Gian Lorenzo Valenti

Flue gas desulfurization gypsum and coal fly ash as basic components of prefabricated building materials.

The manufacture of prefabricated building materials containing binding products such as ettringite (6CaO·Al2O3·3SO3·32H2O) and calcium silicate hydrate (CSH) can give, in addition to other well-defined industrial activities, the opportunity of using wastes and by-products as raw materials, thus contributing to further saving of natural resources and protection of the environment. Two ternary mixtures, composed by 40% flue gas desulfurization (FGD) gypsum or natural gypsum (as a reference material), 35% calcium hydroxide and 25% coal fly ash, were submitted to laboratory hydrothermal treatments carried out within time and temperature ranges of 2h-7days and 55-85°C, respectively. The formation of (i) ettringite, by hydration of calcium sulfate given by FGD or natural gypsum, alumina of fly ash and part of calcium hydroxide, and (ii) CSH, by hydration of silica contained in fly ash and residual lime, was observed within both the reacting systems. For the FGD gypsum-based mixture, the conversion toward ettringite and CSH was highest at 70°C and increased with curing time. Some discrepancies in the hydration behavior between the mixtures were ascribed to differences in mineralogical composition between natural and FGD gypsum.

Use of Industrial Byproducts as Alumina Sources for the Synthesis of Calcium Sulfoaluminate Cements

Calcium sulfoaluminate (CSA) cements show some desirable environmentally friendly features that include the possibility of using several industrial byproducts as raw materials in their manufacturing process. Alumina powder, from the secondary aluminum manufacture, and anodization mud, from the production process of anodized aluminum, have proved to be suitable as partial or total substitutes for an expensive natural material like bauxite. CSA clinker generating raw mixtures, containing limestone, natural gypsum, bauxite, and/or one of the alumina-rich byproducts, were heated 2 h in a laboratory electric oven at temperatures ranging from 1150 to 1300 °C. Conversion of reactants into 4CaO·3Al(2)O(3)·SO(3) (the key component of CSA cements), evaluated using X-ray diffraction (XRD) analysis, increased with an increase of both burning temperature and byproduct concentration. When examined through differential thermogravimetric and XRD analyses, a synthetic CSA clinker (made from the raw mixture incorporating alumina powder as a total replacement of bauxite) mixed with 20% gypsum showed a hydration behavior almost similar to that of an industrial CSA cement containing the same amount of gypsum.

Utilization of Coal Combustion Ashes for the Synthesis of Ordinary and Special Cements

Raw mixes containing pulverized coal fly ash (with limestone and silica sand) or fluidized bed coal combustion ash (fly and bottom, with added limestone, anodization mud, and, when necessary, flue gas desulfurization gypsum), aimed at generating ordinary Portland or calcium sulfoaluminate clinkers, respectively, were heated in a laboratory electric oven at temperatures ranging from 1150° to 1500°C and submitted to X-ray diffraction analysis. The former had the same qualitative phase composition as that of a reference mixture, composed by limestone and clay; furthermore, they exhibited an excellent burnability on the basis of their residual free lime contents, measured after heating at 1350°, 1400°, 1450°, and 1500°C. The latter showed very good results in terms of conversion of reactants and selectivity degree toward the main mineralogical constituent, calcium sulfoaluminate (4CaO·3Al2O3·SO3), even if the behavior of a reference mixture consisting of limestone, bauxite, and natural gypsum was slightly better. The introduction of a fluidized bed coal combustion ash in the raw mix generating calcium sulfoaluminate clinker implies a saving of bauxite and natural gypsum, which can be fully replaced through the addition of anodization mud and flue gas desulfurization gypsum, respectively.

Assessment of ettringite from hydrated FBC residues as a sorbent for fluidized bed desulphurization

Abstract The performance of synthetic ettringite as a sorbent in fluidized bed desulphurization has been assessed and compared with that of a commercial limestone. Experiments have been carried out in a bench scale fluidized bed reactor under simulated desulphurizing (steadily oxidizing) combustion conditions. Sorbent performance has been characterized in terms of desulphurization rate, maximum sulphur uptake and attrition propensity. Fluidized bed sulphation experiments have been complemented by microstructural characterization of solid samples, accomplished via X-ray diffraction analysis, scanning electron microscopy and sulphur mapping of cross-sections of particles embedded in epoxy resin. Experimental results show that both the rate and the maximum extent of sulphur uptake by ettringite significantly exceed those of the limestone. Maximum degree of free calcium utilization is 0.58 for ettringite compared with 0.27 for the limestone. Sulphation tests also indicate that attrition propensity of ettringite is larger than that correspondingly observed for the limestone. Microstructural characterization indicates that sulphation of ettringite takes place evenly throughout the particle cross-section, whereas sulphation of limestone mostly conforms to a core-shell pattern. Along a parallel pathway, the rate and yield of ettringite formation by hydration of fly ash from a utility fluidized bed boiler have been assessed. Formation of ettringite in these experiments appears to be quantitative upon curing of ash at 70 °C for times up to 4 days.

