海洋环境下混凝土与硫酸根离子反应

海水和滨海盐渍土存在的硫酸根离子与混凝土反应将导致保护层膨胀剥落,从而使得氯离子传输距离缩短、钢筋混凝土更易腐蚀。对0～50%粉煤灰掺量、0～65%矿粉掺量混凝土开展实海暴露,同时将混凝土粉末浸泡于1%硫酸钠和海水溶液中,研究混凝土与硫酸根离子的反应量。试验结果表明:海水中的硫酸根离子进入混凝土内部,其1 d的反应量达到总反应量的50%,28 d后将达到反应平衡;其反应的硫酸根离子量超过总硫酸根离子量的84%。当粉煤灰掺量达到50%,矿粉掺量达到65%时,其抗压强度小于普通混凝土,但可以降低混凝土的硫酸根离子反应量5%～15%,从而有助于降低于混凝土的硫酸盐腐蚀风险。海洋潮汐区腐蚀混凝土的硫酸根离子反应量与海沙区相近,但高于大气区腐蚀混凝土。     

混凝土在海洋环境下硫酸盐侵蚀机理研究

对不同配合比混凝土试件在不同海洋区域内进行现场暴露试验,通过硫酸根离子含量测定、SEM扫描电镜试验及X荧光试验,研究实际海洋暴露环境下混凝土内部硫酸盐侵蚀机理。试验结果表明,不同暴露环境下硫酸根作用机理不同,混凝土在不同暴露区域硫酸盐侵蚀程度为潮汐区水下区浪溅区;硫酸根离子由混凝土表层向内部传输,在表层范围内硫酸盐腐蚀产物主要是钙矾石,随着硫酸根离子传输深度增加以及内部游离CaO水化成Ca(OH)2,混凝土硫酸盐损伤程度加剧;混凝土表层SO42-和Ca2+含量较高,而混凝土内部Na+含量较高。     

硫酸盐对氯离子在地下结构混凝土中传输过程的影响

基于地铁工程混凝土实际配合比,进行了水灰比为0.35的双掺(粉煤灰、矿渣)混凝土试件在四种不同浓度的氯盐-硫酸盐复合溶液中的浸泡腐蚀试验,测定了氯离子含量随深度与时间变化的分布以及混凝土结合氯离子能力,运用XRD测试方法对混凝土微观组成试验前后的变化进行了分析。试验结果表明:单一氯盐情况下溶液浓度越高,侵入混凝土中的氯离子含量越多;硫酸根的存在,能够抑制氯离子侵入混凝土,且硫酸根浓度越高,该抑制作用越强;硫酸根离子能够显著降低水泥材料对氯离子的结合作用,并且氯离子结合量变化幅度受其浓度影响较大。XRD测试结果表明,硫酸根影响水泥材料结合氯离子能力的原因在于其反应生成钙矾石,消耗了C3AH6的含量,释放了部分固化的氯离子。     

硫酸盐侵蚀混凝土扩散理论模型研究进展

硫酸盐侵蚀混凝土是研究混凝土结构耐久性的主要问题之一。混凝土内部的硫酸根离子与水泥水化产物发生反应形成石膏和钙矾石,这些可能会影响混凝土内部的孔隙结构的变化和造成结构损坏。从硫酸根离子进入混凝土内部的扩散机理来阐述硫酸盐侵蚀混凝土的研究进展,介绍了国内外典型扩散理论模型的提出,分析了现有理论模型所存在的问题,并提出硫酸根离子扩散过程中的随机性。     

