Sintering and characterisation of ceramics containing paper sludge, glass cullet and different types of clayey materials

Abstract

The present paper reports on the sintering behaviour of several ceramics prepared using a previously selected mixture of incinerated paper mill sludge and glass cullet in the ratio 60/40 which was blended with 10, 20, 30 and 40 wt.% of three different natural materials. The three natural products were: a red quartzitic clay, a yellow quartzitic clay and a kaolin. All mixtures were blended by attrition milling and dried; powders were sieved, pressed into specimens and fired for 1 h at temperatures ranging from 1040 to 1140 °C. The resulting materials were characterized by water absorption, shrinkage, crystallographic composition, microstructure and physico-mechanical properties. It was observed that materials containing kaolin display the best overall behaviour independently of the quantity of kaolin introduced. Conversely the optimal sintering temperature, and consequently the best properties of the materials prepared using red or yellow clay were measured on products fired at temperatures above 1080 °C; materials and temperatures are affected by the amount of clay added.

Introduction

In a previous paper [1] we described the production and characterisation of some ceramics containing mixtures of paper sludge (PS) and glass cullet (GC) from recycled colourless bottles; we demonstrated that materials containing 60 wt.% of PS and 40 wt.% of GC have good physico-mechanical properties independently of the type of PS used. Conversely, their shrinkage exceeds the limits established by the standards for tiles production. On the progress of that research [2], we reported results obtained with materials prepared by mixing the above mixture with increasing amounts of natural red clay. It was demonstrated that the addition of 30 wt.% of a natural red clay enables fast firing production of unglazed tiles. More recently, similar trends were observed on ceramics produced changing the GC origin [3]; if GC from waste colourless bottles is replaced by waste energy saving lamps, the best performing composition is the one containing a mixture PS/GC in the ratio 60/40 just like the binary system already studied.

In this regard, it is worth remembering that GC from waste bottles or windows is classified, in the European Waste Catalogue, as a non-dangerous product and it is presently recycled in glass production. Conversely, GC from exhaust energy saving lamps often contains mercury vapours and, before any eventual landfill disposal or recycling, mercury must be removed by a specific washing. This type of GC mainly derives from industrial buildings and represents a little fraction of the total production of GC, but most of existing waste official norms establish that it cannot be recycled in glass production. It follows that, in order to propose some easy and environmental friendly ways for recycling, alternative methods to landfill disposal are now under study.

The present study deals with the production procedure, followed by materials characterisation of some ceramics containing the blend PS/GC = 60/40 which were added of 10, 20, 30 and 40 wt.% of three different natural raw materials namely: a red quartzitic clay (RC), a yellow quartzitic clay (YC) and a high grade kaolin (K). The goal of the study is to evaluate the influence of their chemical composition on the performances of the resulting fired materials which were characterized by shrinkage, water absorption and X-ray diffraction, microstructure and mechanical properties.