The use of solid residues derived from different industrial activities to obtain calcium silicates for use as insulating construction materials

Abstract

Calcium silicates have obtained through solid state reaction of the different residues generated in various industrial activities. As a source of calcium oxide: marble, mussel shells, and the reagent commercial calcium hydroxide were used. The source of silica was biomass ash and fired ceramic residue formed by crushing pieces of broken and defective ceramic products from a brick factory (chamotte). From the raw materials, biomass ash and marble, biomass ash and commercial calcium hydroxide, and chamotte and crushed mussel shell dust, mixed in a molar ratio CaO:SiO₂ 1:1 and sintered at 1100 °C (24 h), calcium silicates such as wollastonite, gehlenite and larnite were obtained. Both the raw materials and the synthesized material were characterized by XRD, XRF and TGA–DTA. In order to use the calcium silicates obtained as low temperature thermal insulating ceramic materials, the materials obtained were compressed under uniaxial loading at 81.7 MPa to obtain bricks measuring 60 mm × 30 mm × 10 mm. The properties of the bricks were studied. The ceramic materials present conductivity values between 0.10 W/m² K and 0.18 W/m² K and compressive strength of 29.8–59.3 MPa, respectively. The bricks met the UNE guidelines for use as low-temperature structural insulation ceramics.

Introduction

There is a direct relationship between a country's degree of development as represented by its citizens’ per capita income and the generation of large quantities of solid residues. The richer a country is, the more its residues increase. If we are to prevent the collapse of the means of production and of the plants, we cannot continue the current rate at which we generate residues. We must reduce them by fostering reuse and recycling [1], [2], [3], [4], [5].

The Spanish ceramics sector is the largest European producer of construction bricks. Entrepreneurial activity of the ceramics industry extends throughout the national territory, and the region of Andalusia is the second largest producer of bricks and tiles, after Castilla-La Mancha. The city of Bailén (Jaén) is the main centre of production in Andalusia [6]. This industry generates residues of fired products from partially broken or defective and thus unsellable materials (chamotte). At present, their use before grinding is very widespread in all of the ceramics sectors, both the brick and tile production industry and sanitary ware, depending on the amount of the product that can be reintroduced [7], [8], [9], [10]. This material is also being used as a degreasing agent and as a raw material for making Portland cement clinker [11]. Its use is not profitable, however, since special grinding installations are needed to grind the fired materials, with significant additional maintenance costs that do not usually compensate for the savings on raw materials, hence the current problem.

In the wooden board manufacturing industry, the residues generated during processing (constituted primarily of wood scraps from the bark of conifers) are used as fuel to obtain electrical and thermal energy. The combustion process generates a large quantity of ashes as residue, which usually accumulates in authorized dumps. Both the ashes and the chamotte are potential sources of oxides, primarily silica.

The marble industry generates residues from the accumulation of large quantities of dust and white mud during the process of cutting and polishing the material. The materials are constituted mainly of calcium carbonate. To illustrate the magnitude of the waste, sawing 3000 m² of marble alone generates 600 tonnes of residue. Broadly speaking, environmental problems in the marble sector consist of the dumping of residues and mud from the processing industry into public waterways, as well as the serious degradation of the urban environment in population centres that house the industries.

The canning industry is one of the most important industries in the north of Spain, especially in Galicia, and particularly in the conservation of mussels. After China, Galicia is the first world producer of mussels. Every year, the mussel sector produces 256,000 tonnes of mussels, of which 65% is processed by the canning and freezing sector, producing 81,000 tonnes of mussel shell residue. The canneries, boileries, and refineries have great difficulty getting rid of the mussel shells after processing and packaging the mussels. Mussel shell is not a toxic or dangerous residue. The biggest problem is the smell emitted by the decomposing organic material, which enables the shell to be colonized by microorganisms, among them pathogens that can be transmitted to humans or animals. The accumulation of the shells also creates visually unattractive mountains of residue with serious drawbacks from the perspective of the environment, landscape, and even economics. One possible solution to this problem is to deposit the mussel shells in authorized dumps, although this causes problems as well. Both marble residue and mussel shells are composed primarily of calcium carbonate.

Calcium silicates are composed of the system CaO–SiO₂, including wollastonite, which is generated in nature mainly by hydrothermal processes of mixing limestone sediments with silica [12], [13]. Other interesting composites are dicalcium and tricalcium silicate, which are important phases in cement. The most important applications of calcium silicates are found in the ceramics industry, in manufacturing tough ceramics and bioceramics, in metal foundries as melting additives or simply as loads or coating, in the manufacturing of dielectric porcelains, in paints, in the composition of bioactive glass, in the production of resins and plastics (polyester, polyvinyl, nylon or polypropylene), and as a reinforcement material to improve the physical, chemical and electrical properties of the finished product. They are also used in the production of insulating ceramic as a substitute for bones and coatings [14], [15], [16], [17], [18].

Building on these antecedents, the present study used ashes and chamotte as a source of silica, the residue from marble remains and mussel shells, and the reagent commercial calcium hydroxide as a source of calcium oxide from which to obtain calcium silicates. Further, the bricks were formed by uniaxial compression of the silicates obtained. We studied their technological properties to determine their application as low-temperature thermal insulating ceramic materials.