| Super absorbent polymer(SAP)is a kind of common concrete internal curing material.The existing research thinks that SAP can greatly improve the interface of concrete by improving the degree of hydration.The performance of recycled concrete is significantly lower than that of ordinary concrete,mainly due to the multiple interfaces of recycled aggregate.The interface of recycled aggregate is the weak area of recycled concrete,so we can introduce SAP for internal curing.When the internal humidity of recycled concrete is reduced,the moisture will be released in time,and the bonding degree of the new interface area will be improved by improving the secondary hydration of the new cement slurry,so as to improve the interface of recycled aggregate.It is an important exploration to add super absorbent polymer into recycled concrete,and there are many gaps in this research work.The research work of this subject mainly starts from the important mechanical properties of recycled concrete after adding super absorbent polymer,combined with the micro test analysis,reveals the influence of super absorbent polymer on the internal maintenance of recycled concrete and the related mechanism.The work and conclusions are as follows:(1)The slump test was carried out,and the amount of water reducing agent suitable for SAP recycled concrete under different water cement ratio,different SAP content and particle size were determined through repeated test,and the mixing process of SAP recycled concrete was summarized.(2)The orthogonal test was used to test the compressive strength of SAP recycled concrete,and the influence of water-cement ratio,SAP content and particle size on the compressive strength was analyzed by the analysis methods of range and orthogonal test hierarchy analysis.The results showed that the compressive strength decreased with the increase of water-cement ratio,decreased with the increase of SAP content,and decreased with the decrease of SAP particle size.And the influence order of SAP recycled concrete compressive strength is water cement ratio > content > particle size.Therefore,the recommended water-cement ratio is 0.3,the SAP content is 0.1%,and the particle size is 30-50 mesh.The early strength of recycled concrete with SAP is greatly reduced,but due to the secondary hydration of SAP,the compressive strength of 28 d age will be improved to a certain extent.(3)SEM,XRD and TG-DSC were used to analyze the micro morphology and hydration mechanism of SAP recycled concrete.The results showed that the addition of super absorbent polymer will introduce some irregular pores of several hundred microns into the cement paste,and the growth of ettringite inside and around the pores is vigorous,and the growth of ettringite inside the pores has certain centripetality.The release of water from super absorbent polymer improves the hydration degree of cement particles,and the hydration effect is better than that of the samples without super absorbent polymer.The SAP with high content will absorb water in the early stage and affect the hydration degree of cement,meanwhile the SAP release water in the later stage will promote the secondary hydration of cement.However,the promotion effect of SAP with high content on cement hydration at the later curing stage cannot make up for the negative impact of holes on it.The hydration degree of SAP with small particle size added to recycled concrete was relatively higher in each curing age,but its strength was lower,which may be due to the distribution of small particle size SAP.The lower the water cement ratio,the better the effect of adding super absorbent polymer on cement hydration.Therefore,the principle of SAP in recycled concrete is: low water cement ratio,low content and large particle size. |
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