Effect of drying-wetting cycles on pore characteristics and mechanical properties of enzyme-induced carbonate precipitation-reinforced sea sand

Enzyme-induced carbonate precipitation(EICP)is an emanating,eco-friendly and potentially sound technique that has presented promise in various geotechnical applications.However,the durability and microscopic characteristics of EICP-treated specimens against the impact of drying-wetting(D-W)cycles is under-explored yet.This study investigates the evolution of mechanical behavior and pore charac-teristics of EICP-treated sea sand subjected to D-W cycles.The uniaxial compressive strength(UCS)tests,synchrotron radiation micro-computed tomography(micro-CT),and three-dimensional(3D)recon-struction of CT images were performed to study the multiscale evolution characteristics of EICP-reinforced sea sand under the effect of D-W cycles.The potential correlations between microstructure characteristics and macro-mechanical property deterioration were investigated using gray relational analysis(GRA).Results showed that the UCS of EICP-treated specimens decreases by 63.7% after 15 D-W cycles.The proportion of mesopores gradually decreases whereas the proportion of macropores in-creases due to the exfoliated calcium carbonate with increasing number of D-W cycles.The micro-structure in EICP-reinforced sea sand was gradually disintegrated,resulting in increasing pore size and development of pore shape from ellipsoidal to columnar and branched.The gray relational degree suggested that the weight loss rate and UCS deterioration were attributed to the development of branched pores with a size of 100-1000 m m under the action of D-W cycles.Overall,the results in this study provide a useful guidancee for the long-term stability and evolution characteristics of EICP-reinforced sea sand under D-W weathering conditions.

Effect of Carbonation and Drying-Wetting Cycles on Chloride Diffusion Behavior of Coral Aggregate Seawater Concrete

Based on seawater immersion,drying-wetting cycles,carbonation and drying-wetting cycles for coral aggregate sea-water concrete(CASC)with different strength grades,the effect of carbonation and drying-wetting cycles on chloride diffusion be-havior of CASC is studied.The results show that the free surface chloride concentration(Cs),free chloride diffusion coefficient(Df)and time-dependent index(m)of CASC in the drying-wetting cycles is obviously higher than that in seawater immersion.The Df and m of CASC of carbonation and drying-wetting cycles is higher than that in the drying-wetting cycles.Carbonation increases the Df and m of CASC,which is against CASC to resist chloride corrosion.The corrosion possibility of CASC structures in different ex-posed areas is as follows:splash zone(carbonation and drying-wetting cycles)>tidal zone(drying-wetting cycles)>underwater zone(seawater immersion).Besides,the chloride diffusion rate of C65-CASC is 17.8%-63.4%higher than that of C65-ordinary aggre-gate concrete(OAC)in seawater immersion(underwater zone).Therefore,anti-corrosion measures should be adopted to improve the service life of CASC structure in the oceanic environment.

Effect of carbonation-drying-wetting on durability of coral aggregate seawater concrete

Based on the drying-wetting cycles experiment and the carbonation-drying-wetting cycles experiment for coral aggregate seawater concrete(CASC)with different strength grades,the effects of carbonation-drying-wetting on the durability of CASC are studied with the surface state,mass loss rate,relative dynamic elastic modulus,ultrasonic wave velocity and cube compressive strength as indices.Results show that the mass loss rate of CASC increases gradually with the increase in cycle times in the drying-wetting and carbonation-drying-wetting cycles.The mass loss rate increases relatively slowly at the initial stage but it increases remarkably after 10 cycles.The relative dynamic elastic modulus and ultrasonic wave velocity decrease gradually with the increase in cycle times.After 6 cycles,the decrease rate of the relative dynamic elastic modulus and ultrasonic wave velocity of CASC tends to be flat and the surface is slightly damaged.Compared with the initial 28 d cube compressive strength,the cube compressive strength of CASC decreases by 8.8%to 11.0%.Drying-wetting cycles and carbonation can accelerate seawater erosion on CASC,and drying-wetting cycles result in salting-out and accelerate the destruction of concrete.Therefore,the carbonation-drying-wetting accelerates the destruction of CASC.

