| Reactive powder concrete?RPC?is a building material with ultra high strength,high toughness,high durability.It has been used in roads,bridges,and tunnels,et al.Plain RPC is brittle and this property is more obvious when the RPC intensity is higher.In order to improve the toughness and ductility of RPC materials,fiber has become an indispensable raw material for RPC.Although the incorporation of fibers does greatly improve the brittleness problem of RPC,it also causes problems such as deterioration of the fluidity of the RPC slurry and an increase in cost.Basic magnesium sulphate cement is a modified ternary system cementitious material composed of MgO-MgSO4-H2O,and its main hydration product 5Mg?OH?2·MgSO4·7H2O?5·1·7 phase?exhibits exhibits the characteristics of needle-shaped crystals.The length of the needle-shaped crystal is 1020?m,the diameter is 0.20.5?m,and the aspect ratio is between 20100.Based on these,this paper envisages the preparation of RPC materials using the characteristics of in-situ growth of fibers of basic magnesium sulphate cement.Basic magnesium sulphate cement based RPC is prepared without fiber,and the flowability can be improved,and the strength and toughness of the PRC can be guaranteed,and at the same time reduce the cost.First,Basic magnesium sulfate cement is one of the important raw materials for basic magnesium sulfate cement based RPC.The effects of oxygen-sulfur ratio,water-sulfur ratio,modifier content?citric acid,malic acid,tartaric acid?on the mechanical properties and water resistance of basic magnesium sulphate cement were investigated by single factor method,so the optimum formula of basic magnesium sulphate cement can be obtained.The results showed that the optimum formula of basic magnesium sulphate cement was oxygen-sulfur ratio of 12.5:1,water-sulfur ratio of 14:1,and citric acid content of 1.0%?In this case,the 28-day flexural strength and compressive strength of basic magnesium sulphate cement were 16.5 MPa and 95.0 MPa,respectively.Secondly,sand can control the volume change rate and homogeneity of RPC,and sand larger content aggregate in RPC,so the basic range of the basic magnesium sulphate cement based RPC formula can be obtained.Therefore,the effects of oxygen-sulfur ratio,water-sulfur ratio,sand-binder ratio and the dosage of superplasticizer on the mechanical properties and water resistance of basic magnesium sulphate cement mortar were studied by single factor method.The results showed that the optimum range of oxygen-sulfur ratio was 8:112.5:1,the optimum range of water-sulfur ratio was 14:116:1,the optimum range of sand-binder ratio was 0.20.6,and the optimum range of the dosage of superplasticizer was 0.1%0.4%.Then,in order to obtain a basic magnesium sulphate cement based RPC with excellent mechanical properties.The basic mix ratio of RPC was obtained by orthogonal experimental method,and process optimization was carried out in terms of close packing of sand and curing system,etc.Finally,without adding fiber,the 28-day flexural strength of the basic magnesium sulphate cement based RPC was 24.6 MPa,and the 28-day compressive strength was 121.8 MPa.Based on the characteristics of reinforced ordinary cement RPC adding steel fiber,the influence of steel fiber content on the fluidity and mechanical properties of basic magnesium sulphate cement based RPC was also investigated.The results showed that the fluidity decreased with the increase of steel fiber content,but the mechanical properties were continuously improved.When the steel fiber content was 2.0%?volume content?,the 28-day flexural strength was 21.4 MPa and 28-day compressive strength was 120.5 MPa,and the fluidity was 189.7 mm.Finally,in order to explore the application prospect of basic magnesium sulphate cement in civil engineering.Based on the structural beam and under bending and shear conditions,the reinforcement,beam section area and cost of basic magnesium sulphate cement concrete beam and ordinary Portland cement concrete beam were calculated.The results showed that the use of basic magnesium sulphate cement could not only reduce the amount of reinforcement,save the cross-sectional area of the beam,but also reduce the cost.It can save up to 12.7%of the amount of longitudinal reinforcement,31.0%of the cross-sectional area of the beam and 11.4%of the cost.Based on the above experimental results,it can be concluded that the preparation of RPC using basic magnesium sulphate cement is entirely possible and basic magnesium sulphate cement based RPC has great application prospects. |
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