Optimization Design And Application Research Of Gypsum-slag Cementitious Materials

Traditional Portland cement is the most widely used building material,but its production process has brought serious environmental and energy problems.With the promotion of"carbon peak and carbon neutralization"national strategy in China,it’s urgent to develop low-carbon building materials.Besides,China is also suffering from industrial solid waste overstocking because of low recycle rate.Product low-clinker cementitious material by consuming industrial by-products and industrial solid waste is one of the best to reduce the CO₂ emission of cement manufacturing and industrial solid waste cumulating at the same time.For now,this cementitious material has shown good application potential in roadbed laying,soil solidification and tailings filling.gypsum-slag cementitious material,as a kind of low-clinker cementitious material,born with the good characteristics of high utilization rate of industrial solid waste and remarkable environmental benefits,but their application is limited by low early strength and long setting time.Thus,this study focus on the performance optimization and application of gypsum-slag cementitious material.Firstly,we evaluated the macroscopic performance and micro structure of gypsum-slag cementitious material under different material ratios.Secondly,we undertook to modificate to typical gypsum-slag cementitious material using alkali activators Ca O,Ca CO_3,Ca SO₄and water glass,as well as mineral admixtures metakaolin and silica fume.Finally,the optimized gypsum-slag cement were subsequently applied to tailings filling,and their effects were compared with that of traditional Portland cement.Main conclusions are as follows:（1）The radio of gypsum,slag and cement have a significant influence on the setting time and compressive strength of gypsum-slag cementitious material.In general,with a higher content of slag and cement,and a lower content of gypsum,the gypsum-slag cementitious material showed shorter setting time and higher compressive strength due to more AFt and C-S-H were generated in the hydration reaction.Therefore,the low-cement/high-gypsum cementitious material and the low-gypsum/high-cement cementitious material were typical materials with extreme characters in this study.（2）Metakaolin and silica fume both showed good modification effect on the macroscopic properties and microstructure of the two typical gypsum-slag cementitious materials.Mixing 6%metakaolin or silica fume into gypsum-slag cementitious materials can significantly improve their compressive strength and condensation process speed,and the maximum economic benefit were obtained under this condition.Taking one with another,the modification effect of silica fume is better in the same proportion.（3）The modification effects of the four alkali activators on the two typical gypsum-slag cementitious materials are quite different.Taken together,the modification effects of Ca SO₄and water glass were the best on both gypsum-slag cementitious materials,and Ca SO₄ is best matched with the low-gypsum/high-cement cementitious material.（4）In tailings filling experiment,the fluidity,bleeding rate and settlement rate of slurries were mainly affected by slurry concentration,but not hardener material constituent or binder-aggregate ratio.When the slurry concentration is 70%,all of the four hardener slurries’fluidity,bleeding rate and settlement rate were meet the engineering demands.But it should be noted that,when using gypsum-slag cementitious materials as hardener,the bleeding rate and shrinkage rate of slurries were lower than that of traditional Portland cement.（5）The strength of the solidified bodies were always rise along with the increase of binder-aggregate ratio and slurry concentration,but those two factors have different mode of action when hardener changes.Among those four hardeners,low-cement/high-gypsum cementitious material activated by Ca O always show better performance than the traditional Portland cement in tailings filling experiment,which is a superior candidate as a tailings filling hardener.