Determining representative elementary volume size of in-situ expansive soils subjected to drying-wetting cycles through field test

Cracks resulting from cyclic wetting and drying of expansive soils create discontinuities and anisotropy in the soil.The representative elementary volume(REV)defined by the continuous-media theory cannot be applied to cracked expansive soils that are considered discontinuous media.In this study,direct shear tests of three different scales(30 cm^(2),900 cm^(2),1963 cm^(2))and crack image analysis were carried out on undisturbed soil samples subjected to drying-wetting cycles in-situ.The REV size of expansive soil was investigated using the crack intensity factor(CIF)and soil cohesion.The results show that soil cohesion decreased with increasing sample area,and the development of secondary cracks further exacerbated the size effect of sample on cohesion of the soil.As shrinkage cracks developed,the REV size of the soil gradually increased and plateaued after 3−5 cycles.Under the same drying-wetting cycle conditions,the REV size determined using soil cohesion(REV-C)is 1.75 to 2.97 times the REV size determined using CIF(REV-CIF).Under the influence of shrinkage cracks,the average CIF is positively correlated with the REV size determined using different maximum permissible errors,with the coefficient of correlation greater than 0.9.A method for determining the REV-C based on crack image analysis is proposed,and the REV-C of expansive soil in the study area under different exposure times is given.

Chloride Ion Transmission Model under the Drying-wetting Cycles and Its Solution

The chloride ion transmission model considering diffusion and convection was established respectively for different zones in concrete by analyzing chloride ion transmission mechanism under the dryingwetting cycles. The finite difference method was adopted to solve the model. The equation of chloride ion transmission model in the convection and diffusion zone of concrete was discreted by the group explicit scheme with right single point （GER method） and the equation in diffusion zone was discreted by FTCS difference scheme. According to relative humidity characteristics in concrete under drying-wetting cycles, the seepage velocity equation was formulated based on Kelvin Equation and Darcy＇s Law. The time-variant equations of chloride ion concentration of concrete surface and the boundary surface of the convection and diffusion zone were established. Based on the software MATLAB the numerical calculation was carried out by using the model and basic material parameters from the experiments. The calculation of chloride ion concentration distribution in concrete is in good agreement with the drying-wetting cycles experiments. It can be shown that the chloride ion transmission model and the seepage velocity equation are reasonable and practical. Studies have shown that the chloride ion transmission in concrete considering convection and diffusion under the drying-wetting cycles is the better correlation with the actual situation than that only considering the diffusion.

Planning and design of a micro energy network for seawater desalination and regional energy interconnection

With the rapid developme nt of the economy, the continu ously in creasing populati on, and ongoing climate change, the shortage of freshwater resources has become an increasingly important global problem. Seawater desalination tech no logy can effectively alleviate the pressure on freshwater supplies and has bee n in vestigated in many countries. However, the majority of existing projects focus on the research and development of desalination equipment and the use of new tech no logies and pay less atte ntion to the operation optimizati on of the desalinati on process. The micro energy n etwork (MEN) designed in this study is an efficient distributed energy supply system that can be used to simultaneously supply electricity, cooling, heating, and freshwater as photovoltaic power, wind power, combined heat and power (CHP), electric cooling and heating, and a seawater desalinati on device are in teg rated into the MEN. In this study, a model for operati on optimization of a MEN for seawater desalination was developed and the influences of the electric cooling and heating ratios and the operation optimization of the seawater desalination device were studied with the aim of minimizing the life cycle cost. Based on the results of this study, MENs can reduce the operation cost of desalination devices and improve the efficiency of renewable energy sources.

海水干湿循环作用下UHTCC动态压缩性能试验研究

为研究应变率和海水干湿循环周期对超高韧性水泥基复合材料(UHTCC)动态压缩性能的影响,采用直径75 mm的分离式霍普金森压杆系统对0~120 d海水干湿循环作用后的UHTCC试件开展动态压缩试验.通过改进ZWT非线性黏弹性本构模型,提出了一种考虑海水干湿循环作用的UHTCC试件的动态本构模型.结果表明,UHTCC试件的动态抗压强度增强因子随着干湿循环周期的增加逐渐减小.在同一冲击气压下,随着干湿循环周期的增加,UHTCC试件的峰值应力和耗能先增大后减小,并分别在60和90 d时达到峰值.在相同干湿循环周期下,UHTCC试件的峰值应力、动态抗压强度增强因子和耗能均随应变率的增大而增大,表现出明显的应变率效应.改进的动态本构模型能够较好地描述海水干湿循环作用下UHTCC试件的动态压缩应力-应变关系.

