粉煤灰粒径特征对水泥砂浆性能的影响

通过对原状粉煤灰进行粉磨、分级、再混合等操作,获得不同粒径特征的粉煤灰颗粒群,替代部分水泥配制砂浆。测试粉煤灰-水泥-砂干混料的堆积空隙率、新拌及硬化砂浆性能,研究粉煤灰粒径特征对干混料堆积空隙率的影响以及堆积空隙率与砂浆流动性、抗压强度、抗渗性之间的变化规律。结果表明,新拌砂浆的流动性与干混料堆积空隙率间呈现良好的线性变化关系,干混料堆积空隙率对后期硬化砂浆抗渗性的影响作用较粉煤灰比表面积对其的影响更为显著。     

考虑地下水位影响的碱渣土地基半埋混凝土内氯离子传输试验研究

关于碱渣土地基环境下钢筋混凝土结构耐久性问题,尚未见相关研究成果,是有待研究的课题。为探究碱渣土地基混凝土内氯离子传输规律,本工作模拟真实碱渣土地基及地下水侵蚀环境,设计了三种地下水位-0.2 m、-0.5 m、-0.8 m,开展半埋于碱渣土地基的混凝土氯离子自然扩散试验,探究不同地下水位环境下混凝土地表吸附区及地下区域的氯离子分布规律。结果表明,地表吸附区由于其蒸发浓缩及毛细作用,氯离子浓度高于地下区域。氯离子浓度随地下水位的变化呈现较大差异,地表吸附区氯离子浓度随地下水位的升高呈下降趋势,地下区域地下水位的影响主要体现在试件与水位的相对位置,地下水位以下区域氯离子浓度高于地下水位以上区域。在此基础上,探究碱渣土地基半埋混凝土内氯离子传输机理,基于Fick第二定律定量研究了地下水位对表面氯离子浓度及扩散系数的影响,建立考虑地下水位影响的氯离子传输时变模型,通过与实测结果对比印证了模型的正确性,本模型可为实际工程中定量评估不同碱渣土侵蚀环境下混凝土内氯离子浓度以及服役寿命预测提供技术支持。     

电化学除氯和硅酸根电迁移法的联合修复研究

为了抑制氯离子反向扩散造成钢筋二次腐蚀,进一步延长修复后混凝土结构的服役寿命,在电化学除氯(ECR)基础上提出一种通过电驱动SiO_3~(2-)进入孔隙,与孔隙中的Ca(OH)_2反应生成C-S-H凝胶硬化混凝土/砂浆,并同时去除混凝土/砂浆内氯离子的新型的联合修复方法。由对比试验,通过滴定氯离子,利用自腐蚀电位(Ecorr)、腐蚀电流密度(Icorr)和电化学阻抗谱法(EIS)研究了该方法的修复效果及其修复后的长期耐久性,此外,由压汞法和热重分析探讨了该方法的微观作用机制。结果表明:联合修复方法与电化学除氯具有相近的除氯效率;与电化学除氯相比,联合修复方法能更有效延缓钢筋的二次腐蚀;随着初始氯离子含量的减少,联合修复方法延缓钢筋二次腐蚀的效果更为显著;联合修复方法对钢筋二次腐蚀的抑制源于其致密化作用,经联合修复后砂浆总孔隙率与有害孔数量相比于修复前均有所下降。     

海砂混凝土中氯离子固化的影响因素研究

通过海砂混凝土中游离氯离子含量的测定,研究了海砂混凝土氯离子吸附和结合特征.利用X射线衍射、扫描电镜和热分析等测试方法对海砂混凝土的微结构和Friedel盐进行了表征.结果表明:海砂混凝土游离氯离子浓度与溶液的萃取温度密切相关,温度越高,游离氯离子浓度越大,萃取温度为65℃时的游离氯离子浓度约为15℃时游离氯离子浓度的2倍;同时粉煤灰砂浆中结合的物理固化离子在较低温度下溶出率低.TG/DTA曲线上没有出现海砂混凝土的Friedel盐吸热峰,而XRD与SEM图中显示海砂混凝土中存在Friedel盐,可能是因为Friedel盐含量较少,导致生成的Friedel盐不稳定.     

