部分浸泡再生混凝土Mg^(2+)-SO_(4)^(2-)-Cl^(-)复合盐侵蚀耐久性损伤特征与机制

Durability Degradation Characteristics and Mechanism of Partial Immersed Recycled Aggregate Concrete Subjected to Composite Salt Attack of Magnesium-Sulfate-Chloride

为系统研究与揭示西北地区部分掩埋再生混凝土(RAC)结构耐久性损伤特征与机制,以浓度为20%的复合盐溶液为侵蚀介质,开展了14种掺辅助胶凝材料RAC部分浸泡于复合盐溶液的耐久性试验,综合分析RAC动弹性模量、宏观与微观形貌、侵蚀产物物相组成与相对含量的经时变化规律。部分浸泡RAC沿纵向高度分为饱和区、气-液两相界面区、水分传输区及干燥区。侵蚀初期气-液两相界面区损伤程度高于饱和区;侵蚀中后期饱和区的损伤持平或超过气-液两相界面区,水分传输区损伤初现。饱和区侵蚀状态由初期的化学侵蚀转变为中后期的化学-物理双重侵蚀,气-液两相界面区在侵蚀期间均呈现出化学-物理双重侵蚀。化学侵蚀产物为水镁石、硬石膏/石膏、钙矾石、Friedel盐及碱式氯化镁;物理结晶盐包含氯镁石、白钠镁矾、氯化钠、水合硫酸镁、Na_(2)SO_(4)及芒硝,各侵蚀产物与结晶盐的相对含量均随浸泡时间延长而改变。侵蚀后期,Na_(2)SO_(4)和芒硝相互转化使RAC物理力学性能急速退化。粉煤灰-矿渣复掺RAC抗侵蚀性能整体较好,粉煤灰-硅灰复掺最差,后者在浸泡时间180 d时抗压强度损失率高于60%。矿渣-硅灰-偏高岭土三掺RAC耐久性显著高于粉煤灰-矿渣-偏高岭土三掺,后者在侵蚀180 d时已经溃散。四掺辅助胶凝材料RAC性能衰减速度均匀,但抗侵蚀性能仍处于较低水平,相同浸泡时间下,其耐久性指标均与粉煤灰-矿渣复掺RAC差距较大。

In order to investigate the durability degradation characteristics and mechanism of partial burial recycled aggregate concrete(RAC)structure members in Northwest China,the durability experiment of 14 mixtures about RAC with supplementary cementitious materials(SCMs)of fly ash,granulated blast furnace slag,silica fume and metakaolin,respectively,was conducted for partial immersed in compound salt solution with MgSO_(4),Na_(2)SO_(4) and NaCl.Comprehensively analyzed the relative dynamic elastic modulus,macro-and micro-morphology,and mineral composition and its relative content,the partial immersed RAC can be divided into four corrosive zones of saturation zone,air-liquid two-phased transition zone,moisture transport zone and dry zone,respectively,along the longitudinal direction.At initial stage of partial immersed,the degradation of air-liquid two-phased transition zone was higher than that of saturation zone,while the damage in saturation zone was equal to or over than that of air-liquid two-phased transition zone at the middle and late stage.Concurrently,the damage in moisture transport zone was formed and continued to develop slowly.With immersed aging increased,the destruction mechanism of saturation zone changed from chemical attack to chemical-physical attack,while that of the air-liquid two-phased transition zone was due to chemical-physical attack.The chemistry corrosive products of brucite,anhydrite,gypsum and chloromagnesite existed throughout the corrosion,but ettringite,Friedel’s salt and Mg 10(OH)18 Cl_(2)·5H_(2)O disappeared at the late stage of corrosion.And the physical crystalline salts included blodite,halite,MgSO_(4)·x H 2O,thenardite and mirabilite.The phase transformation between thenardite and mirabilite formed the great salt crystallization pressure in pores and micro-cracks,which rapidly decreased of RAC durability.At immersion time of 180 days,the corrosion resistance of RAC with fly ash and GBFS was generally well,however,the loss rate of compressive strength of RAC with fly ash and silica fume was more than 60%.In addition,the durability of RAC with GBFS-silica fume-metakaolin was much higher than that of RAC with fly ash-GBFS-metakaolin,which had spalled off grievously.Although a stable decreased rate,the resistance performance of RAC with four SCMs was still at the low level,compared to that of RAC with fly ash and GBFS at the same immersed time.