Utilization of Recycled Concrete Powder in Cement Composite:Strength,Microstructure and Hydration Characteristics

Recycled concrete powder(RCP)is used more and more in cement-based materials,but its influence on the hydration process is still unclear.Therefore,this paper studied the influence of recycled concrete powder(RCP)on the hydration process of cement and provides a theoretical basis for the hydration mechanism of cement composite materials.The hydration heat method was used to systematically analyze the thermal evolution process of cement paste with or without RCP.Hydration products were identified using X-ray diffraction(XRD)and thermal analysis(TG–DSC).The pore structure change of cement pastes was analyzed by mercury intrusion porosimetry(MIP)method.The mechanical properties of mortar were also evaluated.Four recycled concrete powder(RCP)dosages,such as 10%,20%,30%and 40%are considered.The results indicate that with the increase of RCP content,the hydration heat release rate and total heat release amount of paste decreased,but the second heat release peak of hydration reaction advanced;the proportion of harmful pores and more harmful pores increases,the total porosity and the most probable pore size also increase;the fluidity and mechanical strength of mortar decrease,but the crystal type of hydration products does not change.When the content of RCP is less than 20%,it has little effect on the mechanical strength of mortar.When fly ash and silica fume are mixed,the fluidity difference of mortar decreases,and when the content of fly ash is the highest,the fluidity of mortar is the highest,which is 15mm higher than that of the control group.When RCP content is 15%,fly ash and silica fume content is 15%(FA:SF=3:2),the hydration heat of the clean pulp is the highest among all the compounding ratios,and the hydration reaction is the most complete;the proportion of harmless pores increased by 9.672%,the proportion of harmful pores and more harmful pores decreased,and the compactness of material structure increased;the compressive strength and flexural strength of mortar reached 50.6 MPa and 9 MPa respectively,both exceeding those of control mortar.

Study on the Durability of Recycled Powder Concrete against Sulfate Attack under Partial Immersion Condition

In order to make full use of waste recycled fine powder(RFP)in concrete and achieve the goal of carbon neutrality in the concrete industry,the durability of sulfate resistance is an important aspect of evaluating the performance of recycled powder concrete(RPC).Therefore,the durability of RPC under partial sulfate immersion was studied to provide theoretical guidance for understanding the erosion mechanism of RPC.The compressive strength,mass loss,and microstructure change patterns of RPC under partial immersion of 5%Na2SO4 and MgSO4 solutions were analyzed by cubic compressive strength,mass loss rate,SEM-EDS,and XRD.The results showed that the surface crystalline matter of concrete in Na2SO4 solution was mainly white powders,and that of concrete in MgSO4 solution was mainly transparent paste,both of which had a little spalling on the outer surface of the concrete.The compressive strength and mass loss rate of concrete with 20%RFP was relatively good,indicating that concrete with 20%RFP had better durability against sulfate.The compressive strength of the lower part of the concrete partially immersed in Na2SO4 solution was higher than that of the upper part and the strength of the lower part of RPC-2 was 3.11%higher than the upper part at 180 d;The pattern was reversed in the MgSO4 solution,where the strength of the lower part of RPC-2 was 19.74%lower than the upper part at 180 d.Microscopic analysis showed that the hydration products of RPC were mainly gypsum and ettringite,while the RPC produced more hydration products with the promotion of magnesium ion in the MgSO4 solution.The higher the replacement rate of RFP,the more frequent the gypsum-type failures in the concrete.