Fifty percent of the studied papers introduce residues classified under the EWC codes 01, 10 and 19. Wastes classified in the EWC code 01 are wastes resulting from exploration, mining, quarrying and physical and chemical treatment of minerals. The main source processes generating this type of waste are physical and chemical processing of ornamental rocks, mining and quarrying works and alumina production. Wastes belonging to the EWC code 10 are wastes from thermal processes: power stations and other combustion plants (except those classified as EWC 19), iron and steel industry and aluminum, lead and zinc thermal metallurgy. In this category are included, among others, coal or biomass fly ash, metallic sludge and slag and foundry sand. Wastes classified in the EWC code 19 are wastes from waste management facilities, off-site wastewater treatment plants and the preparation of water intended for human consumption and water for industrial use such as sewage sludge or incinerated sewage sludge ash. In addition, a high percentage of the wastes introduced into ceramics comes from agriculture (EWC 02) and from construction and demolition waste including excavated soil from polluted sites such as waste bricks or river and marine sediments (EWC 17).

Figure 7.2 shows seven different categories of roles (A = fluxing agents, B = fillers, C = clay substitutes, D = body fuels, E = pore formers, F = property affecting wastes) that the waste could play in the ceramic matrix during the firing process. A single alternative material may have different roles in the firing step. For instance, fly ash with high content of carbonaceous matter and alkaline compounds may act as both body fuel and fluxing agent, forming liquid phases at relative low temperatures, contributing to the sintering consolidation. On the other hand, fly ash with low carbon content and negligible content of alkaline compounds may act as clay substitute or as a filler when these ashes are predominantly composed of quartz (Vieira & Monteiro, 2009).

The role A of fluxing agents is composed of materials with a relatively high amount of alkaline oxides, mainly K₂O and Na₂O, which in reaction with silica and alumina promote liquid phase formation at relatively low firing temperatures and, thus, contribute to the sintering consolidation and densification of the ceramic structure. Among the wastes included in this category are glassy wastes, boron-containing residues, Waelz slag, steel slag, bone ash, ash from the gasification of coal and sludge from the ornamental stone industry. For instance, granite is considered a flux material due to its large amount of alkaline oxides. These oxides derive from feldspars and micaceous minerals that are common constituents of granite rock.

The role B of fillers includes wastes which can partially substitute the effect caused by the sand in the vitrification process; this means the dissolution of the inorganic waste material in the glassy phase that occurs during the sintering of clay or the reaction with clay minerals to form new mineral phases in the fired clay structure. Among the wastes possibly found in this category are sewage sludge, ash generated in the incineration of sewage sludge, steel dust, steel refining sludge, foundry sand and spent catalyst rejects.

The role C of clay substitutes is made up of waste with a certain amount of clay minerals that confer the plastic behavior to the ceramic matrix. Among the wastes included in this category are municipal solid waste incineration slag, grog or chamotte, water treatment residues and fly ash from the paper industry. The role D of body fuel agents contains wastes including combustible carbon-containing matter, which has a relative heating power, and is therefore desirable for saving energy. Among the wastes included in this category are oily residues, blast furnace sludge which still has a significant amount of coke, sludge from the paper industry, ash with high carbon content, sewage sludge and sawdust.

The role E of pore formers includes wastes that usually possess a high content of organic matter that burn out to form pores. It is important to observe that the inclusion of alternative materials in the ceramic process normally changes the properties of the fired product with substances that modify the ceramic behaviour and cannot be included in the previous role categories. Therefore, an additional role F of property affecting wastes has been included that contains all the summarized wastes.

The most common roles played by wastes in the ceramic matrix are role E of pore formers, role A of fluxing agents and role D of body fuels. Porosity is a desirable property to obtain acoustic and thermal isolating products, while fluxing agents and body fuels promote energy savings by reducing considerably the firing temperature of the bricks. This leads not only to a much lower heat requirement as compared to the traditional process but also to higher economic profits.