Experimental Study Of Alkali-activated Steel Slag Recycled Concrete

By 2022,China’s total construction waste storage capacity reaches 23 billion tonnes.The massive accumulation of construction waste not only takes up land resources,but also the environment also suffers serious damage.One of the ways to better realise the resourceful use of construction waste is to produce and research and develop recycled concrete,which is of great significance for China’s sustainable development.Steel slag as a major industrial waste discharged from the smelting process of steel.currently,the cumulative volume of steel slag piled up in China is close to 1 billion tonnes,but the resource utilisation rate is much lower than other countries,less than 30%.If the piles of steel slag are not treated,there will be a lot of waste of land resources and mineral resources in China,and the chemical substances emitted from steel slag will pollute the air,environment,water quality and soil,which will aggravate the burden on the environment in China.In this thesis,an orthogonal test was designed to determine the optimum proportion of cementitious materials by taking the slag admixture,water glass modulus and alkali content as factors;and on the basis of the optimum proportion,the effects of different coarse aggregate replacement rates and additional water use methods of recycled aggregates on the concrete properties,including mechanical properties and durability performance,were studied;finally,XRD and SEM microscopic tests were conducted to analyse the hydration mechanism of the cementitious materials and the microscopic changes of the recycled concrete.The main research findings are as follows.（1）The most influential factor on the strength of the alkali-activated steel slag cementitious material is the alkali content,with the increase in alkali content,the strength of the cementitious material also shows a trend of growth.The optimum mix ratio of alkali-activated steel slag cementitious material is:35%slag,Na₂O·n Si O₂modulus of 1.6,alkali content of 7%,the optimum mix ratio of cementitious material7d compressive strength of 45.58MPa and 28d compressive strength of 51.63MPa.（2）The process of pre-wetting the aggregate using additional water for the aggregate is more beneficial to the performance of the recycled concrete.When the ASRC was pre-wetted with additional water,the 7d compressive strength was reduced by 2.2%,4%,7.8%,13.9%and 20.6%at 20%,40%,60%,80%and 100%respectively,and the 28d compressive strength was reduced by 1.3%,3.1%,6.4%,12.1%and 19.8%.The mechanical properties of alkali-activated steel slag recycled concrete show a decreasing trend with the increase of recycled aggregate replacement rate,and to ensure the good mechanical properties of ASRC,the recycled aggregate replacement rate should not exceed 60%.（3）As the number of freeze-thaw cycle tests is increased and the recycled aggregate replacement rate increases,the ASRC strength loss rate and mass loss rate gradually increase and the frost resistance gradually decreases.As the replacement rate of recycled aggregates increases,the ASRC compressive strength corrosion resistance factor gradually decreases and the resistance to sulphate erosion decreases.（4）Study and analysis of alkali-activated steel slag cementitious materials using XRD and SEM microscopic techniques,the results show that the main hydration products of alkali-activated steel slag gelling materials are Ca（OH）₂,C-S-H gels,C-A-H gels and small amounts of AFt and zeolite-like hydration products.In addition,the addition of slag to the alkali-activated steel slag cementation system can accelerate its hydration reaction and increase the degree of hydration,which has a certain promotion effect on the hydration of steel slag.