自然扩散条件下SO4^2-、Cl^-在砂浆中的沉积特性

通过模拟自然扩散条件下,测试不同砂浆试件内部各个位置硫酸根离子、氯离子的沉积数量,着重讨论了硫酸根离子、氯离子在掺粉煤灰、矿渣水泥砂浆中的沉积特性。结果表明,矿渣、粉煤灰对砂浆中硫酸根离子、氯离子的沉积数量存在较大影响,粉煤灰和矿渣复掺能显著降低砂浆表层沉积的离子数量。砂浆中氯离子、硫酸根离子的沉积量随浸泡龄期延长而呈现不同的变化趋势,砂浆表层的氯离子沉积量随浸泡龄期而增加的速率大于硫酸根离子。研究成果为改善水泥基材料耐侵蚀性盐类的腐蚀作用提供科学依据。     

电化学脱盐对结合氯离子稳定性的若干影响因素研究

采用电化学脱盐方法对砂浆试块进行电化学除氯,研究了电场作用下若干影响因素(水灰比、阳离子类型、氯离子浓度、矿物掺合料等)对结合氯离子失稳释放的影响。结果表明:随着水灰比增大,结合氯离子在电场作用下的稳定性减弱,结合氯离子释放能力增强。电场作用下阳离子类型不同的砂浆试块结合氯离子失稳释放能力的大小顺序为:Ca2+Mg2+≈Na+K+。在1%~5%的氯离子浓度范围内,3%氯离子浓度下结合氯离子转化率最高,结合氯离子释放能力在高氯离子浓度和低氯离子浓度下都会出现下降的情况。硅粉会降低电场作用下结合氯离子的释放能力;随着矿粉产量的增加,结合氯离子转化率也逐渐增大,说明矿粉可促进电场作用下结合氯离子的释放,增强脱盐时结合氯离子的释放能力;粉煤灰对电场作用下结合氯离子的释放也有促进作用,且粉煤灰掺量在20%左右时结合氯离子的释放能力最强。     

油页岩渣对混凝土抗压强度和抗氯离子侵蚀的影响

油页岩经干馏或燃烧后留下大量油页岩渣(OSR),OSR的堆弃占用土地和污染环境.以OSR干馏渣为粗骨料制备了油页岩渣混凝土(OS-RC),测试了其工作性能、微观结构、抗压强度和氯离子渗透性能.试验结果表明,OSRC的工作性能、力学性能和抗氯离子侵蚀性能均随油页岩渣取代率(Vr)的增加而降低,Vr为25％和43％时,OSRC的立方体抗压强度分别为29.9 MPa和25.5 MPa,氯离子渗透性能评价分别为很低和低.Vr大于50％时,OSRC的通电量随Vr的提高而快速增加.通过微观形貌测试发现,离OSR与界面过渡区(lTZ)界线越远,lTZ中的钙矾石晶体越少,水化硅酸钙凝胶越多,但氢氧化钙晶体较少,OSR具有"返水"特性.OSRC的受压破坏形态为OSR折断且断面平整,天然骨料周边的水泥砂浆剥落.OSR和lTZ是OSRC的薄弱区域,是后续研究之关键.OSR等多孔废弃物可以用作混凝土的填充料和内养护材料.     

