超细矿渣粉和偏高岭土对硫铝酸盐水泥水化和强度的影响

掺入矿物掺合料是改善硫铝酸盐水泥(CSA)混凝土凝结硬化性能和降低生产成本的主要技术途径之一。研究了水胶比为0.4时,单掺超细矿渣粉(UFS)、偏高岭土(MK)与复掺超细矿渣粉、偏高岭土对硫铝酸盐水泥凝结时间、流动度、电阻率、抗压强度的影响,并对其1 d、28 d龄期时的水化产物进行XRD半定量分析。结果表明,单掺和复掺缩短了水泥浆体的凝结时间,但单掺偏高岭土时的缩短效果更明显,且水泥浆体的流动度随着超细矿渣粉和偏高岭土掺量的增加而减小。掺入超细矿渣粉、偏高岭土缩短了水泥浆体电阻率变化速率曲线峰值出现的时间,峰值大小与掺量成递减关系。当掺量从0%(质量分数,下同)增大到20%时,单掺超细矿渣粉试样的28 d抗压强度减小了24.7%,单掺偏高岭土试样的28 d抗压强度减小了17.7%,两者复掺试样的28 d抗压强度减小了17.3%。超细矿渣粉和偏高岭土对水泥水化产物没有明显影响,但促进了硅酸二钙(β-C₂S)的早期水化。     

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.     

水泥浆体早期收缩的试验研究

开发了一套采用高精度LVDT位移传感器测定水泥浆体长度变化的仪器,并用该仪器对不同水灰比和不同混合材的水泥浆体的收缩行为进行了研究。结果表明,该仪器能够准确、连续地测量水泥浆体终凝后的自收缩和干燥收缩的发展过程。早期自收缩和干燥收缩随着水灰比降低而明显增加,在20℃相对湿度50%的环境下,水泥浆体的自收缩占干燥收缩的10%左右;掺粉煤灰可以少量降低水泥浆体的自收缩和干燥收缩,而掺入石灰石和矿渣则不同程度的增大了水泥浆体早期收缩率。     

矿物超细粉对水泥浆体化学收缩的影响研究

通过笔者自行设计的化学收缩测量装置分别对掺硅灰、磨细矿渣、超细粉煤灰的水泥浆体化学收缩进行了测定。结果表明，硅灰的掺入增大了水泥浆的化学收缩；磨细矿渣的掺入使水泥浆体的化学收缩稍有增长，但不十分明显；而超细粉煤灰则可以较好地抑制水泥浆体的化学收缩。对掺矿物掺合料的水泥浆体强度的测试结果从另一个角度论证了化学收缩是由水化引起的，即水化程度的大小反映了化学收缩的大小。     

沸石粉对硫铝酸盐水泥水化行为的影响机理研究

研究了沸石粉对硫铝酸盐水泥浆体流动度、凝结时间和抗压强度的影响规律,并通过自收缩、电阻率和XRD测试分析了沸石粉在硫铝酸盐水泥水化行为中的作用机理。结果表明,掺入沸石粉后水泥浆体的流动度明显降低,凝结时间显著延长,且延长时间随掺量的增大呈先增大后减小的趋势。当沸石粉掺量为5%~15%(质量分数)时,硬化水泥浆体的1 d、3 d、7 d抗压强度均有显著提高;沸石粉掺量为10%时,水泥浆体3 d、7 d、28 d的强度增长幅度最大,和空白组相比,分别增长了21.6%、13.9%和5.4%。掺入沸石粉后水泥浆体的24 h电阻率显著增大,硬化浆体的7 d自收缩减小,且在相同龄期时,硬化浆体的自收缩随沸石粉掺量的增大而减小。XRD分析显示沸石粉的掺入能有效促进硫铝酸盐水泥的水化,有利于1 d、3 d和28 d龄期内钙矾石的形成。     

粉煤灰对硫铝酸盐水泥水化历程的影响

研究了粉煤灰(FA)及其掺量对硫铝酸盐水泥(CSA)浆体的凝结时间、抗压强度和化学收缩的影响规律,并通过XRD、SEM等方法对72 h龄期时的水化产物进行分析.结果表明,粉煤灰缩短了硫铝酸盐水泥的凝结时间,当粉煤灰掺量为40％时,初凝时间和终凝时间分别缩短了76 min和94 min;掺入粉煤灰使得硫铝酸盐水泥的抗压强度降低,但在28 d龄期时,粉煤灰掺量为20％的硫铝酸盐水泥复合浆体的抗压强度仅略微降低;在硫铝酸盐水泥体系中掺入粉煤灰时,其浆体化学收缩随着粉煤灰掺量的增加逐渐减小,当粉煤灰掺量为20％和40％时,72 h龄期时的化学收缩值分别为0.138 mL/g和0.088 mL/g,较未掺粉煤灰时分别降低了26％和49％;微观分析表明,掺入粉煤灰后,72 h龄期时的水化产物主要是钙矾石和水化硅酸钙凝胶,并未发现氢氧化钙晶体.     

