Reuse of Woody Biomass Fly Ash in Cement-Based Materials: Leaching Tests

Mario Berra, Giancarlo De Casa, Marcello Dell’Orso, Luigi Galeotti, Teresa Mangialardi, Antonio Evangelista Paolini, and Luigi Piga

Abstract The feasibility of using woody biomass fly ash (WBFA) as a mineral admixture in cement-based materials was investigated. This fly ash was character­ized for chemical composition and used to prepare a cement blend with 70 wt% Portland cement and 30 wt% WBFA. Cubic specimens were cast from a blended cement paste (water-to-binder ratio 0.50) and, after 28 days of curing at 20°C and 100% relative humidity, these specimens were tested for heavy metal leachability through the use of a sequential leaching protocol, at a constant pH of leachant (deionized water; pH 6.0). It was found that, except for the chloride content, the WBFA is able to meet the European chemical requirements established for reuse of coal fly ash in cement-based materials. Although the WBFA is characterized by a significant content of heavy metals of particular environmental concern (Cd, Cr, Cu, Ni, Pb, Zn), the results of the monolith leaching test have shown a good immobiliza­tion capacity of such metals by the cementitious matrix and, consequently, a good environmental quality of the blended cement investigated.

10.1 Introduction

In March 2007, the European Commission undertook an approach to climate and energy policy in order to fight climate change and increase EU energy security while strengthening its competitiveness. The European Commission committed itself to transform Europe into a highly energy efficient, low-carbon economy.

M. Berra

ERSE S. p.A., Milan, Italy

G. De Casa, L. Galeotti, T. Mangialardi, A. E. Paolini, and L. Piga (H)

Faculty of Engineering, Sapienza University of Roma, Rome, Italy e-mail: luigi. piga@uniroma1.it

M. Dell’Orso

Chemical Laboratory of DGERM-Economic Development Ministry, Rome, Italy

H. Insam and B. A. Knapp (eds.), Recycling of Biomass Ashes,

DOI 10.1007/978-3-642-19354-5_10, © Springer-Verlag Berlin Heidelberg 2011

To achieve this goal, the European Commission aimed to carry out, by 2020, what is known as the 20:20:20 project, namely:

— A reduction in EU greenhouse gas emissions of at least 20% below 1990 levels

— Twenty percent of EU energy to be produced from renewable resources

— A 20% reduction in primary energy use to be achieved by improving energy

efficiency

In this context, the use of biomasses in place of traditional fuels represents a suitable way of reducing greenhouse gas emissions. In fact, the biomasses may be regarded as clean and renewable energy resources with no net CO2 production, since the amount of CO2 produced from biomass combustion is approximately equivalent to that taken up from the environment during biomass growth.

The most important biomasses are the residues from woodworking or forest activities, the wastes from farms and agrobusiness, the organic fraction of munici­pal solid wastes, and the plants deliberately grown for energy production purposes. In Italy, the most utilized biomasses for burning in power plants are chipped wood, and, to a minor extent, rice-husk and olive residues (GSE 2009).

Although the use of biomass in Italy is less than the European average, the high potential of burnable biomass along with the fast increase in the number of biomass-based thermal plants calls for a solution to the disposal problems asso­ciated with ash production. Both the quality and the quantity of ash depend on the type of biomass used as a fuel. The amount of ash produced per unit weight of original biomass can vary from about 2% (w/w) (chipped wood) to 15% (w/w) (rice husk) (Lokare et al. 2006).

Irrespective of the type of biomass used, two kinds of ashes are produced: fly ash and bottom ash. Fly ash is generally trapped by electrostatic precipitators or sleeve filters located downstream of the combustion process, before the gas and the very fine particles are released to the environment. Bottom ash is collected in the bottom of the boiler. The relative amount of fly ash and bottom ash depends on the type of boiler. Powder boilers produce more bottom ash than fly ash; fluidized bed boilers produce more fly ash than bottom ash. Grate boilers produce about the same quantity of both ashes.

According to the European waste catalog and hazardous residues list (Commis­sion of the European Communities 2000), both fly ash and bottom ash originating from combustion of untreated wood are classified as nonhazardous wastes and are listed with codes 10.01.03 and 10.01.01, respectively. The former code also includes fly ash from peat; the latter also includes slag and boiler dust.

Woody biomass ash, being a waste, has to be disposed of in authorized landfills. Alternatively, this waste may be reused as a fertilizer or for building purposes, provided it passes the tests prescribed by the environmental laws. Bottom ash may be used directly as a building material to replace granular material in geotechnical works, such as road foundations. Fly ash may be reused as a filler in cementitious mixes. However, the high content of alkalies and chlorides could prevent the reuse of fly ash in cementitious mixes.

From the environmental point of view, reuse of biomass fly ash in concrete would be very profitable as partial replacement of Portland cement. This may (1) solve the problem of fly ash disposal, (2) reduce the CO2 emissions involved in the industrial production of cement from traditional raw materials, namely, limestone and clay (0.83 t CO2 is emitted for each ton of Portland cement produced), and (3) preserve the natural resources involved in cement production, with further benefi­cial effects on the environment.

A recent study (Rajamma et al. 2009) has shown that the replacement of Portland cement with woody biomass fly ash (WBFA) up to 20% (w/w) of cement does not negatively affect the development of the mechanical properties of cementitious mixes. The practical inference of such fly ash reuse would be a 20% reduction of CO2 emission related to cement production, and this would be an innovation in line with what is expected by the European Commission.

However, the reuse of biomass fly ashes in cement-based materials is strongly related to their chemical and environmental characteristics. Generally, fly ashes originating from traditional and innovative fuels may contain significant amounts of heavy metals that pose severe limitations for their disposal in landfills or for their reuse in agricultural/industrial applications. In particular, cadmium appears to be the most problematic heavy metal in biomass fly ashes, and chromium, mainly in the Cr(VI) state, may be problematic in many ash stabilization processes because of its mobility at high pH values (Lima et al. 2008). Presently, little it is known about the environmental compatibility of blended cements made with Portland cement and biomass fly ash.

In this study, the leaching behavior of a mixture of Portland cement and WBFA was investigated in view of the possible reuse of this kind of fly ash as a mineral admixture in the formulation of blended cements. The eco-compatibility of such a mixture was assessed through the use of a monolith leaching test on hardened cement pastes under constant pH conditions (pH 6.0).

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