氯盐-硫酸盐共存环境中杂散电流作用下提升砂浆中氯离子结合性能的研究

长期浸泡在地下水中的地铁混凝土结构,不仅遭受氯盐、硫酸盐的双重侵蚀,而且存在杂散电流腐蚀破坏.由于孔溶液中的自由氯离子是导致钢筋锈蚀的首要因素,通过提高混凝土中氯离子的结合能力可有效降低氯离子对钢筋混凝土的危害.本工作通过选择合理的外掺料种类及掺量,提出了提高地铁工程混凝土中氯离子结合性能的最优外掺料组合,并采用电位滴定法测定结合氯离子含量,以及结合XRD、SEM和DTG等微观测试方法对其机理进行分析.结果表明:杂散电流作用下偏高岭土对氯离子结合性能的提升效果优于硅灰,粉煤灰微珠优于沸石粉,复掺10％偏高岭土、20％微珠、1.5％PVA可再分散性乳胶粉的试件中氯离子的结合性能最优,砂浆内部氯离子的化学结合能力明显提高.     

结构混凝土耐久性影响因素的研究进展与探讨

综述了不同因素对结构混凝土耐久性的影响。主要讨论了服役环境对结构混凝土耐久性的影响,包括荷载、氯离子、冻融、硫酸盐及碳化;还分析了如粉煤灰、细晶粒钢筋等材料对氯离子临界值浓度的影响;对今后研究结构混凝土耐久性问题提出了一些建议。     

掺合料粉体种类对泡沫混凝土性能的影响

以普通硅酸盐水泥为结合剂,用粉煤灰和硅灰取代砂和部分水泥,研究掺和料种类对泡沫混凝土抗压强度、吸水率以及抗冻性的影响。结果表明:泡沫混凝土的性能不仅与孔隙率有关,还与基体材料中掺合料的种类有关。加入硅灰可引起泡沫混凝土的成型水胶比增加,显著提高了泡沫混凝土的早期强度,但同时引起吸水率的增加,也不利于抗冻;当掺合料为粉煤灰时,提高了泡沫混凝土的抗冻耐久性,当将原状粉煤灰磨细,使泡沫混凝土的后期强度增长较快,并大幅度的降低了吸水率,但对抗冻性影响不大。     

Sustainable utilization of pre-treated and nano fly ash powder for the development of durable geopolymer mortars

Developments in geopolymer construction are gaining more interest nowadays due to the elimination of cement and the consequent effects such as carbon dioxide emission, greenhouse effect, etc. Although the use of fly ash as a binder in the geopolymer system acts as a key solution for the major hazardous effects like land dumping, soil contamination, groundwater pollution, and respiratory diseases, the slow reactivity of the fly ash resulted in the considerable reduction in the strength. In this paper, a novel pretreatment method was employed on the fly ash binder in terms of thermal and mechanical means. Also, a cost-effective nano fly ash powder was synthesized and used as filler material on the geopolymer system. The efficiency of the fabricated geopolymer mortar was assessed by examining the workability, compressive strength, and resistance against chloride ion penetration. The geopolymer mortars with pre-treated fly ash exhibited a highly workable mix of 130% improved flow rate without adding any superplasticizer. Further, the addition of 1% nano fly ash, exhibited the highest compressive strength of 71.22 MPa, confirmed almost nil chloride ion permeability, and sustained 90% residual strength after immersing in the brine solution for 60 days which explored the development of sustainable and cost-effective geopolymer construction in the marine environment.     

Corrosion behaviour of rebars in fly ash mortar exposed to carbon dioxide and chlorides

Addition of fly ash has beneficial effects on some mechanical properties of concrete, as well as on the corrosion process induced by the chloride ion. The aim of this study was to investigate the effect of fly ash addition on the corrosion process occurring in reinforced concrete exposed simultaneously to carbon dioxide and chloride. The corrosion process of steel rebars embedded in mortar with 15% and 30% of fly ash was tested under carbon dioxide and sodium chloride contamination. Monitoring of open circuit potential and electrochemical impedance spectroscopy (EIS) were used to follow the corrosion process. Results have shown that under accelerated carbonation fly ash mortar shows higher corrosion rates. The chloride content in mortar exposed to accelerated carbonation increases with the amount of fly ash. However, under natural carbonation it decreases with the addition of fly ash.     