水泥加固淤泥土力学与抗海水腐蚀性能研究

为了分析水泥掺量和海水腐蚀对水泥加固淤泥土力学性能的影响规律,制备了水泥掺量为4%、8%、12%、16%和20%水泥加固淤泥土试件,分别经历了0、2、5、8、12、18次海水腐蚀干湿循环后,测试试件的单轴抗压强度、内聚力和内摩擦角,分析水泥掺量和海水腐蚀对水泥加固淤泥土力学性能的影响规律。研究结果表明,(1)随着水泥掺量的增大,水泥加固淤泥土的单轴抗压强度和内聚力呈线性增大,而内摩擦角呈指数衰减;(2)随着海水干湿循环次数的增加,水泥加固淤泥土的抗压强度、内聚力和内摩擦角均呈现不断变小的变化规律;(3)水泥掺量的增大能够有效提高加固淤泥土的抗海水腐蚀能力。

海洋环境条件海水海砂混凝土加速试验力学损伤评估

伴随着海洋开发与岛礁建设的进行,海砂、海水作为建筑材料具有巨大的潜在市场。海水海砂混凝土在海洋环境中的应用通常采用部分浸没温湿循环加速试验模型进行研究。文章建立海水海砂混凝土力学损伤理论计算模型,并根据动弹性模量预测加速试验后海水海砂混凝土的劣化时间。这项研究对部分浸没温湿循环加速试验模型的应用具有重要的科学意义。

渔船船体用HDPE材料的耐久性能

通过调整光辐照强度、海水温度、浸泡时间及冻融循环等因素,研究了其对渔船船体用HDPE力学性能的影响。结果表明,光辐照强度越高,HDPE性能劣化越严重,当强度为1600 W/m^(2)时的辐照1920 h后,HDPE的拉伸强度和弯曲强度分别降低了25.3%和23.7%;热氧作用对HDPE力学性能的整体影响较小,在温度为80℃条件下老化1920 h后,HDPE的拉伸强度和弯曲强度分别下降了8.5%和5.7%,而在常温条件下,HDPE的力学性能基本无影响;海水浸泡作用使HDPE发生轻微的吸湿现象,最大吸湿率仅为0.7%,随着海水浸泡时间的延长,材料的力学性能出现明显降低,拉伸强度和弯曲强度的最大降低幅度均达到了15%,而海水的盐度对HDPE的吸湿率及力学性能影响均较小;海水冻融循环作用使HDPE的力学性能出现小幅度增大,材料的耐冻融能力较强。

基于随机森林算法的海水循环冷却系统钙离子浓度预测模型

海水循环冷却系统中钙离子的检测问题已成为阻碍系统精细化管理的重要限制因素。文章将随机森林算法与海水循环水中钙离子浓度预测结合,充分总结运行经验,采用易实现在线检测的电导率值、pH值和系统日期作为模型特征,结合随机森林算法,实现海水循环水中钙离子浓度预测。利用随机森林算法和宁海电厂5#海水循环冷却系统2014年与2015年海水循环水水质监测数据,建立海水循环水中钙离子浓度预测模型。通过五数概括法进行数据清洗,将样本划分为训练集和测试集,采用五折交叉验证和网格搜索法优化模型参数。训练集和测试集上模型的验证和评价效果良好,该模型可用于预测海水循环水中钙离子浓度。

荧光示踪剂在监测二级水浓缩倍数中的应用

文章针对反渗透海水淡化二级产品水作为循环冷却系统补充水时,水中离子含量低、药剂干扰大,导致浓缩倍数计算结果偏差大的问题,采用添加荧光化合物的方法监测系统浓缩倍数,并探讨了荧光示踪剂1,3,6,8-芘四磺酸钠在去离子水中的荧光特性,考察了杀菌剂、阻垢缓蚀剂对荧光特性的影响;采用静态阻垢性能试验方法,模拟冷却系统条件,综合评价了浓缩条件变化对其荧光特性的影响。结果表明,1,3,6,8-芘四磺酸钠荧光信号稳定,能够有效监测循环冷却系统浓缩倍数变化。

Numerical Analysis of Moisture Influential Depth in Concrete During Drying-Wetting Cycles

The influential depth of moisture transport in a concrete surface subject to drying-wetting cycles was analyzed numerically. The moisture transport was described by a diffusion model with different diffusivities for drying and wetting. A finite difference scheme was developed to solve the partial differential equations The influential depth was then investigated numerically for initially saturated and unsaturated concretes exposed to drying-wetting actions in marine environments using an equilibrium time ratio concept. The equilibrium time ratio was calculated numerically for a saturated condition and the moisture influential depth is shown to be a linear function of the square root of the drying time. However, this equilibrium time ratio does not exist for an unsaturated condition and the moisture influential depth depends on the initial saturation as well as the drying-wetting time ratio. The results indicate that this model gives more realistic predictions of moisture transport of in situ structural concrete and its durability.

