Properties Of Alkali-activated Steel Slag Cementitious Material And Concrete

Steel slag is the slag discharged during the steel-making process,which accounts for about 25%of crude steel production.At present,steel slag is mainly used as aggregate or filler in practical engineering,but it has low utilization as cementitious material.Most of the steel slag in China is converter steel slag.Although converter steel slag has its own hydraulicity,the hydration rate is very low.The cementitious property of converter steel slag can be improved obviously by means of alkaline activation,and the preparation of clinker-free concrete with alkali-activated steel slag as cementitious material can not only reduce the cement output,but also solve the environmental pollution problems caused by cement production and steel slag storage.In this paper,a kind of converter steel slag is selected to study its hydration characteristics under alkaline environment.The aim of present research work is to explore the influence of the change of alkaline environment and cementitious material composition on the hydration and hardening of alkali-activated steel slag,and to evaluate the performance of alkali-activated steel slag concrete.The main contents and conclusions are listed as follows:Firstly,compared to that of Portland cement,this paper investigates the hydration characteristics of alkali-activated steel slag.In terms of compressive strength,the activation effects of 4 common activators:water glass>Na2CO3>NaOH>Na2SO4.The hydration of steel slag is similar to that of cement under the activation of water glass.The crystalline hydration product of alkali-activated steel slag is Ca(OH)2 produced by hydration of calcium silicate,and the amorphous product is C-A-S-H gel produced by hydration of calcium silicate and C3A.Compared with Portland cement,although the hydration rate of alkali-activated steel slag is faster,the content of hydration products is lower due to fewer active components.The crystallization of Ca(OH)2 produced by alkali-activated steel slag is poor.The[SiO4]4-tetrahedron in the C-A-S-H gel mainly exists as the structure of Q2.Ca/Si ratio in C-A-S-H gel is smaller but Al/Si ratio is similar.There are many micro-cracks and unreacted particles with smooth surface in the alkali-activated steel slag matrix,which is not conducive to the development of pore structure,but also harmful to the adhesion of the matrix,resulting in low compressive strength.Secondly,the alkaline environment is changed by transforming the modulus of water glass,and this paper investigates the effect of the change of modulus on the hydration and hardening of alkali-activated steel slag.Although the increase of modulus does not affect the type of hydration products,it slightly reduces Ca/Si ratio and Al/Si ratio in the C-A-S-H gel,and also increases the degree of polymerization of the gel.Although the increase of modulus reduces the dissolution and reaction rate of steel slag particles,thus reducing the degree of hydration of the system and the content of hydration products,it makes the distribution of hydration products more uniform and refines the pore structure,resulting in improving the compressive strength.The increase of modulus also makes the fluidity of the fresh paste worse and the initial setting time longer.Then,this paper investigates the influence of slag content on hydration and hardening of alkali-activated steel slag.The addition of slag does not change the type of alkali-activated steel slag hydration products,but changes the composition of C-A-S-H gel.The addition of slag significantly reduces Ca/Si ratio in the gel and increases Al/Si ratio.The addition of slag increases the degree of polymerization of[Si04]4-tetrahedron in the gel.Although the addition of slag delays the hydration reaction,it has little effect on the 96 h accumulated hydration heat.The fluidity of fresh paste becomes worse with the addition of slag.The initial setting time is obviously shortened by the addition of 10%slag,and not greatly affected by the addition of 20%slag.Although slag admixture has little effect on the content of hydration products,it makes the distribution of hydration products more uniform and improves the pore structure,significantly leading to higher compressive strength.Finally,this paper investigates the mechanical properties,volume stability and durability of alkali-activated steel slag concrete.Increasing the modulus of water glass or slag content can reduce the connected porosity of alkali-activated concrete and improve the pore structure of matrix.resulting in improving the compressive strength,splitting tensile strength and static elastic modulus of concrete.With the increase of water glass modulus,autogenous shrinkage,dry shrinkage and chemical shrinkage increased obviously.With the increase of slag content,drying shrinkage and chemical shrinkage decrease,while spontaneous shrinkage first increases and then decreases.Alkali-activated steel slag concrete has good early cracking resistance and belongs to low cracking risk grade.The sulfate resistance of alkali-activated steel slag concrete is better than that of cement concrete.There is no AFt formation in alkali-activated steel slag concrete exposed to Na2SO4 attack,only a small amount of CaSO4·2H2O generated by chemical reaction and a small amount of Na2SO4·1OH2O generated by physical crystallization.The fire resistance of alkali-activated pure steel slag concrete is not as good as that of cement concrete,and the higher the modulus of water glass is,the worse the fire resistance will be.Adding slag can improve the fire resistance of alkali-activated steel slag concrete below 500?.The higher the slag content is,the better the fire resistance of concrete is.