水改性程度,结果表明得到了预期的改性nano-C-S-H。其次,对改性nano-C-S-H的低温早强性能进行了评价,结果表明：在10 ℃的低温养护条件下,养护龄期为24 h时,掺有3.0%改性nano-C-S-H水泥石的强度为16.7 MPa,而掺未改性nano-C-S-H的水泥石强度为11.2 MPa,且改性nano-C-S-H对固井水泥浆的综合工作性能无任何不良影响。最后,采用低场核磁、水化热、扫描电镜等分析手段研究了nano-C-S-H的低温早强机理,结果表明,在低温条件下,改性nano-C-S-H能有效缩短水泥诱导期,提高水泥早期水化反应速率,促进水化C-S-H凝胶、氢氧化钙等物相的生成,增强水泥石基体的致密性,进而提高水泥石早期强度,并满足深水低温固井要求。综上所述,改性nano-C-S-H在深水低温固井工程中具有良好的应用前景。     

Effect of pore solution calcium and substrate calcium on PMMA/cement paste interface during early stages of hydration

The interfacial shear strength of polymer thin film coating on calcium–silicate–hydrate (C–S–H) or cement paste significantly depends on the time-point, from addition of water, when the polymer is coated. This is because the physico-chemical properties of C–S–H surface evolve over time as hydration progresses. The instant of coating thus determines the interfacial performance of a thin film coating in an organic-inorganic material system. In this work, we provide molecular insights into the interactions of poly(methyl methacrylate) (PMMA) functional groups with the pore solution calcium and bound substrate calcium present in hydrating C–S–H. The experimental observation reveals that PMMA thin film coated on second day of hydration exhibits larger interfacial shear strength than when coated on the 28th day. The possible reasons for this observations are: a) chemical crosslinking of two oxygen atoms from two different PMMA polymer chains by pore solution calcium ions and b) the physical interaction between double bond oxygen (O1) present in –COO functional group of PMMA with pore solution calcium. In this work, physical interaction between double bond oxygen (O1) present in –COO functional group of PMMA with pore solution calcium and substrate calcium in C–S–H is addressed.     

Interfacial adhesion between embedded fibre optic sensors and epoxy matrix in composites

Fibre optic (FO) sensors are becoming increasingly popular for different applications in structural monitoring. Among their excellent properties, a strong interest for this type of sensors are represented by the possibility of embedding FOs inside composite components. In this case, one of the factors that significantly influence the efficiency of the whole Structural Health Monitoring (SHM) system is the interfacial adhesion between FO sensors and the host material. The main objective of this work is to investigate the interfacial adhesion between embedded fibre optic sensors and epoxy matrix to find the best type of optical fibre to be used in epoxy matrices to produce smart composites. Four types of optical fibres with different diameters and coatings (i.e. polyimide, polyacrylate and ormoceramic) were used. Pull-out tests were carried out and different methods were used to obtain the composite/optical fibre interfacial properties. Finally, an optical microscopy and Scanning Electron Microscopy (SEM) analysis were performed to characterize the fibre/matrix interfaces. It was found that the optical fibre that presented the highest energy required for interface rupture and, consequently, less invasiveness to the host material was the ormoceramic fibre with the smallest diameter.     

Microstructure of calcium hydroxide depleted portland cement paste I: Density and helium flow measurements

Hydrated portland cement contains both free and combined Ca(OH)₂ together with C-S-H. Removal of the Ca(OH)₂ in controlled amounts, first by leaching with an aqueous solution of Ca(OH)₂ and then with water, allows the role of C-S-H to be studied. Helium pycnometric and inflow measurements were made on unleached, leached, and overleached material. Leaching of the free Ca(OH)₂ did not alter the C-S-H material and did not change any physical properties. Possible engulfing of C-S-H by Ca(OH)₂ is not effective in preventing helium or water flow. Overleaching to the extent that 50% of the CaO was removed from the cement broke down CaO₂ sheets, causing ready access of helium to collapsed interlayer positions. Evidence of a change in the orientation of interlayer water was observed.     

Modification of epoxy resin by polysulfone to improve the interfacial and mechanical properties in glass fibre composites. I. Study of processes during matrix/glass fibre interface formation

The rheology of epoxy resin-polysulfone blends and wetting at the blend/glass fibre interface have been studied. Measurements were made in a rotary viscometer and in a modified Wilhelmy apparatus. It was shown that none of the blends investigated revealed non-Newtonian behaviour in the range of shear rates used. The viscosity of the blends increased as polysulfone content increased. Introduction of hardener resulted in a significant increase of the blends viscosity up to 2-3 orders of magnitude. Rheological tests suggested that 15 wt% polysulfone was the highest concentration useful for obtaining composites by solvent-free impregnation technique. These tests suggested that the structure of the cured epoxy-polysulfone blends depended on the modifier concentration. The structures of the blends differed for the blends containing 5 wt% polysulfone and 10-15 wt% polysulfone. All the blends (with the hardener) required at least 30 min at 180°C to achieve final values of the mechanical properties such as storage and loss moduli, loss tangent and complex viscosity. For all epoxy resin-polysulfone/glass fibre systems a complete wetting of the fibres was observed. Surface tension vs. polysulfone content dependency was found to be nonadditive. Surface tension measured was minimal for epoxy resin-5% polysulfone blend, while for other systems the values were close to that of epoxy resin. Modification of epoxy resin by polysulfone did not change the kinetics of the fibres wetting by the blends.     

