276 / 2022-09-23 17:02:00

Alkali-activated Artificial Aggregates Made from Red mud

Alkali-activated Artificial Aggregates; Red Mud; Fly Ash; Granulated Blast Furnace Slag

Shortage of aggregates is becoming a prominent issue in the construction industry due to rapid urbanization. Meanwhile, recycling of various solid wastes has called for ecological sustainability. Preparing artificial aggregates with solid wastes is expected to address the above engineering and environmental problems synchronously. In this research, red mud was adopted as the raw material for alkali-activated aggregates preparation considering its high alkalinity, huge reserves and low utilization ratio.



Currently, there are generally two methods for the preparation of alkali-activated artificial aggregates: sintering and cold-bonding pelletization. Sintering aggregates usually behavior the high strength, low density, and high performance of aggregates, but its production leads to high energy consumption and a large CO₂ footprint, which is harmful to the environment and limits the further application of this manufacturing technique. To avoid environmental burden, cold-bonding aggregates have been proposed and studied by many researchers. In the present work, artificial aggregates were produced using an alkali-activated cold-bonding pelletization process with a high-volume of red mud (RM) as the precursor, fly ash (FA)/ granulated blast furnace slag (GGBS) as the silico-calcium additive, and a mixture of sodium silicate and sodium hydroxide as the activator (Fig. 1).



The mechanical properties, densities, and internal structures of the fabricated artificial aggregates were characterized, as well as the microstructure analysis and product identification (Fig. 2 and Fig. 3). The average single particle crushing strengths of the RM-FA aggregates were in the range of 1.46–6.18 MPa and the mechanical properties of the R80F20 aggregates were optimal (6.18 MPa). Based on the results of the 1-h water absorption tests and the densities, all the RM-FA aggregates can be classified as lightweight aggregates. The internal structural characteristics of the aggregates were porous in the center but dense near the surfaces. The polymerization and hydration products of ettringite (AFt) and C-N-A-S-H gel were found through the analysis of microstructure and chemical composition of the aggregates. For RM-GGBS aggregates, the single particle crushing strengths and cylinder compressive strengths were characterized and the crushing strength of the optimal RM-GGBS aggregates was 10.61 MPa and the compressive strength was 21.17 MPa. The bulk densities and 1-h water absorption results show that the RM-GGBS aggregates are also lightweight aggregates. The morphology and microstructure analysis and chemical composition analysis results show that the internal structures of RM-GGBS aggregates are similar to that of RM-FA ash aggregates and the polymerization and hydration products of RM-GGBS aggregates are more.



The results showed that the optimal artificial aggregates could be possible candidates for mortar and concrete preparation.