Ultrasonic testing and microscopic analysis on concrete under sulfate attack and cyclic environment

The damage process of concrete exposed to sodium sulfate attack and drying-wetting cycles was investigated. The water to binder(W/B) ratio and the concentration of sulfate solution were taken as variable parameters. Through the experiment, visual change, relative dynamic modulus of elasticity(RDME) and the surface damage layer thickness of concrete were measured.Furthermore, SEM and thermal analysis were used to investigate the changing of microstructure and corrosion products of concrete.The test results show that the ultrasonic velocity is related to the damage layer of concrete. It approves that an increase in damage layer thickness reduces the compactness and the ultrasonic velocity. The deterioration degree of concrete could be estimated effectively by measuring the surface damage layer and the RDME of concrete. It is also found that the content of gypsum in concrete is less than that of ettringite in test, and some gypsum is checked only after a certain corrosion extent. When the concrete is with high W/B ratio or exposed to high concentration of sulfate solution, the content of ettringite first increases and then decreases with corrosion time. However, the content of gypsum increases at a steady rate. The content of corrosion products does not correspond well with the observations of RDME change, and extensive amount of corrosion products can be formed before obvious damage occurs.

过冷法海水冰浆制备系统能耗特性分析

海水制冰浆技术可应用于海岛及海上风电的负荷转移和削峰以及冷冻保存海产品等,具有广阔应用前景。此外,海水制冰的原料近乎免费,并且利用海水作为冷却介质可以降低冷凝温度,进而提高系统能效,具有明显优势。通过建立双回路单级压缩循环制冰系统模型,分析利用过冷法制取海水冰浆的经济性和可行性,为海水制冰浆技术的实际应用提供理论指导。结果显示,该系统在计算条件下冬季循环性能系数平均值为4.76,夏季平均值为3.61,在蒸发器内较宽的海水流速范围内可维持较长时间正常运行。系统采用板式水冷冷凝器,并与空气源机组进行定量对比,结果表明循环机组采用水冷冷凝器在各指定工况点下的效益均高于采用空冷器,在设计工况下水冷机组单位投资蓄冷量为2.85kW·h/元,对应的空冷机组仅为1.37kW·h/元。

太阳能驱动的制冷循环海水淡化系统性能分析

建立太阳能驱动的制冷循环海水淡化系统数学模型,通过改变驱动热水流量、驱动热水供回水温差、集热器面积和所在城市这4个变量,进行系统性能分析,利用Matlab语言编制程序进行求解。结果表明:提高驱动热水流量和增大驱动热水供回水温差,均有利于提高淡水产量,但所需辅助热源热量增加,太阳能供能比例下降;增大集热器面积,有利于降低辅助热源热量,增大太阳能供能比例,但淡水产量微降;选择大连、北京和深圳3个城市,在相同的条件下,深圳地区太阳能供能比例最高,北京次之,大连最低。

渔船柴油机余热利用系统设计与热力学分析

为充分利用渔船柴油机余热,提出一种以CO_(2)为工质的新型余热利用技术,通过能源梯级利用,实现发电、制冷、制淡水的三联供系统。建立船舶余热驱动的CO_(2)超临界发电、跨临界制冷及蒸馏制淡水循环系统的热力学稳态模型,并以排气温度293℃、排气热量1668 kW的Wärtsilä6L45B型船舶柴油机为例,模拟分析关键热力参数对系统性能、余热利用的影响。研究结果表明,该余热利用系统可为渔船提供6.7~284.0 kW的净发电量,同时可为船上-40℃的速冻库和4℃的食品保鲜库分别提供22~150 kW和194~1311 kW的冷量用于速冻、保鲜或舱内空调,并提供5.25~12.75 t/d的淡水;通过控制膨胀机入口压力和海水淡化温度,灵活控制系统输出的净发电量、中低温冷量和淡水量以满足渔船在航行、捕捞等工作阶段对不同能量的需求。通过柴油机余热利用技术提供的电能、制冷量以及淡水,可减少渔船对柴油的消耗,具有明显的节能效果,为远洋作业渔船的余热利用提供参考。

