Effect Of Mineral Admixtures On The Performance Of Recycled Concrete With Low Shrinkage

With the massive mining of sand and gravel aggregate,the high-quality natural resources are increasingly depleted,and the demolition of a large number of waste-old buildings results,in the accumulation of construction waste,the use of construction waste to produce recycled aggregates and prepare the recycled concrete has become an important development trend in the concrete industry.The large shrinkage of recycled concrete makes the structure easy to crack,which becomes the main reason for restricting the application and dissemination of recycled concrete.High belite sulphoaluminate cement(HBSAC),as a new type of low carbon cement,has the advantages of low shrinkage and low production energy consumption,which can effectively reduce the shrinkage of recycled concrete.However,the cement has low alkalinity and is not suitable for adding a large number of mineral admixtures(SCMs).How to improve the usage of SCMs in high belite sulphoaluminate cement recycled concrete(SHRAC)to reduce the cost and improve the performance has become an urgent problem to be solved.In this study,the SHSA is used to improve the properties of composite cementitious materials.The effects of SHSA content and replacement ratio of SCMs on the physical properties,working properties,and mechanical properties of composite cementitious materials were comprehensively analyzed under three kinds of cementitious series: fly ash,mineral slag powder,and fly ash + mineral slag powder.The optimum content of SHSA and SCMs was determined,and the influence mechanism of SHSA on hydration was clarified.Meanwhile,the SHRAC was prepared by using HBSAC-SCMs composite cementitious material and I-class-recycle aggregate.The effect of SCMs on the working performance,mechanical properties,and durability of SHRAC under SHSA was systematically studied,and the formula for compressive strength of SHRAC was established.The main conclusions from the whole research work can be drawn as follows:(1)Single or mixed with fly ash and mineral powder will reduce the water consumption of standard consistency,prolong the setting time,increase the fluidity of mortar,and reduce the compressive strength.In the single-doped fly ash series,with the increase of SHSA content,the water consumption for standard consistency increases,the setting time shortens,the fluidity of mortar decreases,and the compressive strength increases first and then decreases.When the SHSA content is 4%,the compressive strength reaches the maximum.The changing trend of standard consistency water consumption,setting time,and mortar fluidity of single mineral powder and compound fly ash + mineral powder series is the same as that of single fly ash series.The compressive strength increases with the increase of SHSA content,but the increase of SHSA content slows down the growth of compressive strength.Comprehensive analysis of the performance of composite cementitious materials,the best SCMs content of the three cementitious series is 20 %,and the maximum content of SHSA is4 %.(2)The effects of SHSA on the hydration process of different cementitious materials were studied using microscopic tests including hydration heat,X-ray diffraction(XRD),TG-DTG analysis,and scanning electron microscope(SEM).The results show that SHSA promotes the hydration of HBSAC and SCMs,reduces the early hydration heat of cementitious material system,but increases the cumulative hydration heat.The number of AFt and C-S-H gel increased,and the AFt was arranged more compact,and combined with the cluster C-S-H gel to make the hardened paste denser.The number of AFm decreased significantly,and SHSA effectively prevented the conversion of AFt to AFm.(3)With the increase of SHSA content,the water consumption of SHRAC in different cementitious systems increases,and the compressive strength also increases significantly.When the SHSA content is 4 %,the compressive strength of SHRAC of each cementitious material system increases to the highest.Meanwhile,two compressive strength formulas of SHRAC are established on the basis of the Bolomey and Iaffectoi formula.Error analysis shows that the accuracy of the two compressive strength formulas is both above 95 %.(4)The durability of SHRAC was systematically studied,and the durability difference between SHRAC and HBSAC-RAC,OPC-RAC was compared and analyzed.The results show that SCMs have degradation effects on carbonation resistance,Chloride ion penetration resistance,and frost resistance,but the addition of SCMs can effectively reduce the shrinkage ratio.In addition,although SHSA slightly increases the shrinkage rate of recycled concrete,the SHRAC still has good shrinkage performance under the combined action of SCMs and SHSA,and its largest shrinkage ratio at 120 d is only 82.2% of HBSAC-RAC and 36.1% of OPC-RAC.The carbonation resistance and Chloride ion penetration resistance of SHRAC are greatly improved after adding SHSA.When the SHSA content reaches 4%,the carbonation resistance and Chloride ion penetration resistance of SHRAC are better than those of HBSAC-RAC and OPC-RAC.The SHSA has limited improvement on frost resistance of SHRAC,and the number of freeze-thaw cycles of SHRAC is lower than OPC-RAC.