Design And Preparation Of Porous Fine Aggregates And Internal Curing Effects In The Cement-based Materials

With the rapid development of large-scale urbanization in China,the application of concrete consumes a large number of natural aggregates,and the massive exploitation of sand and gravel has caused the destruction of land resources.Compared with natural aggregates,lightweight aggregate(LWA)has a porous structure.The lightweight concrete prepared reduces the dead weight of the structure and has the advantages of low density,good thermal insulation,and frost resistance.The studies have found that the development of autogenous shrinkage of cement-based materials with low water-to-binder(w/b)ratio can be effectively inhibited by using the water-releasing function of pre-absorbed LWA.Howev er,the design goal of LWA s uch as lightweight and thermal insulation reduces the density and water absorption rate of LWA.As a result,high content of LWA is added to meet the amount of internal curing water required for the chemical shrinkage of cementitious materials,which will reduce the compressive strength of concrete.In addition,fine LWA has a poor water retention capacity due to the large pore size.It is easy to release water before the initial setting of the paste,resulting in an increase in the porosity of the concrete.Although some LWA has high water absorption,the low density makes it easy to float in the concrete with high workability,which causes the deterioration of the overall strength of concrete.At present,there are many studies on the factors affecting the internal curing of LWA,such as the dosage and treatment method,but there are few studies on the characteristics of LWA,especially on its pore structure.On the basis of the traditional preparation technology of LWA,the design principles and performance requirements,raw materials,and preparation technology of porous fine aggregate(PFA)w ere studied by reusing solid waste resources in this work.The effect of pore structure on the water absorption and desorption performance of PFA was clarified.At the same time,the effects of PFA on the flowability,autogenous shrinkage,compressive strength,hydration,and microstructure of the mortars with low w/b ratio were investigated.It is revealed that the w ater release behavior of PFA and its distribution in the paste are the key factors affecting its internal curing.This research will provide reference suggestions for the design,preparation,and selection of PFA for internal curing.Bauxite tailing was used as the main raw material for the preparation of PFA through analyzing the chemical composition,mineral composition,and thermal properties,and silica fume were used as supplementary material to optimize the material composition.The effects of preheating conditions,calcinating conditions,silica fume amount,Ca F2 and C a CO3 amount,and paper sludge amount on the properties of PFA were systematically studied.Combined with microstructure analysis,the pore formation mechanism of PFA was clarified.During the firing process,calcination temperature had the most obvious effect on the properties of PFA.The amount of silica fume significantly changed the pore formation mechanism of PFA,thus affecting the properties of PFA.In order to ensure that the PFA had high water absorption and density,the low amount of silica fume and low calcination temperature should be considered.With the increase of Ca F 2,Ca CO3 and paper sludge amount,the water absorption of PFA gradually increased.However,compared with the use of Ca F 2 and Ca C O3,the pore size of PFA could be reduced by replacing part of the bauxite tailing with paper sludge.PFA can achieve a higher water absorption ratio and quicker wa ter absorption rate.The water absorption performance of PFA varied with the composition of raw materials and calcination temperature,while the aggregate size of PFA had little influence on the water absorption and release performance of PFA.When the environmental relative humidity was 97%,the PFA could release more than 93% of the total water absorption,and basically released most of the water at 72 h,with little change in the water release rate.The pore size of PFA had little difference,and the pore size was concentrated in the range of 100-3000 nm.The formation of pore structure wa s mainly related to the particle gap formed by the gas released from the gas-generating component.The main phases of PFA w ere mullite,corundum,and anorthite,which play an important role in improving aggregate strength.The internal curing effects of PFA without additives,PFA w ith Ca F2 and Ca CO3,and PFA w ith paper sludge in the high-strength mortar(HSM)were investigated.It was found that compared with other kinds of PFA,PFA with paper sludge had less influence on the workability of fresh HSM.The PFA with Ca F2 and Ca C O3 reduced the autogenous shrinkage of HSM,but also reduced the compressive strength of HSM,while PFA with paper sludge could not only effectively inhibit the autogenous shrinkage of HSM,but also improved the compressive strength of HSM,and this effect was enhanced with the increase of water absorption ratio of PFA.T he test results of h ydration heat showed that the PFA wi th paper sludge improved the hydration heat release rate and the total heat release,but did not change the hydration process.It was indicated that the PFA with paper sludge had the ability to release water in a timely manner,and the increase of hydration degree reduced the porosity of HSM.The effects of the w ater absorption ratio,amount,and particle size of PFA and w/b ratio on the flowability,autogenous shrinkage,hydration degree,pore structure,and compressive strength of ultra-high strength mortar(UHSM)were studied.It was found that the slump flow of UHSM with PFA increased with time within 120 min,and this effect increased with the increase of the water absorption ratio of PFA.The early water release behavior of PFA not only shortened the induction period,but also increased the hydration heat release rate and total heat release,and this effect was enhanced with the increase of the water absorption ratio of PFA.With the increase of water absorption ratio and particle size of PFA,the hydration degree of cementitious materials increased first and then decreased,and with the increase of PFA amount,the hydration degree of cementitious materials also increased accordingly.The influence of the water absorption ratio of PFA on the autogenous shrinkage of UH S M is different from that of HS M.Too low or too high the water absorption ratio of PFA was not conducive to improving the shrinkage reduction effect.With the increase of the PFA amount,the autogenous shrinkage of UHS M further decreased.However,with the increase of the particle size of PFA,the reduction amplitude first increased and then decreased.PFA increased the total porosity of UHSM but decreased the porosity of the paste.With the increase of water absorption ratio and particle size of PFA,the porosity of the paste decreased first and then increased.With the increase in the PFA amount,the porosity of the paste decreased accordingly.Too low or too high a wa ter absorption ratio of PFA was not conducive to improving the compressive strength of U HSM.With the increase in the amount and particle size of PFA,the compressive strength of UHSM decreased gradually.In a word,when the volume of internal curing water was constant,the internal curing effects of PFA with the water absorption of 10.8%and particle size of 1.18-2.36 mm in UH SM were the best.In the UHS M with a lower w/b ratio,the use of PFA with a lower water absorption ratio was helpful to improve its internal curing effects.