粉煤灰对双膨胀水泥膨胀性能的影响

本文研究粉煤灰对双膨胀水泥化和膨胀性能的影响。结构表明,粉煤灰对双膨胀水泥早期和后期膨胀都有显著的抑制作用,对早期膨胀的抑制作用与形成钙化矾石的化学环境改变以及钙矾石形成提前结束有关,对后期膨胀的抑制作用与其抵制水泥浆体中水镁石的形成一致。     

粉煤灰对双膨胀水泥膨胀性能的影响

本文研究粉煤灰对双膨胀水泥水化和膨胀性能的影响。结果表明 ,粉煤灰对双膨胀水泥早期和后期膨胀都有显著的抑制作用 ,对早期膨胀的抑制作用与形成钙矾石的化学环境改变以及钙矾石形成提前结束有关 ,对后期膨胀的抑制作用与其抑制水泥浆体中水镁石的形成一致。     

CaSO4·2H2O-C3A压实体水化产生膨胀应力的机理

本工作采用自行设计的一套膨胀应力测试装置测试了在三维约束条件下CaSO_4·2H_2O-C_3A压实体在40℃的水溶液、饱和Ca(OH)_2溶液和1 mol/L NaOH溶液中水化生成钙矾石过程中产生的膨胀应力,并测试了压实体中水化生成的钙矾石的吸水肿胀应力。结果表明,CaSO_4·2H_2O-C_3A压实体在水溶液、饱和Ca(OH)_2溶液和1 mol/L NaOH溶液中水化生成钙矾石产生的最大膨胀应力为23.9 MPa、27.3 MPa和26.4 MPa,在水溶液中水化膨胀应力最小,表明Ca(OH)_2溶液和OH~-均会促进钙矾石产生的膨胀应力;生成的钙矾石45℃干燥再吸水,在约束条件下40℃的吸水肿胀应力分别为1.23 MPa、1.27 MPa和1.26 MPa,远小于钙矾石形成过程产生的膨胀应力,表明钙矾石吸水肿胀应力对CaSO_4·2H_2O-C_3A压实体水化膨胀应力的贡献很小,钙矾石形成时产生的膨胀应力主要源于结晶压力。     

双膨胀水泥研究

双膨胀水泥同时利用了钙矾石和氧化镁两个膨胀源 ,钙矾石膨胀主要发生在 2 8天以前 ,氧化镁膨胀主要发生在 2 8天以后 ,两者恰当组合 ,获得适宜的膨胀量和膨胀分布 ,从而可以比较有效地补偿大坝等大体积混凝土的体积收缩。     

双膨胀水泥研究

双膨胀水泥同时利用了钙矾石和氧化镁两个膨胀源，钙矾石膨胀主要发生在２８天以前，氧化镁膨胀主要发生在２８天以后，两者恰当组合，获得适宜的膨胀量和膨胀分布，从而可以比较有效地补偿大坝等大体积混凝土的体积收缩。     

双膨胀低热矿渣水泥的膨胀过程研究

本文借助水泥强度和净浆膨胀测定方法与XRD对高镁水泥熟料-矿渣-石膏系统的双膨胀低热矿渣硅酸盐水泥进行了研究。得到(1)低热矿渣水泥净浆膨胀随SO3含量和时间的变化规律;(2)低热矿渣水泥强度随SO3含量的变化规律;(3)在水泥中适量SO3和熟料中MgO含量小于5%时,钙矾石膨胀和水镁石膨胀具有连续性、整体性和稳定性;(4)矿渣的掺入有助于钙矾石的形成及其数量的增加和水泥膨胀率的增加与稳定。     

混凝土中钙矾石的重结晶

钙矾石是水泥重要的水化产物之一,也是铝酸盐类膨胀剂产生膨胀的主要物质,但它在混凝土中并不稳定,容易发生重结晶.提出了混凝土中钙矾石重结晶的理论基础,并从钙矾石的溶解和析晶两方面探论了混凝土中钙矾石重结晶的影响因素,重点讨论了混凝土原料组分中碱和石膏对钙矾石析晶的影响.此外,还概述了钙矾石重结晶与膨胀之间的关系.最后,提出了一些有待进一步研究的问题.     

硅酸盐水泥早期体积稳定性的调控研究

分别研究了利用硬石膏、粉煤灰制备的钙矾石型膨胀剂和硬石膏、石灰和粉煤灰三元组分制备的钙矾石-羟钙石复合膨胀刑对水泥体系膨胀性能的影响.利用XRD、TG-DSC-DTG和SEM等测试方法对水泥体系中的钙矾石和羟钙石进行了定性和定量分析,在微观上对钙矾石和羟钙石的形态进行了分析.结果表明,由硬石膏、粉煤灰制备的膨胀刺,膨胀主要发生在前7d,由硬石膏、石灰和粉煤灰制备的复合膨胀剂,膨胀效果较好,28d时仍有微膨胀.对硅酸盐水泥早期的体积稳定性进行调控,可以有效减小水泥体系早期开裂的几率.     