公路用石灰石粉水泥基材料的性能研究

为促进石灰石粉在公路水泥基材料中的应用,测试了掺0～30%废弃石灰石粉水泥基材料的凝结时间、流变性能、强度和干燥收缩,并分析了石灰石粉的影响机制。结果表明：掺入石灰石粉能延缓水泥浆体的凝结时间,减少水泥浆体的剪切应力和黏度,提高水泥浆体的流动度,但会增加水泥浆体的流动度损失率。掺入10%以内的石灰石粉可以显著提高水泥砂浆的强度,但掺入超过10%石灰石粉会降低水泥砂浆的强度。而且,石灰石粉的掺入会明显增大水泥砂浆的干燥收缩。     

复掺偏高岭土-矿渣对轻骨料混凝土性能的影响

轻骨料混凝土具有收缩较大和渗透性较差的特性,矿物掺合料的加入可以改善轻骨料混凝土的性能.将偏高岭土(MK)和矿渣(S)以低于25％的总掺量复掺到轻骨料混凝土中,研究复掺掺合料及不同掺量对轻骨料混凝土的力学性能、氯离子扩散系数和干燥收缩的影响.利用XRD、TG、SEM等测试方法,对其影响机理进行探讨.结果表明,复掺10wt％偏高岭土与10wt％矿渣对轻骨料混凝土力学性能和抗氯离子渗透性能有显著的改善;复掺10wt％偏高岭土和15wt％偏高岭土能有效降低轻骨料混凝土干燥收缩.偏高岭土复合矿渣能优化水泥浆体的组成与结构,增强骨料与水泥浆体间的结合,降低界面过渡区的钙硅质量比,从而提高轻骨料混凝土各龄期抗压强度的抗渗性能.     

粉煤灰部分取代石灰石粉的水泥胶砂干缩性能研究

通过测定水泥胶砂不同龄期干缩率,分别研究了石灰石粉、粉煤灰单掺以及石灰石粉和粉煤灰双掺时水泥胶砂干燥收缩性能的影响。结果表明：随着石灰石粉掺量增加,胶砂干燥收缩先增大后减小,石灰石粉掺量为10％左右时胶砂干燥收缩最大;胶砂干燥收缩随粉煤灰掺量增加先减小后增大,粉煤灰掺量10％左右时,胶砂干燥收缩最小;石灰石粉掺量一定时掺入粉煤灰可以减小胶砂干燥收缩。     

磷建筑石膏对超硫酸盐水泥水化的影响

本文采用不同掺量的磷建筑石膏(CPG)、粒化高炉矿渣和熟石灰制备超硫酸盐水泥(SSC),通过测试水泥浆体的水化热、电阻率、化学收缩、水化产物、孔溶液pH值和抗压强度的变化规律,研究了CPG掺量对SSC水化性能的影响规律。结果表明：当CPG掺量从0%(质量分数,下同)增大到20%时,水泥浆体的第三放热峰出现时间延迟,3 d放热量与14 d化学收缩均增大,3 d电阻率减小,28 d孔溶液pH值从11.95减小到10.80;掺入CPG会促进钙矾石的生成;当CPG掺量为10%时,试件的28 d抗压强度最大,达到23.8 MPa。     

Autogenous relative humidity change and autogenous shrinkage of high-performance cement pastes

In this paper, the effects of water to cementitious material ratio (w/cm), silica fume (SF) and ground blast-furnace slag (GBFS) on autogenous relative humidity (RH) change and autogenous shrinkage (AS) of high-performance cement pastes were studied. The mechanism of self-desiccation caused by mineral admixture and reduction of w/cm were studied by the parameters of mineral admixture self-desiccation-effect coefficient k and efficient w/cm r_e proposed. Furthermore, the relationship between autogenous RH and AS of high-performance paste was established. The results indicate that w/cm is a chief factor that affects autogenous RH change and AS of cement pastes. The lower the w/cm of paste is, the more reduction the autogenous RH and the increment of AS are. SF increases autogenous RH reduction and AS increment of cement paste at early ages, and GBFS increases autogenous RH reduction and AS increment at later ages. The effect of mineral admixtures on autogenous RH change of paste resulting from self-desiccation can be reflected effectively by the nonlinear equation with the parameters of k and r_e. There exists a good linear correlation between autogenous RH change and AS of cement pastes.     

Effect of water-to-cementitious materials ratio and silica fume on the autogenous shrinkage of concrete

This paper presents an experimental study on the autogenous shrinkage of Portland cement concrete (OPC) and concrete incorporating silica fume (SF). The results were compared with that of the total shrinkage (including drying shrinkage and part of the autogenous shrinkage) of the concrete specimens dried in 65% relative humidity after an initial moist curing of 7 days. The water-to-cementitious materials (w/c) ratio of the concrete studied was in the range of 0.26 to 0.35 and the SF content was in the range of 0% to 10% by weight of cement.The results confirmed that the autogenous shrinkage increased with decreasing w/c ratio, and with increasing SF content. The results showed that the autogenous shrinkage strains of the concrete with low w/c ratio and SF developed rapidly even at early ages. At the w/c ratio of 0.26, the autogenous shrinkage strains of the SF concrete were more than 100 micro strains at 2 days. For all the concretes studied, 60% or more of the autogenous shrinkage strain up to 98 days occurred in the first 2 weeks after concrete casting. The results indicated that most of the total shrinkage of the concrete specimens with very low w/c ratio and SF exposed to 65% relative humidity after an initial moist curing of 7 days did not seem to be due to the drying shrinkage but due to the autogenous shrinkage.     

