Strength And Volume Stability Of Chemico-activated Steel Slag Based Cementitious Materials

The resource utilization of steel slag has important economic,ecological and social benefits,which integrates energy saving and emission reduction,environmental protection and sustainable development.The preparation of cementitious materials by steel slag provides a green recovery channel for the comprehensive utilization of large scale steel slag solid waste.The chemical composition,mineral composition and hydration characteristics of steel slag are very similar to those of Portland cement clinker and have great potential for use as cementitious material.However,low cementitious activity,slow hydration rate of steel slag and the delayed expansion produced by?-CaO of its composition limit its application in cementitious materials.Based on the technical advantages of resource utilization on alkali-activated steel slag principle,steel slag based cementitious materials were prepared by chemico-activated and slag-intensified steel slag.By means of X-ray diffraction?XRD?,infrared spectroscopy?FT-IR?and scanning electron microscopy?SEM?,the hydration products and microstructure of chemico-activated steel slag/steel slag-slag under different curing conditions were characterized,and the influence of curing conditions on the mechanical properties and volume stability of chemico-activated steel slag based cementitious materials were explored.The main research contents and results are as follows:?1?When steel slag powder is activated by using Na₂CO₃,Na₂SO₄,NaOH and Na₂SiO₃,it is found that Na₂SiO₃ has the best excitation effect,followed by NaOH and Na₂CO₃,and Na₂SO₄ has the worst excitation effect.Under Na₂SiO₃ excitation,The[SiO₄]tetrahedrons and[AlO₄]tetrahedrons of the steel slag vitreous body are depolymerized,and the Si-O bonds and Al-O bonds are broken,dissolved,and polymerized,and then react with Na⁺and Ca²⁺in solution to form C-S-H?I?,C-A-S-H and N-A-S-H gel and other zeolite hydration products.In addition,when the Na₂SiO₃ content?Na₂O equivalent?is 7%,the gelling activity of the steel slag can be fully stimulated.?2?After the chemico-activated steel slag specimens are cured by different curing methods,it is found that natural curing causes drying shrinkage of chemically excited steel slag cementitious materials,resulting in a large number of small cracks inside the specimens,which will lead to the later strength reduce.And under standard curing conditions,the later strength growth rate of the test blocks is high,and the gel water absorption in C-S-H hydration products causes macroscopic volume expansion.Furthermore steam curing can improve the early strength of chemico-activated steel slag.However,it makes a small amount of?-CaO in steel slag participate in hydration reaction and cause volume expansion at the same time.This produces some microcracks inside which destroys the slurry structure,so that the later strength of chemico-activated steel slag under steam curing condition is lower than that of standard curing.Autoclaved curing promotes the chemico-activated steel slag to produce more C-S-H?I?,C-A-S-H,N-A-S-H gels and tobermorite crystals,which enhances the gelation properties between the hydration products of steel slag,thus greatly improving the mechanical property of steel slag cementitious materials.Moreover,autoclaved curing can promote the?-CaO in steel slag to participate in hydration reaction in the early stage,which basically eliminates the hidden trouble of late stability caused by?-CaO.?3?The introduction of slag powder promotes the hydration of the cementitious system of chemico-activated steel slag,and increasing the content of slag powder enhances the strengthening effect on the strength and volume stability of the cementitious system.In addition,the addition of slag powder can inhibit the later strength inversion of chemico-activated steel slag cementitious material under autoclaved conditions.