| The floating lightweight aggregate planting concrete is a kind of environment-friendly material.Its porous structure provides the space for aquatic plants and microorganisms to attach and grow,increases the biodiversity of water body,and is conducive to the improvement of water ecosystem.Using floating lightweight aggregate planting concrete to effectively and scientifically repair and purify eutrophic water body has gradually been recognized by people.This paper mainly focuses on the preparation of light aggregate planting concrete and its performance of nitrogen and phosphorus removal.First of all,the matching of sulphoaluminate cement and flocculant was studied.Three flocculants,poly ferric sulphate,poly aluminium sulphate and ferric chloride,were added to sulphoaluminate cement to study the effect of flocculant on the performance of sulphoaluminate cement.Then,the preparation and performance characterization of foamed lightweight aggregate,flocculated foamed lightweight aggregate and core-shell structure foamed lightweight aggregate are studied.Through static immersion method,the change of nutrient elements in eutrophic water body is analyzed to characterize the nitrogen and phosphorus removal performance of lightweight aggregate.Finally,the preparation and water purification performance of light aggregate planting concrete are studied.The conclusion is as follows:（1）Both PFS and FeCl3 can prolong the setting time of sulphoaluminate cement and increase the fluidity of cement paste,which shows that PFS and FeCl3 delay the early hydration rate of sulphoaluminate cement and reduce the early strength of sulphoaluminate cement.On the contrary,polyaluminium sulphate can shorten the setting time of sulphoaluminate cement,reduce the fluidity of slurry,promote its early hydration,and increase the early strength of sulphoaluminate cement.The addition of three flocculants did not affect the hydration products of sulphoaluminate cement.（2）Porous foamed lightweight aggregate was prepared with hydrogen peroxide as foaming agent,and its bulk density was as low as 492.87 Kg/m3,far lower than the density of water.The numerical tube pressure can reach 8.62 MPa。The maximum softening coefficient of foamed aggregate after soaking is 94.13%After soaking,it still has good mechanical properties.The maximum water absorption of aggregate reaches 26.05%High water absorption rate is conducive to the absorption of nutrients by light aggregate.After soaking the foamed lightweight aggregate for 45 days,the alkalinity of the water body does not change greatly,and the maximum pH value is only 9.3.The analysis of water quality shows that the calcium and magnesium ions released from cement can precipitate with nitrogen and phosphorus,and the adsorption of porous foamed lightweight aggregate.The removal rates of total nitrogen and total phosphorus of aggregate to water body are respectively 43.36%and48.96%.（3）Compared with hydrogen peroxide,the mechanical properties of foamed lightweight aggregate prepared with aluminum powder as foaming agent are better,the maximum numerical tube pressure and softening coefficient are 11.57 MPa and 96.11%,respectively.However,the inner pore size of the aggregate is small and its density is high.The lowest loose bulk density is 810.22 kg/m3,which is close to the density of water and the water absorption is only 16.09%.After soaking for 45 days,the removal rates of total nitrogen and total phosphorus are 24.85%and 31.14%respectively,which are lower than that of hydrogen peroxide series.（4）The loose bulk density of PFS-foamed lightweight aggregate is only 443.53 kg/m3,which is far less than the density of water.The maximum water absorption can reach 26.99%.The high water absorption is beneficial to the adsorption performance of lightweight aggregate.Under the condition of low density,the foamed lightweight aggregate still has good mechanical properties.The numerical tube pressure is higher than 5.4 MPa,and the softening coefficient is not lower than 81.87%after the aggregate is soaked.When the PFS-foamed lightweight aggregate is soaked in the eutrophic water,the pH value of the water is between 7-9,and the dissolved oxygen is stable between 8-9 mg/L.It was found that the formation of polynuclear complex after the dissolution of polymeric ferric sulfate could play a role of sweeping and adsorption in eutrophic water.When the soaking age was 60 days,the maximum removal rates of total nitrogen,ammonia nitrogen,nitrate nitrogen,total phosphorus and phosphate were 83.70%,66.06%,71.31%,94.74%and 87.72%,respectively.（5）The minimum loose bulk density weight of PAS-foamed lightweight aggregate is426.11 kg/m3,the maximum water absorption is 26.82%,the numerical tube pressure is not less than 5.43 MPa,and the softening coefficient of aggregate after soaking is not less than81.17%.After soaked in eutrophic water,polyaluminium sulfate foamed lightweight aggregate has little effect on alkalinity and dissolved oxygen.After dissolving,polyaluminium sulfate can also form a polynuclear complex,which can also adsorb or flocculate and precipitate the nitrogen and phosphorus elements in the water body.When the immersion age reaches 60 days,the maximum removal rates of total nitrogen,ammonia nitrogen,nitrate nitrogen,total phosphorus and phosphate are as high as:72.08%,65.60%,70.40%,96.36%,100%.（6）The core-shell structure of foamed lightweight aggregate changed the structure of aggregate,but did not affect its density.The lowest loose bulk density of aggregate was only562.98 kg/m3.The core-shell structure still has good mechanical properties.Its compressive strength is not less than 5.56 MPa,and the minimum softening coefficient is not less than85.08%.The maximum water absorption of foamed lightweight aggregate with core-shell structure is 25.97%,indicating that the aggregate has strong adsorption performance.（7）Through the electronic scanning analysis and energy spectrum analysis of aggregate shell,it can be concluded that polymeric ferric sulfate can be released into eutrophic water through the pore wall.Combined with the calculation of the dissolution rate of polymeric ferric sulfate after the aggregate soaking,the dissolution rate of each experimental group is between 2-4%every 5 days after the core-shell foamed lightweight aggregate soaking,which shows that the core-shell structure foamed lightweight aggregate can control the uniform release of polymeric ferric sulfate.（8）As the core-shell structure controls the release of polymeric ferric sulfate,compared with the flocculated foamed lightweight aggregate.The removal rate of nitrogen and phosphorus is slow.The purification of eutrophic water by aggregate is stable,efficient and long-term.（9）The 28 day compressive strength of the floating planting concrete made of foamed lightweight aggregate is 6.23 MPa;when the void ratio is 25%,the dry density is 785.83kg/m3,and the water permeability coefficient is 5.72 cm/s.After soaking,the alkalinity of the water body slightly increases.Because of its adsorption and release of metal ions,the foamed lightweight aggregate planting concrete still has the ability of nitrogen and phosphorus removal,although there is no added polymeric ferric sulfate.After 60 days of immersion,the total nitrogen removal rate is 7.74%,and the total phosphorus removal rate is 43.86%.（10）The compressive strength of the planting concrete prepared by flocculating foamed lightweight aggregate can reach about 6.2 MPa in 28 days,and different aggregates have little effect on the compressive strength of the planting concrete.When the void ratio is 25%,the dry density is about 750 kg/m3.The water permeability coefficient is less than 5.88 cm/s,which can meet the space needed for plant roots to penetrate the concrete.After soaking,the alkalinity of the eutrophic water will be reduced and polymeric ferric sulfate can be released.The removal efficiency of nitrogen and phosphorus is more obvious than that of the blank group.After 60 days soaking,the removal efficiency of total nitrogen and total phosphorus are 87.50%and 91.23%,respectively. |
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