减缩剂外涂对水泥基材料性能的影响

对比研究了减缩剂内掺和外涂对砂浆收缩、抗裂及力学性能的影响.结果表明:减缩剂外涂对砂浆试件有比较明显的减缩效果;减缩剂外涂的溶液浓度越高,减缩效果越好,适宜浓度为50%:相比内掺,减缩剂外涂可提高砂浆的抗折强度和抗压强度,提高砂浆抗裂性能,开裂面积下降了35%;减缩剂外涂增加水泥石中C-S-H凝胶及钙矾石的生成量,减少了水泥石中20～50nm毛细孔的数量.     

减缩剂预饱和轻骨料对水泥砂浆自收缩的影响及机理

使用内养护(IC)和减缩剂(SRA)是两种可有效降低水泥基材料自收缩的方法,但两者单独采用时存在明显不足.试验研究了减缩剂预饱和的轻骨料对水泥砂浆自收缩、强度、水化程度以及骨料一水泥石基体界面过渡区域的影响.结果表明:由于减缩剂降低溶液表面张力并增加溶液黏度,减缩剂溶液相比水从轻骨料中释放速率明显降低;与直接内掺减缩剂...     

水化热调控剂与氧化镁复掺对混凝土抗裂行为的影响

为了研究氧化镁膨胀剂与水化热调控剂复掺对混凝土抗裂性能的影响,探讨水化热调控剂的作用机理,开展了单掺氧化镁及水化热调控剂与氧化镁复掺对混凝土水化热、绝热温升、限制膨胀率、体积变形、自收缩、弹性模量及微观结构的影响研究。结果表明:水化热调控剂可显著改变水泥水化放热历程,净浆抑温率可达31.2%,随入模温度升高,抑温效果增强;水化热调控剂可激发氧化镁的水化,促进膨胀能的释放,随养护温度提升,作用效果更显著;水化热调控剂可改变混凝土温升历程,特别是降低混凝土早期水化热释放速率;水化热调控剂与氧化镁复掺可降低混凝土自收缩,当水化热调控剂掺量为0.2%和0.4%(质量分数)时,混凝土收缩值分别减少了84 με和54 με;水化热调控剂会导致混凝土早龄期弹性模量降低,但对长龄期弹性模量影响不大;水化热调控剂对水泥水化历程、水化产物生成及微观结构密实程度的改变是提高混凝土抗裂性能的根本原因。     

抑制水泥浆体早期自收缩的方法

水泥浆体早期自收缩已受到越来越多的关注,本文分别检验了掺减缩剂、膨胀剂和纤维抑制浆体3种样品早期自收缩的效果.试验结果表明,单掺钙钒石类膨胀剂对抑制早期自收缩作用不明显.单掺二元醇醚类减缩剂减缩效果最高可达80%,但强度随龄期增长而损失较大.在不需要浆体早期强度很高时,可以用二元醇醚类减缩剂和JM Ⅲ膨胀剂复掺来抑制浆体早期自收缩;在需要考虑浆体早期强度时,可以用二元醇醚类减缩剂和长度3mm的聚丙烯纤维复掺来抑制浆体早期自收缩.     

减缩型聚羧酸减水剂提高混凝土早期抗裂性的作用研究

减缩型聚羧酸减水剂(SRPC)是一类兼具减缩与减水双重作用的新型混凝土外加剂,它对混凝土早期抗裂性的作用将影响其在工程中的应用。本文利用平板抗裂试验对比测试了掺SRPC、聚羧酸减水剂(PCE)及小分子减缩剂(SRA)的混凝土早期抗裂性,并通过对表面张力、收缩率、孔结构、水化热等参数的分析,讨论了SRPC影响混凝土抗裂性的作用机理。试验结果表明,尽管SRPC在掺量0.15%(质量分数)下的减缩效果弱于掺量1.5%(质量分数)的SRA,但SRPC表现出更佳的早期抗裂性能。机理分析表明,SRPC除了与SRA都是通过降低孔溶液表面张力及改变毛细孔结构从而减少混凝土的收缩外,还通过延缓水泥水化速率、降低早期水化放热量、抑制混凝土中水分蒸发等作用,改善混凝土的早期抗裂性。     

