Abstract: The red mud has a huge stock and high increase, which brings serious problems such as safety, environmental protection and land occupation. To expand red mud resource utilization and advance green composite foundation technology, we propose a red mud–fly ash geopolymer (RFG) pile technology. This study evaluated the material preparation, construction method, pile quality, vertical bearing characteristics, and environmental impact of the RFG pile, based on laboratory tests, an in-situ pile installation test, and a static load test on a silty-clay foundation. The results show that: 1) RFG piles can be formed by pumping and underwater casting using conventional wet drilling. When the mass ratio of red mud: fly ash: medium sand: gravel: blast furnace slag powder is 1:2:2:4.94:0.1, with a liquid-solid ratio of 0.55, the RFG mixture exhibits good workability suitable for pumping and underwater pouring. The slump can be controlled in 220−225 mm, and the strength can reach 13.9 MPa after 28 days of curing at room temperature. 2) RFG piles exhibit strong compressive bearing performance. For a single RFG pile 10 m long and 0.52 m in diameter, the vertical ultimate bearing capacity is 753 kN; the bearing capacity of a single-pile composite foundation with a spacing of 3 diameters is 240 kPa. 3) On-site formation quality of the RFG pile is good, and core samples collected along the pile length are largely intact. The coring rate of the six piles is more than 99%, and the compressive strength of the core samples at different depths is more than 10 MPa, up to 19.5 MPa. 4) The implementation of RFG piles has no adverse impact on the quality of surrounding water bodies and has not changed the evaluation results of groundwater corrosivity. The research results lay a foundation for the engineering application of RFG pile, and also provide a new idea for improving the utilization rate of red mud

Key words: red mud, red mud-fly ash geopolymer (RFG) pile, composite foundation, field test, compressive bearing capacity