低品位粉煤灰的改性及其对水泥浆体强度和自收缩的影响

采用石灰石粉对低品位粉煤灰进行煅烧改性,利用X射线衍射、扫描电镜和能谱分析等方法对改性粉煤灰的矿物组成和化学组成进行表征.同时测定了掺改性粉煤灰的水泥浆体的抗压强度和自收缩,并采用背散射扫描电镜和压汞测孔仪研究了掺改性粉煤灰水泥浆体的微观结构.结果表明,粉煤灰经煅烧改性生成了水硬性矿物β-C2S,水化可生成CSH凝胶,改善了等外粉煤灰颗粒与水泥基体的界面粘接,降低了复合水泥浆体的孔隙率和自收缩,提高了复合水泥浆体的强度.     

Prediction of self-desiccation in low water-to-cement ratio pastes based on pore structure evolution

High-performance concrete is sensitive to early-age cracking, mainly due to its rapidly-developing autogenous shrinkage. Autogenous shrinkage and internal relative humidity (RH) decrease are direct consequences of the emptying of capillary pores due to cement hydration in low water-to-cement ratio concretes. To predict early-age cracking, it is desirable to model the evolution of the internal RH, and the accompanying autogenous shrinkage, based on the microstructure evolution.In this paper, the pore size distribution of cement pastes measured by mercury intrusion porosimetry and the chemical shrinkage are used as input data for calculating the internal RH of Portland cement pastes with different water-to-cement ratios (0.30, 0.35 and 0.40). The composition of the extracted pore solution is also taken into account. The measured RH can be predicted with good accuracy (mostly within 1–2% RH). However, a number of sources of error both in the experiments and in the analysis need to be identified and controlled.     

Influence of Cement Particle-Size Distribution on Early Age Autogenous Strains and Stresses in Cement-Based Materials

The influence of cement particle-size distribution on autogenous strains and stresses in cement pastes of identical water-to-cement ratios is examined for cement powders of four different finenesses. Experimental measurements include chemical shrinkage, to quantify degree of hydration; internal relative humidity development; autogenous deformation; and eigenstress development, using a novel embedded spherical stress sensor. Because the latter three measurements are conducted under sealed conditions, whereas chemical-shrinkage measurements are made under "saturated" conditions, the National Institute of Standards and Technology cement hydration and microstructure development model is used to separate the effects of differences in hydration rates (kinetics) from those caused by the different initial spatial arrangement of the cement particles. The initial arrangement of the cement particles controls the initial pore-size distribution of the cement paste, which, in turn, regulates the magnitude of the induced autogenous shrinkage stresses produced by the water/air menisci in the air-filled pores formed throughout the hydration process. The experimental results indicate that a small autogenous expansion (probably the result of ettringite formation), as opposed to a shrinkage, may be produced and early age cracking possibly avoided through the use of coarser cements.     

Autogenous and drying shrinkage of alkali-activated slag mortars

Shrinkage of alkali-activated slag (AAS) cement is a critical issue for its industrial application. This study investigated the mechanisms and effectiveness of shrinkage-reducing agent (SRA) and magnesia expansive agent on reducing autogenous and drying shrinkage of AAS mortars that were activated by liquid sodium silicate (LSS) solution with modulus (SiO₂/Na₂O molar ratio) of 0-1.5. The results showed that the autogenous shrinkage of AAS mortars increased with the increase of LSS modulus from 0 to 0.5, then decreased as modulus increased up to 1.5. The drying shrinkage consistently increased with the increase in the modulus of LSS. The oxyalkylene alcohol-based SRA could significantly reduce the autogenous and drying shrinkage of AAS mortars while the magnesia expensive agent was comparatively less effective. The autogenous shrinkage of AAS mortars was inversely proportional to the internal relative humidity, while the drying shrinkage was more related to the mass loss of samples. Mathematical models were established to describe the autogenous and drying shrinkage behavior of AAS mortars.     

The shrinkage of alkali activated fly ash

In this study, the autogenous and drying shrinkage of alkali activated fly ash (AAFA) pastes prepared with different contents of sodium silicate solution are reported. A higher amount of both Na₂O and SiO₂ resulted in a larger autogenous and drying shrinkage. Although a large autogenous shrinkage was obtained during the first 1–3 days, cracking was not observed in the ellipse ring tests. In AAFA pastes, water was not a reactant, but mainly acted as a medium. The experiment results indicate that the autogenous shrinkage of AAFA is not caused by the well-known self-desiccation process that happened in cement paste, but related to the continuous reorganization and polymerization of the aluminosilicate gel structure. AAFA pastes with a larger drying shrinkage exhibited a higher weight loss. The different microstructures lead to the different drying shrinkage of these AAFA mixtures.