硫铝酸盐基促强减缩剂对水泥性能的影响

分别研究了随硫铝酸盐基促强减缩剂(SP-SRA)掺量变化,P·Ⅰ和P·O两种水泥的标准稠度用水量、凝结时间、28 d干空收缩以及胶砂强度的变化规律,分析了不同掺量的SP-SRA对P·Ⅰ水泥水化热、水化产物物相和微观形貌的影响。结果表明:随SP-SRA掺量的增加,两种水泥的初凝、终凝时间明显缩短,28 d干空收缩减少,各龄期抗折、抗压强度增大;早期水化放热速率随SP-SRA掺量的增加而增大,XRD和DSC分析表明SP-SRA使钙矾石含量明显增多,MIP分析结果显示随水化的进行,掺SP-SRA的水泥浆体小孔数量逐渐增多,说明钙矾石等水化产物填充了水泥石毛细孔,使得大孔数量减少,小孔数量增加,水泥石更加密实,强度增大,并且钙矾石的膨胀性抵消了部分收缩,使得掺SP-SRA的水泥干空收缩减小。     

减缩剂对混凝土收缩性能影响的试验研究

针对掺高效减水剂导致早期混凝土自收缩增大,收缩时间提前等问题,提出一种掺加减缩剂的方式。采用立杆式收缩测量方法,对两种不同减缩剂进行干缩试验,从而得到效果比较明显、混凝土力学性能得到明显改善的一种复合外加剂材料。通过试验研究表明,减缩剂B-SAR较减缩剂D-SAR对混凝土在抗压、抗折、收缩率方面要明显具有优势,并确定缩减剂B的最佳掺加量。     

高强自密实混凝土的减缩措施

观察了不同掺量的减缩剂和高吸水性树脂对于C70自密实混凝土的工作性、抗压强度和体积稳定性的影响;对含有高吸水性树脂的硬化混凝土的微观结构进行了分析,探讨高吸水性树脂降低混凝土自收缩的机理。结果显示,掺加减缩剂和高吸水性树脂都能降低自密实混凝土的自收缩;高吸水性树脂的减缩效果更好,对于混凝土工作性的影响很小;掺加适量高吸水性树脂使高强混凝土的自收缩开始发展时间延后,第一天内的收缩量降低,有利于提高混凝土的抗裂性能;在硬化混凝土中高吸水性树脂颗粒分散分布,水分释放后留下的孔洞的直径大多处于50~70 nm范围内,并使混凝土的总孔隙率有所上升,但对其抗压强度的影响较小。     

建筑垃圾复合粉体材料对小型混凝土构件收缩性能和微观结构的影响

针对我国建筑垃圾堆积如山,再生利用率较低的现状,将建筑垃圾磨细砖粉与矿粉、粉煤灰和激发剂复合形成建筑垃圾复合粉体材料(以下简称CWCPM),从宏观和微观两方面研究了CWCPM对小型混凝土强度和收缩性能的影响.结果表明:掺CWCPM的混凝土具有较好的强度效应和收缩性能;当替代量小于30％时,混凝土早期强度有小幅度降低,而后期强度高于基准试样;60d龄期时,CWCPM的掺入提高了混凝土的收缩性能,随其掺量的增加收缩系数先减小后增大.DSC-TG、压汞试验结果表明:孔隙率并不是影响混凝土性能的决定性因素,混凝土各项性能与其内部孔径分布有密不可分的关系;CWCPM良好的填充效应和火山灰效应改善了水泥水化产物组成和内部孔结构,提高了混凝土密实度.     