干湿循环作用下再生混凝土中硫酸盐传输性能研究

研究了在干湿循环作用下,硫酸根离子在再生混凝土中的传输规律。采用分光光度计法定量表征不同深度处的水溶硫酸根离子浓度,研究再生粗骨料取代率(0,30%,50%,70%,100%)、矿物掺合料、水胶比、腐蚀龄期及干湿循环制度对其传输规律的影响。结果表明,再生粗骨料的掺入对硫酸盐传输有较为明显的影响。试验还表明,硫酸盐-干湿循环耦合作用下,粉煤灰和矿粉等矿物掺合料可阻碍硫酸根离子在再生混凝土中的传输,再生混凝土水胶比越小其水溶硫酸根离子浓度越小。随着腐蚀龄期增长,再生混凝土水溶硫酸根离子浓度大幅增加。与单一盐溶液侵蚀相比,干湿循环加剧了硫酸根离子在再生混凝土中的传输,其水溶硫酸根离子浓度随着循环次数的增加而增大。     

水泥砂浆表层硫酸盐物质渗透和反应产物的分析

环境介质中的硫酸盐物质向水泥混凝土材料内部的渗透对于其耐久性的研究具有重要意义。采用精密分层取样方法结合化学成分分析和X射线衍射分析方法对浸泡于硫酸盐溶液中的水泥砂浆试样表层SO3含量的分布以及反应产物进行了仔细研究。结果表明:对于硫酸盐溶液中浸泡180 d的水泥砂浆试块SO42-渗透深度大约3~5 mm;Na2SO4溶液浓度越高,SO3渗透量越多,意味着腐蚀加剧;硫酸盐物质渗入后形成的反应产物主要有钙矾石和石膏,反应产物种类与硫酸盐溶液种类关系密切,5%Na2SO4溶液浸泡主要产物为钙矾石,5%MgSO4或5%(NH4)2SO4溶液浸泡同时形成钙矾石和石膏;超细矿渣粉掺量较少时似乎增加了SO42-的渗透,而较多掺量时对SO42-的渗透似乎能起到扼制作用。     

硫酸钠侵蚀下掺粉煤灰砂浆的体积膨胀规律及其机理研究

体积膨胀是水泥基材料硫酸盐侵蚀的主要劣化模式之一，一般认为体积膨胀的机理是由于外部硫酸根离子与水泥基材料内部水化铝酸钙、单硫型硫铝酸三钙、来水化的铝酸三钙和氢氧化钙等易受侵蚀化合物反应，形成膨胀性的石膏或钙矾石——侵蚀生成物所致。本文采用室温下5％硫酸钠溶液浸泡试验，研究浸泡后砂浆试件的线长度变化，探讨掺粉煤灰水泥砂浆受硫酸盐侵蚀后的体积膨胀规律；采用XRD微观分析和化学分析，揭示其侵蚀机理。试验结果表明，未掺粉煤灰的水泥砂浆在硫酸盐溶液侵蚀下，出现其线长度不断增长的现象．砂浆试件内部膨胀性产物一钙矾石和石膏形成量不断增加，且水胶比越大，其膨胀现象越严重；但掺粉煤灰的砂浆的线长度在9个月内变化很微小，粉爆欢掺量越大膨胀越小。其试件内部膨胀性产物的量很少。且渗入试件内的硫酸根离子SO4^2-的量很少是粉煤灰抑制砂浆膨胀的主要原因。     

硫酸钠激发尾矿充填材料的性能与微观结构

以铅锌尾矿、硅酸盐水泥、矿粉为原材料,硫酸钠为碱性激发剂,设计制备了高流动性早强尾矿充填材料(以下简称充填材料),研究分析了硫酸钠掺量、矿粉含量对充填材料流动度、凝结时间、力学性能、反应产物物相组成和微观结构的影响.结果表明:随着矿粉含量的增加,充填材料的凝结时间延长,流动性增加,早期强度降低;硫酸钠可以缩短充填材料的凝结时间,并提高充填材料的早期强度;硫酸钠提供的硫酸根离子促进早期钙矾石(AFt)的形成,减少了自由水数量,增加了颗粒间的胶结程度;硫酸钠可显著提高充填材料孔溶液pH值,加速矿粉的早期水化.     

