Development of insulating backfill materials produced from the ternary mixtures of red mud, fly ash and preformed foam

Many public utility lines to transport power, water, natural gas, water, sewer, and communication are placed beneath trafficable areas. However, insufficient or inadequate backfill induces sudden subsidence, damage, or pothole on the road. This study aims to develop lightweight controlled low-strength materials (CLSM) with low thermal conductivity using the ternary mixtures of red mud to replace aggregate sand, high carbon fly ash, and preformed foam. Changes in flow consistency, air void characteristics, bulk unit weight, unconfined compressive strength (UCS), and thermal conductivity (k) of the tested materials with varying red mud contents were investigated as a function of the foam volume ratio (FVR). The results demonstrate that the UCS and k of tested materials decreased with increasing FVR due the decrease in unit weight, and a greater UCS but smaller k was observed at a given FVR with increasing red mud content because the inclusion of red mud in the lightweight CLSM mix design helps improve the stability of air bubbles and achieve uniform distribution of air voids. In addition, the red mud can act as a NaOH supplier, leading to the developed material had additional strength gain from the alkali activation. Thus, the developed insulating backfill material showed 43 % decrease in k while maintaining UCS similar to non-foam CLSM without red mud.