Production of cellular concrete from waste

Imagine, in a construction, being able to choose the density of the sealing blocks and the subfloor.

In this sense, a low cost aluminum slag expanding agent is being developed for incorporation into mortars, aiming at replacing aluminum powder (used in the manufacture of autoclaved cellular concrete) and foaming agent (used in concrete molded cell phone “in loco”). This blowing agent can be produced from primary slag grinding. The resulting material contains finely divided metallic aluminum and aluminum carbides that release, when in alkaline medium, hydrogen and methane. These gases are responsible for the expansion of the mortar, decreasing its density.

Tests were carried out to determine the properties of autoclaved cellular concrete with the addition of an expanding agent (dry bulk density – NBR 13440 and compressive strength NBR 13439), maintaining the same industrial processing conditions. In the case of non-autoclaved concrete, the compression rupture performance (NBR 5739) was evaluated, for densities between 500 and 1400 kg / m3 and ages 7, 14 and 28 days. In financial terms, this technique represents a viable alternative for replacing common expanders.

INTRODUCTION

Light concrete in relation to concrete sealing blocks, the main acceptance criterion is to obtain a compressive strength of the order of 2.5 MPa (1), with the lowest possible density. With regard to home screeds, the vast majority of them can have half the density normally used. When using the technology of incorporating air into light concrete, the gain in material and in the reduction of efforts to which the structures are subjected are very significant.

However, currently, few options are available on the market for the production of light mortars, the main ones being autoclaved cellular concrete and foamed cellular concrete. Autoclaved cellular concrete is a material that starts from the mixture of fine sand, micro lime and fine metallic aluminum powder. When in a basic medium, aluminum reacts with lime, releasing hydrogen gas 2Al + 2OH- – by expanding the mortar. This expanded mortar is autoclaved, that is, it is a processing that causes a phase transformation to occur, resulting in a highly resistant component: tobermorite.

The autoclaved cellular concrete blocks reach 2.5 MPa of compressive strength with a density of 550 kg / m3 (this density value corresponds to 30% of the density of conventional mortars). In foamy cellular concrete, the incorporation of air is achieved by adding foam, obtained in a generator. This foam is then mixed with the mortar. Foamy concrete is cast “in loco”, its density varies from 1000 to 1800 kg / m3 and the respective compressive strength, after 28 days, from 4 to 25 MPa (2). Thus, it is only possible to obtain products based on foamy concrete with a resistance of 2.5 MPa with twice the density of autoclaved concrete, that is, 1100 kg / m3. In this case, autoclaving is not performed, but a foam generator is required. The recycling of aluminum slag The slag resulting from the production of aluminum can be classified into two types:

• Primary, from the processing of bauxite, whose main constituents are: alumina (Al2O3), aluminum nitride (AlN), spinel (MgAl2O4) and metallic aluminum (Al) from 20 to 80% by weight. The salt content is low, on the order of 2%.

• Secondary, which can come either from recycling primary slag or from recycling aluminum scrap, contains KCl, NaCl and silica, in addition to the constituents present in the primary slag. The aluminum content ranges from 5 to 20% by weight.

The salts contained in the secondary slag are easily leachable by water and, therefore, must be deposited in industrial landfills. Due to the high cost of this operation, around US $ 50 / ton, this practice is not always observed, mainly by small companies in the sector.

In addition to salts, other residues are generated in the recycling process, which involves grinding and washing slag. These residues are, in general, a mixture that contains free metals, metal oxides, carbides and nitrides, among others, and when in contact with water they produce gases such as acetylene (C2H2), methane (CH4) and ammonia (NH3) , in addition to hydrogen (H2). The main reactions that occur during slag leaching are:

2AlN (s) + 6H2O (l)  2Al(OH)3 (aq) + 2NH3 (g) (1)

2NH3 (g) + H2O (l)  NH4+(aq) + OH-(aq) (2)

2Al + 2H2O (l) + 4OH-(aq)  2Al(OH)3 (aq) + H2 (g) (3)

Al4C3 (s) + 12H2O (l)  4Al(OH)3 (aq) + 3 CH4 (g) (4)

CaC2 (s) + 2H2O (l)  Ca(OH)2 (s) +C2H2 (g) (5)

Reactions (1) and (2) promote an increase in the concentration of hydroxyl ions,
increasing the pH of the solution to values ​​greater than 8, which favors the occurrence of the reaction (3). This reaction, in addition to releasing a large volume of hydrogen, is strongly exothermic and raises the temperature of the system, favoring the increased release of gases from reactions (1), (4) and (5).