减缩剂对水泥基材料水化和孔结构的影响(英文)

通过对水泥净浆水化放热量、Ca(OH)2的含量和化学结合水量及水泥砂浆的孔结构的测定,研究减缩剂对水泥基材料水化和孔结构的影响.结果表明:减缩剂会延缓水泥的水化,且随着龄期的发展,延缓作用渐弱.具体表现为减缩剂会降低水泥水化放热的峰值,延迟峰值出现的时间,减小水化放热量;减缩剂能够减小水泥净浆的化学结合水量;龄期为3d时,掺减缩剂的净浆中Ca(OH)2的含量明显低于空白样的,28d时与空白样的相当;减缩剂能够减小水泥砂浆中多害孔和有害孔的孔体积,增加无害孔的孔体积.另外还对减缩剂在水泥基材料中存在的形态提出设想.     

Interactions between shrinkage reducing admixtures (SRA) and cement paste's pore solution

Shrinkage reducing admixtures (SRA) have been developed to combat shrinkage cracking in concrete elements. While SRA has been shown to have significant benefits in reducing the magnitude of drying and autogenous shrinkage, it has been reported that SRA may cause a negative side effect as it reduces the rate of cement hydration and strength development in concrete. To examine the influence of SRA on cement hydration, this study explores the interactions between SRA and cement paste's pore solution. It is described that SRA is mainly composed of amphiphilic (i.e., surfactant) molecules that when added to an aqueous solution, accumulate at the solution-air interface and can significantly reduce the interfacial tension. However, these surfactants can also self-aggregate in the bulk solution (i.e., micellation) and this may limit the surface tension reduction capacity of SRA. In synthetic pore solutions, SRA is observed to form an oil-water-surfactant emulsion that may or may not be stable. Specifically, at concentrations above a critical threshold, the mixture of SRA and pore fluid is unstable and can separate into two distinct phases (an SRA-rich phase and an SRA-dilute phase). Further, chemical analysis of extracted pore solutions shows that addition of SRA to the mixing water depresses the dissolution of alkalis in the pore fluid. This results in a pore fluid with lower alkalinity which causes a reduction in the rate of cement hydration. This may explain why concrete containing SRA shows a delayed setting and a slower strength development.     

海水环境下矿物掺合料对硫铝酸盐水泥的水化影响

研究了海水环境下掺入硅灰、粉煤灰、矿渣对硫铝酸盐水泥抗压强度、化学收缩和水化产物的影响规律.结果表明:当硅灰的掺量为2.5%时,水泥浆体的抗压强度比空白组高.矿渣掺量为10%的水泥浆体28 d抗压强度明显超过掺入硅灰和粉煤灰时的强度,60 d强度高于空白组.掺入2.5%硅灰后,水泥浆体的化学收缩增大;在水化早期,粉煤灰和矿渣的火山灰活性很低,导致水泥浆体的化学收缩降低.掺入10%硅灰加快了硫铝酸盐水泥3 d水化反应,钙矾石生成量增多,水泥浆体早期强度比掺其它掺合料有所提高,但体积过快膨胀会破坏其内部结构,对水泥浆体的强度发展不利.     

内掺硅烷乳液憎水剂对混凝土性能的影响

通过强度测试、水化热实验、热重分析、毛细吸水实验以及压汞实验研究了内掺硅烷乳液憎水剂对混凝土强度以及毛细吸水性能的影响规律及其机理.结果表明:高掺量硅烷乳液一定程度上抑制水泥水化,从而导致混凝土早龄期抗压强度和劈裂抗拉强度下降以及28 d养护后水泥净浆试块中毛细孔的体积增大.毛细吸水实验表明:28 d龄期混凝土毛细吸水...     

Combined effect of expansive and shrinkage reducing admixtures to obtain stable and durable mortars

In order to improve the dimensional stability of cement based mortars, the effects produced on cement hydration of a shrinkage reducer (propyleneglycol ether based—SRA) and an expansive admixture (calcium oxide based—EXP) were investigated. Mortar samples (prepared without admixtures or with SRA or EXP or SRA and EXP) were compared through compressive strength measurements, water evaporation, restrained shrinkage and restrained expansion measurements. Setting time and free expansion were also detected on cement paste specimens.