Effect of ground fly ash and ground bagasse ash on the durability of recycled aggregate concrete

This research aims to study the effect of ground fly ash (GFA) and ground bagasse ash (GBA) on the durability of recycled aggregate concrete. Recycled aggregate concrete was produced with recycled aggregate to fully replace crushed limestone in the mix proportion of conventional concrete (CON) and GFA and GBA were used to partially replace Portland cement type I at the rate of 20%, 35%, and 50% by weight of binder. Compressive strength, water permeability, chloride penetration depth, and expansion by sulfate attack on concretes were investigated.The results reveal that the use of GFA and GBA to partially replace cement in recycled aggregate concrete was highly effective in improving the durability of recycled aggregate concrete. The suitable replacement of GFA or GBA in recycled aggregate concrete to obtain the suitable compressive strength, low water permeability, high chloride penetration resistance, and high sulfate resistance is 20% by weight of binder.     

纳米SiO2与粉煤灰协同改性水泥基材料性能研究

通过调整纳米SiO_2与粉煤灰的比例,研究了两者协同作用对水泥基材料性能的影响。结果表明,纳米SiO_2(NS)和粉煤灰协同作用效果优于NS单一掺加,3%(质量分数,下同)纳米SiO_2和不大于30%的粉煤灰同时掺加可以补偿粉煤灰引起的早期强度降低,且砂浆28d抗压强度不降低。随着NS掺量增加水泥基材料的干燥收缩增大,粉煤灰可以改善纳米SiO_2对干燥收缩的不利影响。随着NS掺量的增加,试件的抗冻性和抗氯离子渗透性能均得到提升,掺加3%NS与30%粉煤灰使水泥基材料达到最佳耐久性能。NS可以缩短水泥水化诱导期,加速水泥水化进程,且使胶凝体系总放热量增加。在水泥粉煤灰体系中掺入NS后,非蒸发水含量在早期明显增多,但在后期增长缓慢。     

The effect of processed fly ashes on the durability and the corrosion of steel rebars embedded in cement–modified fly ash mortars

This paper deals with the study of corrosion level of reinforcing steel bars embedded in Portland cement mortars containing different types of fly ash. Fly ashes used were obtained by physico-chemical treatments of an original F class fly ash to modify their magnetic properties and reduce their particle size. An original fly ash (T0) and three types of modified ashes were tested according to treatment duration and magnetic properties (T60, ground fly ash; TNM, non-magnetic fraction; TM, magnetic fraction). Corrosion tests on reinforced mortar specimens with and without different types of fly ashes, cured at 40 °C, and under accelerated carbonation conditions and seawater immersion, have been performed in order to obtain conclusions on durability. From the corrosion point of view the addition of TNM in mortars showed to be much more effective than addition of the original T0 fly ash.     

Freeze–thaw resistance and transport properties of high-volume fly ash roller compacted concrete designed by maximum density method

The aim of this study is to evaluate the effect of high-volume fly ash on some durability characteristics of roller compacted concrete (RCC). In addition to a control mixture without fly ash, two different series of mixtures were prepared by partial replacement of either cement or aggregate with fly ash. The mixtures were designed by a maximum density method. A total of 28 mixtures having four different water/binder ratios (0.30, 0.35, 0.40 and 0.45 by mass) were prepared to determine the optimum water/binder ratio. Among these, seven mixtures containing the optimum water content were selected for further experimental study. It was observed that in the mixtures where cement was substituted with fly ash, increasing the fly ash content adversely affected the durability performance up to 90 days. However, fly ash substitution for a part of the aggregate improved the durability characteristics of the mixture as the amount of fly ash increased.     