Analysis of iron oxides accumulating at the interface between aggregates and cement paste

Iron oxides formed by the corrosion of a steel reinforcing bar were found to accumulate at interfaces between aggregate and cement paste in Portland cement-based concrete. Microstructural characterization of the zones of oxide was carried out using optical microscopy, scanning electron microscopy (SEM), and energy dispersive X-ray (EDX) spectroscopy. The study showed that the oxide layer has an average thickness of about 1.5 mm, and consists of inner and outer layers with different types of oxides in each layer. A mechanism based on the Schikorr reaction is proposed for the preferential formation of iron oxides in the two layers taking into account local variations in pH of the pore solution during the early ages of the cement paste.     

TEM study of carbon fibre reinforced aluminium matrix composites: influence of brittle phases and interface on mechanical properties

The microstructure of A357 aluminium alloy (7 wt% Si+0.6 wt% Mg) reinforced by 1D-M40J carbon fibres is characterised using different techniques of transmission electron microscopy (diffraction, HRTEM, EDX, EELS). The microstructure of the high modulus PAN based fibre and of the pyrolytic carbon coating (Cp) is fully characterised. Silicon and Mg₂Si grains which determine matrix-reinforcement adhesion are investigated. On the basis of the microstructural features, the mechanical properties of the composites are discussed. The mechanical behaviour of composites prepared with and without Cp interphases corresponds to a brittle matrix reinforced by brittle fibres. In the case of the composite without Cp interphase, the most influent parameter is the high resistance to sliding at the interface between silicon and fibres which leads to a strong fibre-matrix “bonding” and thus, to a weak and brittle material. The interfacial resistance to decohesion and to sliding is lower in the composite with Cp interphase resulting in higher strength and limited pull out. This lower interfacial resistance is due to the successive microporous and layered microstructures of the pyrolytic carbon coating.     

The correlation between compressive strength and ultrasonic parameters of calcium aluminate cement materials

Calcium aluminate cement (CAC) mortars were investigated by nondestructive ultrasonic measurement in the through-transmission mode and compressive strength measurements. The detected profile of ultrasonic signal was fitted as a sine wave modulated with the Gauss function. The linear relationship between compressive strength and the product of the amplitude and angular frequency of the signal was established. A qualitative explanation of the proposed correlation based on the existing theories was given.     

Binding of calcium and magnesium by modified onion skins

The use of modified onion skins for binding of Ca²⁺ and Mg²⁺ from solutions has been investigated. The effect of time of equilibration, temperature, and pH on the sorption of the metal ions have been studied. Batch and column experiments have been performed and the adsorption isotherms have been plotted. The capacities with respect to Ca²⁺ and Mg²⁺ were found to be 4 and 16 meq, respectively, per gram of the substrate when separate column experiments were conducted using 1 L of solution containing 1000 meq of the respective metal ions at pH6. With a solution containing 10 meq each of Ca²⁺ and Mg²⁺ together, however, the substrate seems to exhibit greater preference for Ca²⁺ than Mg²⁺. The sorbed metal ions from the substrate can be leached into solution with a decinormal solution of HCl and the washed bed can be reused. In view of the complex organic nature of the onion skin and its considerable capacity to bind Ca²⁺ and Mg²⁺, the possibility of its use for preventing scale formation in boilers is indicated.     

硫铝酸盐水泥膨胀性能与水化胶凝特性的关系研究*

利用XRD和TG/DTG等实验方法,对硫铝酸盐水泥膨胀性能与水化产物及抗压强度的关系进行研究,并通过Rietveld分析法对水化产物进行定量分析。结果表明,二水石膏与硫铝酸钙物质的量比达到2.0及以上时,试件膨胀速率可达0.05%/d以上,且膨胀持续时间延长;试件强度是影响膨胀量大小的外在因素,强度增大会制约水泥试件膨胀发展,强度小的试件膨胀率较大;钙矾石（AFt）增长速率是影响膨胀率变化的内在因素,铝凝胶（AH3）不具有膨胀特性。     

Preparation and properties of viscose staple fibre modified by grafting polyvinyl chloride

Conclusions 1. The authors have obtained modified viscose fibres containing PVC in amounts reaching 25–230% of the weight of cellulose. The molecular weight of the grafted chain varies between 40,000 and 140,000; the degree of conversion of cellulose does not exceed 13–22%.2. It is shown that grafting of PVC increases the absolute strength and elongation of fibres, reduces the loss of wet strength, and reduces the resistance to repeated bending. A marked reduction of the fibre number reduces their strength.3. The results are explained by the probability that graft copolymerization takes place on a supramolecular level.Moscow Textile Institute. Translated from Khimicheskie Volokna, No. 5, pp. 30–32, September–October, 1969.