海水稻根际效应对滨海盐碱地土壤氨氧化微生物的影响

滨海盐碱地的特殊环境严重限制了土壤氮素转化和利用。微生物介导的水稻根际氨氧化过程是盐碱稻田土壤氮循环的关键过程,但限于研究盲点和技术不足,海水稻根际效应对滨海盐碱地土壤氨氧化微生物群落结构的影响仍少有报道。据此,本研究以“海稻86”为研究对象,分别设置低盐浓度(2 g·kg^(-1))和高盐浓度(6 g·kg^(-1))两组处理进行盆栽试验。结果显示:种植海水稻70 d后,高盐和低盐处理根际土壤的pH分别下降了0.82和0.70个单位,土壤有机质(SOM)含量下降了6.41和4.46 g·kg^(-1),腐殖质(HU)含量提高了5.76和4.45 g·kg^(-1),全氮(TN)含量减少0.46和0.37 g·kg^(-1),表明海水稻可通过降低盐碱地土壤pH,加速有机质分解转化,提高土壤氮循环速率。水稻根际作用可显著提高土壤微生物生物量碳(MBC)、微生物生物量氮(MBN)和微生物呼吸强度,并在种植第55天达到最高,在高盐处理中分别达到850.0 mg·kg^(-1)、72.2 mg·kg^(-1)和231.9 mg·kg^(-1)·d^(-1),低盐处理中达到546.1 mg·kg^(-1)、53.7 mg·kg^(-1)和171.2 mg·kg^(-1)·d^(-1),说明水稻根际作用对土壤微生物数量和活性的影响在55 d最强。海水稻根际效应对滨海盐碱地土壤氨氧化古菌(ammonia-oxidizing archaea,AOA)丰富度、多样性和丰度无显著影响,AOA优势菌为norank_c__environmental_samples_p__Crenarchaeota、unclassified_k__norank_d__Archaea和Nitrososphaera。海水稻根际效应显著提升了滨海盐碱地土壤氨氧化细菌(ammonia-oxidizing bacteria,AOB)丰富度和多样性,提高土壤AOB优势菌environmental_samples_f__Nitrosomonadaceae和Nitrosospira的丰度。相关性分析发现environmental_samples_f__Nitrosomonadaceae和Nitrosospira与土壤pH呈显著负相关,与腐殖质呈显著正相关。本研究结果表明种植海水稻可提高滨海盐碱地养分循环,并且在酸性土壤中耐盐碱水稻根系效应主要影响AOB的群落结构。

岁差驱动的全新世热带太平洋—印度洋水汽输送

大洋内部蒸发、水汽输送和降水过程构成了全球水文循环的主体,但长久以来对全新世大洋内部水文循环的演变历史和驱动机制缺乏认识。利用全球热带海区98个站位混合层浮游有孔虫壳体的δ18O和Mg/Ca温度记录,计算了全新世以来表层海水δ18O以及海水剩余δ18O的波动历史。发现最显著的特征是全新世以来热带西太平洋和东印度洋表层海水剩余δ18O具有不同的变化趋势。早—中全新世时期(11.5~6.0 ka BP),热带西太平洋表层海水剩余δ18O比东印度洋偏重约0.2‰,而晚全新世(2.0~0 ka BP),两个海区的重建值几乎相同。结合同位素数值模拟结果,发现岁差通过一系列机制控制了两个大洋不同的剩余δ18O变化信号。早全新世较低的岁差值可以驱动西太平洋往印度洋的大气水汽净输送并降低印度洋降水同位素,同时强盛的南亚季风通过河流体系向孟加拉湾倾注了大量陆地冲淡水。这些机制都有利于东印度洋海水剩余δ18O出现相对负偏移信号。但较低的岁差值导致开放的西太平洋净降水量下降,并通过大气传输损失淡水,因此海水剩余δ18O值较为偏重。本次研究结合大空间尺度上的δ18O重建记录和模拟结果,较为可靠地刻画了岁差调控下洋盆之间的水汽转移过程和机理。

MICP加固钙质砂的耐久性试验研究

南海岛礁建设中,钙质砂是易于获取的原材料,但也存在孔隙多、易破碎等不足。为保证岛礁建设的安全稳定,设计经微生物诱导碳酸钙沉积(MICP)加固后钙质砂试样在海水、纯水环境下分别进行浸泡与干湿循环试验,以探究不同环境与处理方式对MICP加固钙质砂样耐久性能的影响。结果表明:(1)经MICP加固的钙质砂具有较好的抗侵蚀能力,通过增加钙质砂的加固轮次能够有效地提高试样的耐久性能,其在海水中干湿循环时劣化最快,在纯水中浸泡时劣化最慢;(2)持续浸泡与干湿循环均会对试样耐久性造成不利影响,干湿循环的劣化作用更大;(3)控制其余变量不变时,海水环境下试样耐久性能的劣化比纯水环境下更严重。