赤泥基高强度免烧透水砖制备及影响因素与构效机制

Preparation of High-Strength Permeable Bricks Based on Red Mud:Influencing Factors and Structure-Performance Mechanism

随着赤泥堆存量不断增加和城市化进程加快,赤泥资源化利用和低碳透水砖开发,成为协同解决2大问题的关键。基于赤泥和透水砖材料基因共性,以赤泥为原料,采用造粒工艺将赤泥制成免烧骨料,结合混料、成型、养护等步骤,成功制备了高强度免烧透水砖。为系统阐明透水砖制备过程中材料性能的影响因素与构效关系,进一步探究了水灰比、目标孔隙率以及发泡剂占比对透水砖性能的影响,发现水灰比和目标孔隙率主要通过改变砖体内部浆料的流动性,以及连通孔隙数量来显著影响砖体的性能。在发泡剂添加量低于4%(质量分数)时,可以有效提升砖体内部的孔隙率,但是过高的添加量会使透水砖的水灰比增大,弱化了发泡剂本身的造孔作用。采用X射线粉末衍射仪、扫描电子显微镜研究透水砖制备过程中的强度形成机理,发现在砖体制备过程中,赤泥以及水泥反应形成了硅酸钙凝胶,可以有效凝结骨料,赋予透水砖强度以及透水性能。本工作体现了“变废为宝”的资源化利用理念,为赤泥的资源化利用与海绵城市建设对高端建材的需求,提供了新的思路和方法。

Introduction Red mud is a solid waste generated from alumina production,its massive accumulation not only occupying extensive land areas but also leaching alkalis and heavy metals,leading to significant environmental issues such as soil alkalization and groundwater pollution.This study responds to the current demand for sponge city construction by innovatively utilizing red mud as a raw material to produce unsintered aggregates through a novel pelletization process that leverages similarities between red mud and permeable brick materials.By integrating mixing,molding,and curing techniques,high-strength unsintered permeable bricks were successfully developed.These bricks improve overall compressive strength and permeability while drastically reducing energy consumption anpd environmental impact during production.This approach presents a fresh perspective on red mud utilization and sponge city development,promising superior environmental and economic benefits.Methods The preparation of red mud-based unsintered permeable bricks involves four steps.First,pelletization:the aged red mud chunks are crushed,then mixed uniformly with P·O42.5 ordinary Portland cement and water,and fed into the pelletizer for pelletization.The aggregated particles are removed and naturally cured for one week.Second,mixing:different proportions are designed based on the volumetric method,with red mud-based unsintered aggregates,cement,water,and additives calculated and weighed according to these proportions,then added to a basin and mixed uniformly.Third,pressing:the uniformly mixed slurry is placed into the mold of a semi-automatic brick machine for pressing,with mold dimensions of 20 cm×15 cm×10 cm.Fourth,curing:the pressed permeable bricks are demolded and naturally cured for 1 to 2 days,then placed in a standard curing box for 28 days.The material’s phase structure and micro-morphology are analyzed using X-ray Powder Diffraction(XRD)and Scanning Electron Microscopy(SEM).Heavy metal leaching analysis of the permeable bricks is performed following the method described in national standards(HJ557-2010).The concentrations of ten metal elements in the leachate are measured using an Inductively Coupled Plasma Mass spectrometry(ICP-MS).Finally,the environmental risk of heavy metal elements in the leachate is evaluated according to national standards(GB/T 5085.3-2007).Results and discussion The bulk density(ρbv)of red mud-based aggregates measures 1078.50 kg/m³,with an apparent density(ρap)of 1724.14 kg/m³and a porosity(v)of 37.45%.Analysis of the water-cement ratio(wc)on permeable brick performance revealed that increasing wc from 0.20 to 0.35 gradually enhances compressive strength.This improvement stems from the increased fluidity and adhesive properties of the cement paste at higher wc,facilitating better aggregate encapsulation and thereby improving connectivity.However,increasing wc beyond 0.35 to 0.40 resulted in a significant decrease in compressive strength,likely due to excessive fluidity causing settling of the cement paste.Investigating the effects of target porosity on permeable bricks demonstrated a negative correlation with compressive strength and a positive correlation with permeability coefficient.Greater porosity led to increased interconnected pores within the bricks,enhancing water infiltration but reducing contact points crucial for stress transmission.This leads to localized increases in pressure and higher susceptibility to fracturing,thereby impacting mechanical properties.Evaluation of foaming agent content on permeable brick performance indicated that increasing the foaming agent from 0%to 4%slightly raised the permeability coefficient while marginally decreasing compressive strength.This effect results from hydrogen peroxide decomposition,creating additional pores within the bricks that weaken aggregate bonding and increase porosity.Further increases in foaming agent content reduced the permeability coefficient while gradually enhancing compressive strength.XRD analysis confirmed CaCO3 as a common component in red mud,cement,aggregates,and permeable bricks.Red mud and aggregate The characteristic peaks of red mud and aggregates remained unchanged,indicating no alteration in red mud's primary phases under the unsintered process.In the permeable bricks,characteristic peaks of 3CaO·SiO_(2) were absent,whereas peaks of Ca5(Si6O16)(OH)2 were identified.This suggests that 3CaO·SiO_(2) undergoes a reaction with water under high alkalinity conditions to produce Ca5(Si6O16)(OH)2,consequently boosting the bricks’strength.Microstructural observation of cross-sections and vertical sections revealed abundant micro-voids crucial for permeability.Cross-sections displayed flaky hydration gels distributed along aggregate surfaces,while vertical sections exhibited thinner hydration products and flocculent gels.Heavy metal leaching experiments revealed chromium(Cr)as the highest concentration metal in leachates from red mud-based unsintered permeable bricks,exceeding 40μg/L,while other metals were below 10μg/L.Leachate toxicity was well below national standards(GB/T 5085.3-2007),confirming compliance with environmental safety standards and minimal environmental risk for red mud-based unsintered permeable bricks.Conclusions Red mud-based unsintered permeable bricks can be prepared through the steps of pelletization,mixing,pressing,and curing.The water-cement ratio affects the brick’s performance;an increase in the water-cement ratio enhances the fluidity of the slurry and increases compressive strength,but when the ratio exceeds 0.35,the mechanical properties deteriorate.Increasing the target porosity improves permeability but reduces compressive strength.Adding hydrogen peroxide as a foaming agent effectively increases porosity at levels below 4%,but excessive addition leads to performance decline.XRD analysis shows that the high alkalinity of red mud promotes the formation of C-S-H gel,enhancing compressive strength.SEM images of the cross and vertical sections of the permeable brick reveal flaky and flocculent hydration gel products attached to the surfaces of the aggregates.Heavy metal leaching experiments indicate that the leaching risk of heavy metals from the permeable bricks is extremely low.All these findings provide a new approach to simultaneously address the challenges of red mud resource utilization and sponge city construction.