Cement-free binder systems based on alumina production waste hardened by forced carbonation

Introduction. The disposal of industrial waste to produce a useful product is one of the basic tenets of the concept of sustainable development. In this regard, an urgent task is to study the production of binders and building products based on them from waste from metallurgical industries, which harden in the environment of carbon dioxide due to its absorption, while acquiring the necessary physical and mechanical properties.

Materials and methods. The main raw material component was nepheline sludge, a secondary raw material formed from nepheline concentrates of the Kola Peninsula during the production of alumina by the alkaline method. The crystalline phases were analyzed by measuring X-ray diffraction (XRD) on a Europe 600 (GNR) diffractometer. Thermal analysis to determine the mineralogical composition of nepheline sludge and artificial stone based on it was performed using a STA 8000 analyzer (Perkin Elmer). The dispersed composition of the nepheline sludge was determined by laser diffraction on a laser particle size analyzer Partica LA-960 (Horiba). Porosity was changed using the Automated Standard Porosimeter 3.2 (MPM&P Research Inc.). The mechanical characteristics of the prototypes were determined on an automatic servo-hydraulic MCS8 control console complete with software.

Results. Nepheline sludge consists of at least 85 % by weight from belite (β-C2S). It was revealed that the process of active CO2 absorption by nepheline sludge is observed in the first 120 minutes of carbonation. The compressive strength of the resulting material reaches 58 MPa, and after subsequent hydration, the strength increases to 70 MPa. The increase in strength is due to the formation of calcium carbonate in the carbonizing system. During subsequent hydration, gel-like hydration products of belite are formed, which additionally strengthen the carbonized material. Using the method of reference contact porosimetry, it was found that as a result of forced carbonation, the material is compacted, and the total porosity of the prototypes is significantly reduced. It is determined that the effective pore radii of carbonized specimens are shifted towards lower values in comparison with specimens not subject to carbonation, while this process takes place in the macropore region (0.1–16 µm).

Conclusions. Based on nepheline sludge, it is possible to obtain construction products that harden in a carbon dioxide environment due to its binding and subsequent hydration process, to obtain a material with high physical and mechanical properties.

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