Effect of gypsum on strength performance of cemented backfilling materials of red mud-slag system

Four types of different gypsum,i.e.flue gas desulfurization waste(FGDW) gypsum,natural gypsum,semihydrate building gypsum and chemical pure gypsum were used as active stimulator of the backfilling materials,and the strength of the backfilling material samples affected by these four types of gypsum were studied.The results show that the samples containing gypsum of FGDW have the highest strength of 7 d and 28 d curing.In addition,the strength of the backfilling material samples is also greatly affected by the mixing percent of the FGDW gypsum.The strength increases continuously when the FGDW gypsum content increases in the samples.The maximum strength occurs when the mixing amount of FGDW gypsum reaches 19%(mass fraction).The further increase of FGDW gypsum in the samples causes the decrease of the samples strength.The samples containing gypsum of FGDW have higher strength than those of chemical pure gypsum because of the dissolution rate and ionic atmosphere.Large amount of reaction products is formed by the hydration reaction,and these include ettringite,C—S—H gel and nepheline minerals.