A novel backfilling method for underground coal mines has been recently proposed at the University of Wollongong. Different from traditional backfilling technology (i.e. solid backfilling and paste backfilling), the main feature of this technology is that cementitious material with high water-to-solid ratio is directly pumped into the gob filled with coal reject aiming to fill the large number of voids. To verify the feasibility and potential advantages of this new technology compared to its counterparts, a series of compression tests have been conducted. A total of four cubic samples with the dimension of 300 mm and the height of 150 mm have been tested to better understand the effect of cementitious material on the compressive behaviour of the combined backfill. The experiment results show that the strength of combined material with confinement is significantly affected by the coal reject filling coefficient. Based on the experimental observations, the compressive behaviour of the combined backfilling consisting of three typical stages, namely initial compacting stage, support improving stage and stable sedimentation stage has been determined.