Preparation, properties and microstructure of autoclaved lightweight aggregates using CFBA and magnesium slag as solid waste raw materials

Circulating fluidized bed ash (CFBA) and magnesium slag (MS) are two types of industrial solid waste with low utilization rates. This study innovatively prepared autoclaved lightweight aggregates (ALA) using CFBA and MS, where MS partially or fully substituted cement clinker. The effects of CFBA and MS on granulation performance, mechanical strength, density and water absorption of ALA were investigated, and hydrothermal products were characterized via XRD, FTIR, SEM-EDS and 29 Si NMR. The results showed that despite its lower reactivity, MS can partially or even fully replace clinker to produce ALA. For MS-CFBA-based ALA, compressive strength, bulk/apparent density rose first and then declined with increasing MS dosage, while water absorption presented an opposite trend. Under the present experimental conditions, 100 wt% solid-waste-derived ALA with a maximum cylinder compressive strength of 19.1 MPa was obtained at 55 wt% MS. Excessive MS (>75 wt%) triggered expansive cracking attributed to Mg(OH)₂ formation. Under autoclaving, γ-C₂S in MS reacted with CFBA to form tobermorite. The addition of 55 wt% MS facilitated the generation of densely-stacked plate-like tobermorite; over one-third of Si atoms exist as tobermorite in this case. The clinker-free ALA herein exhibits favorable economic and environmental benefits, providing a feasible route for large-scale utilization of CFBA and MS.