A synergistic effect on the shrinkage reduction was observed when the shrinkage reducing admixture and the expansive agent were used together. In order to clarify this phenomenon, the hydration of cement pastes containing these kinds of admixtures was followed by ESEM-FEG (environmental scanning electron microscopy–field emission gun), TG (thermogravimetry), specific surface area measurements (by BET—Brunauer–Emmet–Teller-method) and XRDS (X-ray diffraction spectroscopy).     

预湿水泥浆颗粒内养护材料对混凝土早期自收缩及抗压强度的影响

制备不同粒径和水灰比的水泥浆颗粒作为低水灰比混凝土内养护材料.以最佳内养护水灰比原则,设计了使用三种水灰比分别为0.6、0.7和0.8的同粒径水泥浆颗粒等体积取代砂子的混凝土.研究了不同水灰比水泥浆颗粒对混凝土早期自收缩、抗压强度和内部微结构的影响.结果表明:颗粒吸水率与其水灰比正相关、与其粒径负相关;预湿水泥浆颗粒可显著降低混凝土早期的自收缩,颗粒水灰比越大,自收缩降低效果越明显;但是掺入水泥浆颗粒也会降低混凝土的抗压强度,颗粒水灰比越高抗压强度降低越多,应用中应优化选择预湿颗粒的水灰比;水泥浆颗粒作为高性能混凝土内养护材料,可改善微观界面的孔隙结构,提高界面的密实性,减少混凝土早期的收缩和开裂.     

超细矿渣粉和偏高岭土对硫铝酸盐水泥早期收缩性能的影响

通过开展化学收缩、自收缩与干燥收缩试验,研究了超细矿渣粉和偏高岭土对硫铝酸盐水泥早期收缩性能的影响。结果表明,掺入超细矿渣粉与偏高岭土会增大水泥浆体的内部相对湿度,能有效抑制水泥浆体的化学收缩、自收缩与干燥收缩,且掺量越大,抑制效果越明显,根据水泥浆体的内部相对湿度能够大致判断其自收缩的变化规律。掺入超细矿渣粉与偏高岭土会加快硫铝酸盐水泥的早期水化,使化学收缩变化速率达到峰值的时间提前。当超细矿渣粉的掺量为20%(质量分数,下同)或偏高岭土的掺量为10%、20%时,与空白组相比水泥浆体的7 d自收缩分别减小了42.21%、35.89%和63.73%,7 d干燥收缩分别减小了24.89%、16.42%和30.87%。在相同掺量条件下,掺入偏高岭土的水泥浆体化学收缩、自收缩与干燥收缩显著小于掺入超细矿渣粉的水泥浆体。自收缩与线性化学收缩的比值随龄期的增长而减小,掺入超细矿渣粉与偏高岭土后,自收缩与线性化学收缩的比值进一步减小。     

Will wind always boost early-age shrinkage of cement paste?

Immediately after casting, the occurrence of early age shrinkage is sensitive to internal stresses, degree of hydration, and tensile strength, which is of concern to engineers. This study investigated the influences of wind speeds on early-age shrinkage and tensile strength (strain) of cement paste, and the results were further interpreted by an investigation of hydration degree and capillary water. The outcomes show that as the wind speed increased, the shrinkages (i.e., drying, chemical, and autogenous shrinkage) experienced various trends depending on several factors. Meanwhile, a variety of wind speeds altered the proportion of the evaporation in the capillary water, the inner moisture distribution, and the humidity difference between specimens’ surface and their interior sections. Further, tensile strength and elastic modulus were deteriorated by wind, whereas with the growth of the wind speed, the ultimate tensile strain of specimens exhibited various scenarios within and after 24 h.