含石灰石粉水泥砂浆在低温环境中的硫酸盐侵蚀

分析了掺30%石灰石粉砂浆与纯水泥砂浆在(5±1)℃的2%MgSO4溶液中浸泡不同时间后试件表面层的矿物成分变化与微观结构,研究了强度与外观的变化.结果表明:掺入石灰石粉使水泥水化产物中的单硫型水化硫铝酸钙转变为稳定的单碳水化铝酸钙,物理填充作用使水泥石结构更加致密,因而在短期的低温硫酸盐侵蚀环境下掺石灰石粉砂浆比纯水泥砂浆表现出更好的耐腐蚀性.在经受低温硫酸盐侵蚀后纯水泥砂浆生成大量的石膏和钙矾石,而掺石灰石粉砂浆在经受同条件210 d侵蚀后的腐蚀产物中除了石膏、钙矾石外,还有少量的硅灰石膏生成,表明水泥石中的CSH凝胶体也受到侵蚀.     

混凝土中钙矾石的重结晶

钙矾石是水泥重要的水化产物之一，也是铝酸盐类膨胀剂产生膨胀的主要物质，但它在混凝土中并不稳定，容易发生重结晶。提出了混凝土中钙矾石重结晶的理论基础，并从钙矾石的溶解和析晶两方面探论了混凝土中钙矾石重结晶的影响因素，重点讨论了混凝土原料组分中碱和石膏对钙矾石析晶的影响。此外，还概述了钙矾石重结晶与膨胀之间的关系。最后，提出了一些有待进一步研究的问题。     

Effects of MgO-based expansive additive on compensating the shrinkage of cement paste under non-wet curing conditions

Expansive additives are widely used to compensate the drying shrinkage of cement-based materials to avoid cracking. However, the expansion of conventional ettringite-bearing expansive additive depends strongly on wet curing and is mainly generated at early age, and hence it may not work well in concretes without sufficient water supply or exhibit long-term shrinkage. MgO-based expansive additive, for which less water is needed for the formation of Mg(OH)₂ in comparison to ettringite, was prepared and its compensating effect on the autogenous shrinkage and late age thermal shrinkage of Portland and fly ash cement pastes at low water-to-cement ratio was investigated. The tests were conducted under sealed condition, so that the moisture exchange with the environment was prevented. Results show that, even under the non-wet curing condition, the shrinkages of cement pastes can be compensated effectively. Microstructure analysis by scanning electron microscope indicates that the macro-expansion of cement pastes is probably caused by the locally restrained expansion of MEA due to the hydration of MgO.     

A state-of-the-art review on delayed ettringite attack on concrete

The paper presents a critical review of the sulphate attack related to ettringite formation. This process is associated with expansion. However, not necessarily any ettringite-related expansion is related to the sulphate attack. For example, early ettringite formation which occurs immediately (within hours) in a plastic fresh mixture does not produce any damaging expansion and is associated with the regulation of setting time of Portland cement paste. Expansion after the hardening of cement paste can be advantageously used for the development of chemical prestress in expansive cements. Delayed ettringite formation (DEF) occurs at late ages and the related heterogeneous expansion in a hardened concrete can produce cracking and spalling. Two different types of DEF are examined depending on the sulphate source: DEF caused by external sulphate attack or internal sulphate attack.     

硫酸盐侵蚀下水泥净浆膨胀应变计算

针对硫酸盐侵蚀过程中水泥净浆体积膨胀问题,运用微孔力学理论,建立了水泥净浆基体与其孔隙内钙矾石晶体相互作用的代表性体积单元(RVE)及其力学分析模型;分析了一定钙矾石生成量下的RVE内膨胀应变在微观尺度上的空间分布规律;通过均匀化方法,将微观尺度上的RVE内膨胀应变转化为宏观尺度上RVE所在位置点的等效应变,分析了该等效应变随钙矾石生成量的变化规律。分析结果表明:微观尺度上,孔隙率为0.1的RVE内钙矾石晶体和侵蚀溶液组成的内球体各向为拉应变,水泥净浆外球壳径向为压应变、环向为拉应变;宏观尺度上,孔隙率为0.1的RVE所在位置点的径向等效应变为压应变,且随钙矾石生成量的增加而增大,而环向等效应变为拉应变,且随钙矾石的生成量的增加而增大。     

