Sintering of MSW fly ash for reuse as a concrete aggregate

doi:10.1016/S0304-3894(01)00286-2

Journal of Hazardous Materials

Volume 87, Issues 1–3, 12 October 2001, Pages 225-239

https://doi.org/10.1016/S0304-3894(01)00286-2 Get rights and content

Abstract

The sintering process of municipal solid waste (MSW) fly ash was investigated in order to manufacture sintered products for reuse as concrete aggregates.

Four types of fly ash resulting from different Italian MSW incineration plants were tested in this study. A modification of the chemical composition of MSW fly ash — through a preliminary four-stage washing treatment of this material with water — was attempted to improve the chemical and mechanical characteristics of sintered products.

The sintering treatment of untreated or washed fly ash was performed on cylindrical compact specimens (15 mm in diameter and 20 mm in height) at different compact pressures, sintering temperatures and times.

The sintering process of untreated MSW fly ashes proved to be ineffective for manufacturing sintered products for reuse as a construction material, because of the adverse chemical characteristics of these fly ashes in terms of sulfate, chloride, and vitrifying oxide contents.

A preliminary washing treatment of MSW fly ash with water greatly improved the chemical and mechanical characteristics of sintered products and, for all the types of fly ash tested, the sintered products satisfied the Italian requirements for normal weight aggregates for use in concretes having a specified strength not greater than 12 and 15 N/mm², when measured on cylindrical and cubic specimens, respectively.

A compact pressure of 28 N/mm², a sintering temperature of 1140°C, and a sintering time of 60 min were the best operating conditions for manufacturing sintered products of washed MSW fly ash.

Introduction

Fly ash resulting from municipal solid waste (MSW fly ash) incineration has to be regarded as a hazardous material, due mainly to its high heavy metals content. Therefore, MSW fly ash needs careful management and its disposal requires either high protection level landfills or treatment before landfilling, so as to reduce contaminants release into the environment.

At present, the most widely applied treatment for the inertization of MSW fly ash is the solidification/stabilization (S/S) process, generally using cement as a binder [1].

The main drawback of the S/S process is represented by the considerable increase in the volume of waste to be disposed of, due to the significant amount of cement utilized in the inertization process. However, a recent work [2] proved the feasibility of a washing pretreatment of MSW fly ash with water as a means of maximizing the incorporation of fly ash in cementitious matrices (ash content up to 75–90% by weight of total solid) without the risk of heavy metals leachability from solidified products.

Thermal processing of MSW fly ash represents a promising technique in view of both immobilizing the heavy metals and reducing the volume of residue to be managed [3]. Moreover, when properly conducted, thermal treatment can produce a suitable material for further utilization, so representing a cost-saving method of treating waste combustion residues.

Recently, thermal treatments such as melting or sintering were applied to MSW incineration residues for producing dense, amorphous material (vitrified products) [4] or coherently bonded particles (sintered products), respectively [5].

The vitrified products proved to be suitable for use as aggregates in Portland cement or asphaltic concretes for roadways, as well as for walking or garden tiles and high temperatures mineral wool insulation [4]. However, vitrification appears to be a too expensive process.

On the other hand, the possibility of using sintered products as concrete aggregates is largely depending on the operating conditions adopted for sintering. It is undoubted that the compaction degree of powders, the sintering temperature and time, as well as the chemical composition of fly ash can significantly affect the type and amount of porosity and, consequently, the specific gravity, mechanical strength and heavy metals leachability of sintered products, as well as their chemical stability in aqueous solutions.

In this regard, Wang et al. [5] found that, for one MSW fly ash collected from a mass burning incineration plant located at Taiwan, changes in the compact pressure (14–35 N/mm²), sintering temperature (1100–1140°C) and sintering time (10–60 min) caused a relevant variation of the apparent density, compressive strength, moisture absorption, and heavy metals leachability of sintered products.

At present, little is known about the effects of the chemical composition of MSW fly ash on the optimum sintering conditions and the properties of sintered products.

Therefore, the present study was undertaken to search thoroughly into this subject. Four types of MSW fly ash with different chemical composition were tested, and the sintering process of these materials was aimed at manufacturing sintered products for reuse as concrete aggregates.

A modification of the chemical composition of these fly ashes — through their preliminary washing treatment with water — was attempted to improve the chemical and mechanical characteristics of sintered products.

Section snippets

Materials and methods

The fly ashes used in this study came from MSW incineration plants located at four different Italian cities (Bologna, Forlı̀, Modena, and Reggio Emilia). These fly ashes were designated by notations indicating the respective place of origin (BO, FO, MO, and RE).

Each fly ash was preliminarily dried in an oven at 105°C and then analyzed for its particle size distribution and chemical composition.

The particle size distribution was evaluated by a laser diffraction technique. The chemical

Sintered specimens of untreated fly ash

Table 1 gives the elemental analyses, LOS-values, and particle size distributions of untreated MSW fly ashes.

It is well known that the particle size and the particle size distribution of powders are two very important factors in affecting the sintering process. In particular, surface diffusion and grain boundary diffusion are very sensitive to particle size [8].

As can be seen from Table 1, all the tested fly ashes were characterized by particle sizes less than 150 μm and did not significantly

Conclusions

The sintering treatment of Italian untreated MSW fly ashes is an ineffective process for manufacturing sintered products for reuse as concrete aggregates, because of the adverse chemical characteristics of these fly ashes in terms of chloride, sulfate, and vitrifying oxide contents.

A modification of the chemical composition of MSW fly ash — through a preliminary four-stage washing treatment of this material with water — represents an effective way of improving the chemical and mechanical

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