防腐混凝土收缩和硫酸盐腐蚀研究

混凝土开裂使得环境中腐蚀离子能快速渗透到内部并诱导钢筋混凝土腐蚀.研究了防腐剂对混凝土抗裂、收缩与抗硫酸盐腐蚀性能的定量影响规律.试验结果表明:防腐剂使得混凝土28d强度提高了6.7%,56d强度提高了9.2%;混凝土氯离子扩散系数降低28%.掺加防腐剂使得混凝土硫酸根离子表观扩散系数降低32%;减少了腐蚀产物生产量,降低了硫酸根反应量30%左右,混凝土抗硫酸盐腐蚀能力提升;防腐剂使得混凝土早期开裂风险增加,其60d收缩率增长了20%~30%.     

海洋环境混凝土对流区形成机理及氯离子迁移速率分析

混凝土在青岛实海暴露3、9、13个月,研究其在干湿交替区(浪溅区、潮汐区)和水下区的表层氯离子浓度分布,确定氯离子在干湿交替区的对流区深度及迁移速率。结果表明,干湿交替区腐蚀13个月混凝土表层氯离子浓度存在一个局部峰值,该峰值位于3~5 mm深处。水下区腐蚀混凝土长期处于饱和状态,不存在对流区海水溶蚀及硫酸根离子作用导致混凝土氯离子结合能力随腐蚀龄期增加而下降,并导致其表层自由氯离子迁移速率高于内部离子迁移速度3倍左右。     

盐渍土环境下混凝土中SO4^2-扩散性试验研究

为了研究西部盐渍土环境下混凝土中SO4^2-的扩散性,测试了在不同溶液、不同干湿循环龄期下,混凝土中不同深度的SO42-含量。采用Fick第二定律为基本模型,考虑SO42-与混凝土结合能力以及混凝土为非均匀介质的特性,对SO42-扩散系数进行了分析讨论。此外,基于XRD结果,分析了侵蚀过程中钙矾石的生长情况以解释说明钙矾石的生成对SO4^2-扩散性的影响。结果表明,Cl-可促进侵蚀前期SO4^2-的侵蚀速度;西部盐渍土环境下混凝土中SO4^2-的扩散系数与结合能力呈负相关关系,与侵蚀深度呈正相关关系;SO4^2-浓度越高,钙矾石生长越快,钙矾石膨胀作用会填充混凝土的既有裂缝,从而抑制SO42-扩散。     

Effect of W/C ratio on covering depth of fly ash concrete in marine environment

This investigation studied the effect of W/C ratio on covering depth required against the corrosion of embedded steel of fly ash concrete in marine environment up to 4-year exposure. Fly ash was used to partially replace Portland cement type I at 0%, 15%, 25%, 35%, and 50% by weight of cementitious material. Water to cementitious material ratios (W/C) of fly ash concretes were varied at 0.45, 0.55, and 0.65. The 200-mm concrete cube specimens were cast and steel bars with 12-mm diameter and 50 mm in length were inserted in the concrete with the covering depth of 10, 20, 50, and 75 mm. The specimens were cured in water for 28 days, and then placed to the tidal zone of marine environment in the Gulf of Thailand. Subsequently, the concrete specimens were tested for the compressive strength, chloride penetration profile and corrosion of embedded steel bar after being exposed to tidal zone for 2, 3, and 4 years. The results showed that the concrete mixed with Portland cement type I exhibited higher rate of the chloride penetration than the fly ash concrete. The chloride penetration of fly ash concrete was comparatively low and decreased with the increasing of fly ash content. The increase of fly ash replacement and the decrease of W/C ratio could reduce the covering depth required for the initial corrosion of the steel bar. Interestingly, fly ash concretes with 35% and 50% cement replacement and having W/C ratio of 0.65 provided better corrosion resistance at 4-year exposure than the control concrete with W/C ratio of 0.45. In addition, the covering depth of concrete with compressive strength of 30 MPa (W/C ratio of 0.65) could be reduced from 50 to 30 mm by the addition of fly ash up to 50%.     