Calcium looping spent sorbent as a limestone replacement in the manufacture of portland and calcium sulfoaluminate cements.

The calcium looping (CaL) spent sorbent (i) can be a suitable limestone replacement in the production of both ordinary Portland cement (OPC) and calcium sulfoaluminate (CSA) cement, and (ii) promotes environmental benefits in terms of reduced CO2 emission, increased energy saving and larger utilization of industrial byproducts. A sample of CaL spent sorbent, purged from a 200 kWth pilot facility, was tested as a raw material for the synthesis of two series of OPC and CSA clinkers, obtained from mixes heated in a laboratory electric oven within temperature ranges 1350°-1500 °C and 1200°-1350 °C, respectively. As OPC clinker-generating mixtures, six clay-containing binary blends were investigated, three with limestone (reference mixes) and three with the CaL spent sorbent. All of them showed similar burnability indexes. Moreover, three CSA clinker-generating blends (termed RM, MA and MB) were explored. They included, in the order: (I) limestone, bauxite and gypsum (reference mix); (II) CaL spent sorbent, bauxite and gypsum; (III) CaL spent sorbent plus anodization mud and a mixture of fluidized bed combustion (FBC) fly and bottom ashes. The maximum conversion toward 4CaO·3Al2O3·SO3, the chief CSA clinker component, was the largest for MB and almost the same for RM and MA.

Low-CO2 Cements from Fluidized Bed Process Wastes and Other Industrial By-Products

ABSTRACT According to the findings of X-ray diffraction analysis, calcium looping (CaL) spent sorbent (as a main source of CaO), anodization mud (as a source of Al2O3 and additional sulfate), and fluidized bed combustion (FBC) fly and bottom ashes, separately or in mixture (as main sources of CaSO4 plus alumina and uncarbonated lime) can replace limestone, bauxite, and gypsum as components of calcium sulfoaluminate (CSA) clinker-generating raw mixes. All of the hydrated CSA cements obtained from the corresponding clinkers and investigated by means of differential thermal-thermogravimetric analysis displayed a similar behavior. It has been demonstrated that the well-recognized environmentally friendly character of CSA cement manufacture can be further improved not only by using industrial wastes instead of natural materials, but also by virtue of the reduced CO2 emission and increased energy saving associated with the limestone substitution with poorly carbonated sources of lime, such as CaL spent sorbent and FBC ashes.

Use of Fluidized Bed Combustion Ash and Other Industrial Wastes as Raw Materials for the Manufacture of Calcium Sulphoaluminate Cements

Calcium sulphoaluminate cements, mainly composed by 4CaO·3Al2O3·SO3 and 2CaO·SiO2, are special hydraulic binders which require limestone, bauxite and gypsum as natural raw materials for their manufacture. In order to save bauxite and natural gypsum, it has been explored the possibility of using, among the raw mix components, FBC waste together with pulverised coal fly ash or anodization mud and, when necessary, flue gas desulphurization gypsum. Mixtures containing limestone (29–39%), FBC waste (30–44%), pulverised coal fly ash (0–13%) or anodization mud (0–32%), bauxite (0–18%) and flue gas desulphurization gypsum (0–8%) were heated for 2 hours in a laboratory electric oven at temperatures ranging from 1150° to 1300°C. The X-ray diffraction patterns on the burnt products generally showed a good conversion of the reactants and a high selectivity degree towards 4CaO·3Al2O3·SO3, particularly at 1250°C.

Enhancement of selectivity toward ettringite during hydrothermal processes on fluidized bed combustion wastes for the manufacture of preformed building components

Due to its lime, alumina and calcium sulfate contents, fluidized bed combustion (FBC) waste is worthy of consideration as a raw material for the hydrothermal synthesis of building elements based on ettringite (6CaO·Al2O3·3SO3·32H2O). Two FBC waste samples (a fly and a bottom ash) were hydrated, alone and in a mixture, at temperatures between 40° and 85 °C for curing times ranging from 2 hours to 7 days, and were submitted to X-ray diffraction and differential thermal-thermogravimetric analyses. It has been found that: (a) the FBC fly ash could be hydrated alone, due to its satisfactory ettringite-generating ability; (b) the FBC bottom ash needed to be used together with other raw materials; (c) progressively better results were obtained by adding (i) FBC fly ash to FBC bottom ash (to give a blend with a 60 : 40 mass ratio), (ii) anodization mud (a by-product of anodized aluminum manufacture, acting as an additional source of alumina and calcium sulfate) in a measure of 10% by mass to the fly-bottom ash blend, (iii) anodization mud (20% by mass) to the FBC bottom ash. Moreover, compressive strength measurements were carried out, according to the EN 196-1 Standard for cements, on the hydrated systems showing the two largest ettringite concentrations, and a maximum value of about 6 MPa was reached at 70 °C and 16 hours of curing. The data obtained in this investigation were consistent with an industrial prefabrication of building components.