硫酸盐侵蚀下石膏的形成及破坏机制研究现状

硫酸盐侵蚀是影响水泥基材料耐久性的重要因素,它不仅会缩短材料的服役寿命,甚至可能危及结构安全。在硫酸盐侵蚀过程中,钙矾石、石膏和碳硫硅钙石等侵蚀产物不断形成,从而导致材料出现膨胀、开裂、软化和剥落等不同形式的破坏。由于不同侵蚀产物的形成条件和对水泥基材料的侵蚀机理存在明显差异,而侵蚀机理是工程实践中指导预防硫酸盐侵蚀的重要依据,因此探明不同侵蚀产物的形成及稳定条件以及各侵蚀产物对材料的作用机理成为该研究课题的主要内容。 从现有研究来看,钙矾石型硫酸盐侵蚀是目前研究最为成熟的一种硫酸盐侵蚀。钙矾石是在高碱性硫酸盐溶液条件下形成的主要侵蚀产物,并且当其在狭小封闭的孔洞中生长时会导致材料发生膨胀、开裂破坏,相应的膨胀机理有吸水肿胀理论、结晶压理论和固相反应理论等。另外,在钙矾石型硫酸盐侵蚀的预防方面,发现通过控制水泥中铝酸三钙含量可有效减小因钙矾石形成而造成的膨胀危害。近年来,世界各地的研究者竞相报道了碳硫硅钙石的形成对混凝土结构造成严重破坏的工程实例,这使得碳硫硅钙石型硫酸盐侵蚀也逐渐受到重视。目前普遍认为碳硫硅钙石的形成主要导致材料出现泥化和分解现象,但其形成条件较为复杂,只在一些特殊环境下才有可能发生。 石膏是水泥基材料在硫酸盐侵蚀下形成的另一种较为常见的腐蚀产物,它的形成同样影响着水泥基材料的耐久性能。研究发现,硫酸盐溶液浓度越高,越利于形成石膏,但后来发现溶液pH值对石膏的形成及稳定影响更为显著,同时溶液温度、离子种类以及腐蚀制度等对石膏的形成也有一定影响。由于石膏的化学组成相对简单且不含铝相,因此采用普通抗硫酸盐侵蚀方法并不能有效抑制石膏的形成及破坏。石膏的形成往往伴随着水化产物的溶解脱钙,从而导致材料出现软化和剥落现象,但在石膏的膨胀问题上仍存在较大争议。 本文综述了硫酸盐侵蚀下水泥混凝土中石膏形成的影响因素,总结了石膏的生长位置及其引起的脱钙作用,最后对石膏的膨胀作用进行了相关探讨。     

A hydration study of various calcium sulfoaluminate cements

The present work studies the hydration process and microstructural features of five calcium sulfoaluminate (CSA) cements and a ternary mixture including also ordinary Portland cement (OPC). The pastes were studied with simultaneous differential thermal-thermogravimetric (DTA-TG) analysis, mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM), and expansion/shrinkage tests. The DTA-TG analysis confirmed the role of the hydration reactions involving the main CSA clinker constituent, tetracalcium trialuminate sulfate, which produced (i) ettringite when combined with lime and calcium sulfate, (ii) ettringite and aluminum hydroxide in the presence of calcium sulfate alone, and (iii) monosulfate and aluminum hydroxide in the absence of both lime and calcium sulfate. The MIP and SEM were able to discriminate between expansive (ternary mixture and CSA cement containing 50% gypsum) and non-expansive cements. Expansive cement pastes had (i) a nearly unimodal pore size distribution shifted toward higher radii and (ii) ettringite crystals smaller in size during the first day of curing. In a SEM image of a hardened paste of the CSA cement containing 50% gypsum, a stellate ettringite cluster was observed.     

Effect of ettringite morphology on DEF-related expansion

In this study, time dependent ettringite formation in heat-cured mortars has been investigated. In order to clarify the effect of formation place and morphology of ettringite on expansion, secondary electron images of cracked surfaces of mortars at three ages were analysed by SEM–EDS. Also, the X-ray microtomography analysis has been performed to observe the crack formation. The expansive role of delayed formed ettringite was related with its time dependent morphology as a function of formation place. From these observations, mechanism of ettringite reformation after heat curing has been proposed. Alumina rich species were the primary sources of ettringite formation as the starting nuclei. At later ages, if S and Al sources are readily available, the mentioned alumina rich nuclei will grow up and build ball ettringite. At long term, ball type ettringites (non-expansive) converted to massive type (expansive). These conversions can only take places if the form of available space is narrow (preformed micro-cracks). Massive ettringites exert pressure in these narrow spaces and cause expansion of mortar. If the form of the available space is spherical (entrapped air voids) ball ettringites preserve their initial form and do not cause any expansion.     

膨胀土路堑基床新型防水层振动荷载下服役性能试验研究

膨胀土路堑基床病害是铁路工程中亟需解决的关键技术问题。针对新建云桂线膨胀土地段高速铁路工程实际,开展膨胀土路堑基床模型室内激振试验,研究新型防水结构层在干燥、降雨、地下水位上升三种极端服役条件下的动力性能及防水效果,并结合现场实测数据进行了分析。分析结果表明：服役环境对新型基床结构的动力特性影响显著,降雨和地下水位上升均引起基床动应力、速度及加速度不同程度的增加;新型防水结构层可加快基床内部动应力的衰减,动应力沿横向距离近似呈“Z”形分布,在轨道正下方出现峰值,距线路中线5.0m以外受振动影响较小;新型防水结构层能满足的防水、隔水、抗振、减振的要求,对提高铁路线路的平顺性和稳定性具有积极意义。研究成果可为新建云桂膨胀土高速铁路工程建设及同类工程实践提供参考。