海工混凝土的氯离子扩散性与寿命评估

通过现场海洋曝露试验和实验室海水浸泡试验,采取分层取样和化学分析方法,应用氯离子三维扩散理论,研究了普通混凝土和高性能混凝土在海洋大气区、潮汐区、水下区和实验室海水浸泡下的C1-扩散系数变化规律.结果表明,混凝土的C1-一扩散系数随着曝露时间的增加而降低,高性能混凝土的抗C1-扩散性优于普通混凝土.在Khatri计算模型的基础上,提出了考虑劣化效应系数的海工混凝土使用寿命计算模型.该模型计算结果与Clear经验模型基本吻合,解决了Khatri计算模型结果与实际寿命不相符的问题.     

Long term performance of chloride binding capacity in fly ash concrete in a marine environment

The capacity of binding chloride ions in fly ash concrete under marine exposure was studied. The free and total chloride contents in concrete were determined by water and acid-soluble methods, respectively. In order to study the effects of W/B ratios, exposure time, and fly ash contents on chloride binding capacity of concrete in a marine site, a class F fly ash was used as a partial replacement of Portland cement type I at 0%, 15%, 25%, 35%, and 50% by weight of binder. Water to binder ratios (W/B) were varied at 0.45, 0.55, and 0.65. Concrete cube specimens of 200 mm were cast and placed into the tidal zone of a marine environment in the Gulf of Thailand. Consequently, acid-soluble and water-soluble chlorides in the concrete were measured after the concrete was exposed to the tidal zone for 3, 4, 5, and 7 years. It was found that the percentage of chloride binding capacity compared to total chloride content increased with the increase of fly ash in the concrete. The percentage of chloride binding capacity significantly decreased within 3–4 years after the concrete was exposed to the marine environment, and then its value was almost constant. The research also showed that the W/B ratio does not noticeably affect the chloride binding capacity of concrete.     

Chemical sulfate attack performance of partially exposed cement and cement + fly ash paste

When concrete elements are partially exposed to sulfate rich environment, the upper part of concrete in contact with air will be deteriorated more severely than the underground part. Fly ash additions seem to accelerate the collapse of concrete in such an environment. Although concrete technologists attribute concrete damage mainly to salt crystallization or physical sulfate attack, the influence of chemical sulfate attack cannot be neglected and should also be studied.The objective of this paper is twofold. First, pore solution expression test was conducted to squeeze pore solution of different parts of cement paste partially exposed to Na₂SO₄ solution. The sulfate concentration and pH value of pore solution were measured. Results showed that the sulfate concentration of the pore solution in the upper part of paste in contact with air was much higher than in the lower submerged part. Fly ash additions could draw more sulfates into the paste in a shorter time, forming a higher concentration sulfate pore solution than in normal concrete.The second test was designed to simulate the effect of severe exposure condition on reactive products of cement paste. Pure cement and cement + fly ash (25% dosage) pastes were immersed in 5%, 15% and 30% at 30 °C and 15% at 40 °C Na₂SO₄ solutions. Thermogravimetric analysis was used to analyze the reaction products of the paste. The results indicate that more ettringite and gypsum were formed in cement + fly ash paste than pure cement paste.     

大掺量磨细矿渣混凝土试验研究

针对挡潮闸面临的耐久性问题,系统研究大掺量磨细矿渣混凝土的物理力学性能、耐久性能和热学性能.在掺量磨细矿渣混凝土中,使用活化剂改善混凝土和易性和体积稳定性,并提高早期强度.使用强纶纤维改善大掺量磨细矿渣混凝土的抗拉与抗裂能力.配制的大掺量磨细矿渣混凝土具有优良的抗氯离子侵蚀、抗硫酸盐侵蚀、延缓钢筋锈蚀等耐久性能,并具有...     

