Conversion of a waste mud into a pozzolanic material

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Abstract

This work investigated the conversion of a waste mud from alum production into a pozzolanic material. X-ray diffraction analysis indicated that the main components in the mud were kaolinite (Al2O3.2SiO2.2H2O), quartz (SiO2) and titanium oxide (TiO2). After calcination at 750oC for 5 h, kaolinite in the mud converted into metakaolinite (Al2O3.2SiO2), and quartz and titanium oxide remained unchanged. The calcined mud showed very high pozzolanic reactivity. The addition of chemical activators such as Na2SO4 and CaCl2 accelerated the pozzolanic reaction between lime and calcined mud and increased the strength of lime–calcined mud very significantly.

Introduction

Most industries produce by-products or wastes which fall outside the usual business line of the manufacturer. The volume of waste continues to grow and approval of facilities for waste processing and proper disposal is becoming more difficult. Many existing disposal facilities are approaching capacity. Furthermore, environmental regulations have become increasingly widespread and restrictive. As a consequence, the cost of waste handling and disposal has increased significantly in recent years. However, some industrial wastes may find applications in other industries. The recycling and reuse of waste materials and by-products in construction is always a popular concept since large quantities of various materials are needed by the construction industry. A recent publication by the Transportation Research Board in the USA has reviewed the recycling and use of various waste materials and by-products in highway construction and maintenance operations [1].

The properties, and chemical and mineral composition of industrial by-products or wastes vary with the characteristics of the raw materials used, the production process and the materials handling process. This causes difficulties for the use of these by-products or wastes in many cases. Characterizing the by-product or waste is an important step before applications are considered.

Red mud is a by-product from the production of aluminum. It is well known that it can be converted into a pozzolan through proper calcination. It is reported that the chemical and mineral compositions of red mud vary with the raw materials used, and have a significant effect on the pozzolanic properties of calcined red mud [2].

In this study, a mud from alum production was characterized and calcined to convert it into a pozzolan. The pozzolanic activity of the calcined mud was evaluated through strength testing by mixing with hydrated high calcium lime. Na2SO4 and CaCl2 were added to the lime–calcined mud mixtures to improve the reactivity of the calcined mud.

Section snippets

Materials

The raw mud has the appearance of natural clay and has an average moisture content of 40%. The typical chemical analysis of the mud is summarized in Table 1. The wet mud was calcined at 750°C for 5 h and then ground to a powder in a rotary ceramic jar with ceramic balls.

A commercial hydrated high calcium lime was used in this study. Reagent grade Na2SO4 and commercial flake CaCl2.2H2O were used as chemical activators for lime–calcined mud mixtures.

Evaluation of pozzolanic activity of calcined mud

To evaluate the pozzolanic activity of the

Effect of calcination on mineralogical composition of the waste mud

The XRD diffractograms of the raw and calcined mud are shown in Fig. 1. The raw mud consists mainly of kaolinite [Al2Si5(OH)4], anatase (TiO2) and quartz. There is an unidentified peak at approximately 2θ=27.4°. After calcination, all peaks for kaolinite disappeared, while peaks for anatase, quartz and the unidentified compound remained.

Strength development of lime–calcined mud pastes

The strength development of the lime–calcined mud pastes is shown in Fig. 2. For the control batch CNP1, there was a continuous steady gain in strength from 5

Discussion

XRD analysis, as shown in Fig. 1, indicates that the one of the main components in the raw mud is kaolinite. The calcination of the mud actually converts kaolinite into metakaolinite:kaoliniteAl2Si2O5OH4560−580CmetakaoliniteAl2O3·2SiO2+2H2O

The changes in the structure of clay during calcination are summarized in Table 2. The detailed effects of calcination on the change in structure of clay depends on the mineralogy of the material, the inherent atomic structure of the clay minerals,

Conclusions

The raw mud from the production of alum consisted mainly of kaolinite, quartz and anatase. After a calcination of the mud at 750°C for 5 h, the XRD peaks for kaolinite disappeared and amorphous metakaolinite formed, while XRD peaks for quartz and anatase remained.

Compressive strength testing of lime–calcined mud mixtures indicated that the calcined mud appears to be a good pozzolanic material. Impurities, such as quartz and anatase, act as inert fillers. The presence of the Na2SO4 accelerated

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