The fluid transport properties of HCWA–DSF hybrid supplementary binder mortar

It has been demonstrated in several past studies that high calcium wood ash (HCWA) can be effectively used in combination with densified silica fume (DSF) as supplementary binder material to enhance the mechanical performance of concrete. The experimental investigation was conducted to study the effect of the inclusion of HCWA and DSF on the durability properties of high strength cement mortar produced. A total of twelve different mix designs of mortar were fabricated with the use of HCWA at various cement replacement levels of 0–20% in combination with 7.5% densified silica fume (DSF) and subjected to various durability tests. The durability assessments performed include tests on water absorption, air permeability, porosity and degree of carbonation. A significantly lower degree of water absorption, porosity and carbonation was observed for cement mortars with HCWA contents of 2–8% used in combination with 7.5% DSF by weight of binder as compared to an equivalent pure cement mortar.     

Influence of the usage of waste oyster shell powder on mechanical properties and durability of mortar

In order to investigate feasibility of waste oyster shell powder (WOSP) as fine aggregate to produce eco-friendly mortar, workability (slump flow and slump flow loss), mechanical properties (compressive strength and flexural strength), durability (sorptivity, volume of permeability coefficient, water permeability coefficient and chloride ion diffusion coefficient) and microstructure (pore size diffusion) were studied. The effect of replacing river sand with different WOSP proportions (0%, 10%, 20% and 30%) in flowability, strength, permeability and microstructure of mortar have been revealed. The results indicate that increasing substitution ratio of WOSP could decrease the mortar slump flow. The utilization of WOSP in mortar enhanced the compressive strength, flexural strength, resistance to water penetration and chloride diffusion. The WOSP addition exhibits a positive contribution to the pore size distribution of the mortar. Furthermore, it is founded that the utilization of WOSP as construction material is a satisfactory way to reduce waste pollution. Based on its superior mechanical property, durability, eco-efficiency and cost-efficiency in mortar, it is recommended to utilize WOSP alternative to river sand at 10–30% in construction engineering.     

Durability of concrete incorporating high-volume of low-calcium (ASTM Class F) fly ash

Research on structural concrete incorporating high volumes of low-calcium (ASTM Class F) fly ash has been in progress at CANMET since 1985. In this type of concrete, the cement content is kept at about 150 kg/m³. The water-to-cementitious materials ratio is of the order of 0·30, and fly ash varies from 54 to 58% of the total cementitious material. A large dosage of a superplasticizer is used to achieve high workability.

This paper presents data on the durability of this new type of concrete. The durability aspects considered are: freezing and thawing cycling; resistance to chloride ion permeability; and the expansion of concrete specimens when highly reactive aggregates are used in the concrete.

The investigations performed at CANMET indicate that concrete incorporating high volumes of low-calcium fly ash has excellent durability with regard to frost action, has very low permeability to chloride ions and shows no adverse expansion when highly reactive aggregates are incorporated into the concrete.     

Use of natural zeolite as a supplementary cementitious material

Natural zeolite, a type of frame-structured hydrated aluminosilicate mineral, is used abundantly as a type of natural pozzolanic material in some regions of the world. In this work, the effectiveness of a locally quarried zeolite in enhancing mechanical and durability properties of concrete is evaluated and is also compared with other pozzolanic admixtures. The experimental tests included three parts: In the first part, the pozzolanic reactivity of natural zeolite and silica fume were examined by a thermogravimetric method. In this case, the results indicated that natural zeolite was not as reactive as silica fume but it showed a good pozzolanic reactivity. In the second part, zeolite and silica fume were substituted for cement in different proportions in concrete mixtures, and several physical and durability tests of concrete were performed. These experimental tests included slump, compressive strength, water absorption, oxygen permeability, chloride diffusion, and electrical resistivity of concrete. Based on these results, the performance of concretes containing different contents of zeolite improved and even were comparable to or better than that of concretes prepared with silica fume replacements in some cases. Finally, a comparative study on effect of zeolite and fly ash on limiting ASR expansion of mortar was performed according to ASTM C 1260 and ASTM C 1567. Expansion tests on mortar prisms showed that zeolite is as effective as fly ash to prevent deleterious expansion due to ASR.