碳化对水泥石中硫元素分布的影响

为合理评价混凝土中发生硫铝酸盐膨胀反应的硫酸根离子浓度,应用电子探针显微分析技术,研究了碳化对水泥石中硫元素分布的影响,阐明了碳化作用下混凝土中硫元素的迁移规律.结果表明:碳化前水泥石截面的硫元素分布比较均匀,碳化后水泥石中的硫元素由碳化区向非碳化区迁移和积聚,硫元素在碳化区浓度较低,非碳化区浓度较高,钙矾石含量也随之增大,这种因碳化作用造成的硫元素分布不均匀可能导致混凝土局部发生硫铝酸盐膨胀开裂.     

海工结构服役寿命预测细观数值模拟研究

在现有二维混凝土随机骨料模型基础上,通过引入正态分布随机数来模拟混凝土界面过渡区厚度的非均匀性,并耦合氯离子扩散、钢筋锈蚀和混凝土损伤,建立了海工结构服役寿命预测细观数值模型.研究表明:由氯离子扩散引起的单根钢筋非均匀锈蚀导致混凝土保护层形成3条主裂缝,3条主裂缝之间夹角约为120°,出现这种现象的主要原因是界面过渡区加速了氯离子扩散,次要原因是界面过渡区的力学性能劣化;在相同钢筋锈蚀量条件下,非均匀锈蚀钢筋混凝土比均匀锈蚀钢筋混凝土的服役年限短,混凝土保护层开裂形态也不一样;数值模拟的氯离子含量分布、裂缝形态与试验数据较为吻合,验证了所建立的数值模型的可靠性.     

Corrosion protection performance of microsilica added concretes in NaCl and seawater environments

In a hardened concrete structure, diffusion of oxygen, carbon dioxide, aggressive ions and moisture from the concrete/environment to the concrete/rebar interfaces takes place through the pores. This results in the failure of the passivation provided by the alkalinity of the cement to the rebars. Microsilica is a mineral that improves the corrosion protection and strength of concrete by reducing the permeability of the concrete and forming more calcium silicate hydrate (CSH) which provides strength and durability to concrete. In order to study the corrosion protection behavior of microsilica in ordinary Portland cement (OPC) and sulfate resistant cement (SRC), carbon steel reinforced concrete specimens of above cements containing 10% each of densified (DMS) and undensified (UDMS) microsilica were exposed to 5% NaCl solution and seawater. Corrosion of rebars was monitored using open circuit potential (OCP) measurements. Diffusion of ions into the concrete was studied by salt ingress, salt fog and rapid chloride permeability tests. It has been noted that densified microsilica provides better corrosion protection to the rebars than the undensified one. In addition to this its protective effect with SRC is much more than with OPC. An attempt has been made to explain the mechanism of corrosion protection behavior of microsilica added concrete immersed in seawater and NaCl solution.     

复合盐侵蚀衬砌喷射混凝土耐久性退化研究

我国西部地区土壤及地下水中含有高浓度、及Cl,与隧道衬砌喷射混凝土发生一系列物理化学反应,造成其结构耐久性能退化。为系统研究复合盐侵蚀喷射混凝土耐久性能退化规律及机理,分别以10%Na2SO4溶液和5%Na2SO45%MgSO43.5%NaCl混合溶液为侵蚀介质,采用干湿交替法,开展喷射混凝土耐久性试验。、及Cl与氢氧化钙和铝相反应生成水镁石、石膏和Friedel盐,延缓钙矾石形成。复合盐侵蚀喷射混凝土物理力学性能退化速度明显小于硫酸盐侵蚀。硫酸盐侵蚀喷射混凝土以表面水泥砂浆和骨料的剥落为主,复合盐侵蚀主要以表面龟裂最终断裂为主,且裂缝中充满白色结晶盐。分析侵蚀喷射混凝土矿物组成和微观结构,硫酸盐侵蚀喷射混凝土产物主要为钙矾石和石膏,而复合盐侵蚀喷射混凝土产物组成复杂,包括碳硫硅钙石、水镁石、石膏、钙矾石、水化硅酸镁和结晶盐。硫酸盐侵蚀喷射混凝土中含量高于复合盐侵蚀,而混凝土pH值低。