Evaluation of time-dependent rheological properties of cemented paste backfill incorporating superplasticizer with special focus on thixotropy and static yield stress

Superplasticizers are widely used to reduce the pipe flow resistance of cemented paste backfill(CPB), which is characterised by high concentration and high yield stress. This study aimed to assess the time-dependent rheological properties of CPB containing superplasticizer, with special focus on static yield stress and thixotropy. The results indicate that with the increase of the superplasticizer dosage, the static yield stress, dynamic yield stress and thixotropy of CPB decreased significantly, while the plastic viscosity decreased slightly. The curing time has a significant effect on the static yield stress, dynamic yield stress and thixotropy of CPB containing superplasticizer, which increase by 46.6%-87.1%,15.2%-35.6% and 79.4%-138.2%, respectively, within 2 h. The static yield stress, dynamic yield stress and thixotropy of CPB without superplasticizer only increase by 4.9%, 6.3% and 16.1%, respectively, within 2 h. The curing time has a significant influence on the plastic viscosity of CPB regardless of superplasticizer addition, the plastic viscosity increases by 13.2%-19.7% within 2 h. Regardless of superplasticizer dosage, plotting of both static yield stress and dynamic yield stress versus thixotropy produces clearly linear curves. The findings of this study are conducive to the design of pipe transportation of CPB containing superplasticizer.

Effect of CO_(2) Curing on the Physical Properties of Recycled Coarse Aggregate with Different Attached Mortar Contents

The effects of carbon dioxide (CO_(2)) curing conditions (temperature,relative humidity and CO_(2) curing time) on the physical properties of recycled coarse aggregate (RCA) with varying attached mortar (AM) contents were studied.Before and after CO_(2) curing,the physical properties in terms of the apparent density,water absorption and crushing value of RCA were tested and the quality of RCA was determined.Besides,scanning electron microscope was used to observe the microstructure of RCA.Results show that the physical properties variation of RCA with higher AM content are more significant,and the quality of RCA with lower AM content is easier to be upgraded during CO_(2) curing.The physical properties of RCA with 40.8% AM content are earlier stable than that with no less than 44.5% AM content during CO_(2) curing.The optimal temperature and relative humidity are 50 ℃ and 55% for CO_(2) curing,respectively.CO_(2) curing is incapable of upgrading the quality of RCA with AM no less than 50.6%.The quality of RCA with 44.5% AM content can be upgraded only under the optimum CO_(2) curing conditions.Under relative humidity higher than 40% and the CO_(2) curing time more than 12 h,CO_(2) curing upgrades the quality of RCA with 40.8% AM content.

Influence of Selected Curing Techniques on Compressive Strength of Concrete From Palm Kernel Shell Ash and Ordinary Portland Cement

This paper discusses the findings of an experimental study on the effect of various curing procedures on the compressive strength of concrete pro­duced by partially substituting portland cement with Palm Kernel Shell Ash(PKSA).Palm kernel shell ash was utilized in a 1:2:4 mix ratio as a partial substitute for ordinary Portland cement(OPC)at percentage levels of 0%,10%,and 15%.River sand with particles passing a 4.75 mm BS sieve was used,as well as crushed aggregate with a maximum size of 20 mm,and palm kernel shell ash with particles passing a 212μm sieve.The compressive strength of the test cubes(150 mm × 150 mm × 150 mm)was determined after 7,28,and 56 days of curing.The results demonstrated that test cubes containing Palm kernel shell ash developed strength over a longer curing period than ordinary Portland cement concrete samples and that the strength changes depending on the amount of PKSA in the cube samples.The findings showed that at 28 days,test cubes with 5%,10%,and 15%PKSA content in all curing procedures utilized obtained a greater compressive strength.Curing by immersion produced the highest compres­sive strength in all replacement level while the concrete cured by sprinkling and spraying gives a lower strength in all replacement level.

Key parameters controlling electrical resistivity and strength of cement treated soils

The improvement of question soils with cement shows great technical, economic and environmental advantages. And interest in introducing electrical resistivity measurement to assess the quality of cement treated soils has increased markedly recently due to its economical, non-destructive, and relatively non-invasive advantages. This work aims to quantify the effect of cement content （aw）, porosity （nt）, and curing time（T） on the electrical resistivity （p） and unconfined compression strength （UCS） of cement treated soil. A series of electrical resistivity tests and UCS tests of cement treated soil specimen after various curing periods were carried out. A modified Archie empirical law was proposed taking into account the effect of cement content and curing period on the electrical resistivity of cement treated soil. The results show that nt/（aw·T） and nt/（aw·T^1/2） ratio are appropriate parameters to assess electrical resistivity and UCS of cement treated soil, respectively. Finally, the relationship between UCS and electrical resistivity was also established.

The Mechanical Properties of Coastal Soil Treated with Cement

The influences of cement type, cement content, and curing time on the unconfined compression strength （UCS） of soil-cement were investigated. The influence of groundwater on UCS of soil- cement was also studied. The experimental results indicate that the soil treated with high grade cement presents a higher UCS. Additionally, the UCS of soil-cement presents linearly increased with the cement content. A logarithm correlation between UCS and curing time presents to forecast the strength development. Compared with the UCS of samples immersed in distilled water, those immersed in groundwater oresent a hizher value.

Potentials of Balanite Endocarp Pod Ash as a Cement Replacement Material

The exponential growth of agricultural wastes such as Balanite seed pod has resulted in waste management issues and finding alternatives through waste recycling is an interesting area of research.Balanite seed pods were collected in Yobe state,washed,dried,crushed.Balanite endocarp pod ash(BEPA)was calcined at 600°C for 3 hours in a muffle furnace and the biomass was characterized with X-ray fluorescence spectrometer to determine its chemical composition.The analysis showed that the ash composed of silica,alumina and ferric oxide(74.24 wt.%)indicated a strong pozzolan based on American Standard.The BEPA possessed a high loss on ignition of 8.24 wt.%owing to the presence of unburnt carbon in the ash.The ash obtained was sieved with 75µm sieve and cement was replaced with ash between 2.5 wt.%~12.5 wt.%at interval of 2.5 wt.%to assess its impact on cement properties such as setting time,consistence,mortar strength and water absorption.Results showed that as percentage replacement increases,the water absorption increased while the mortar strength diminished whereas as the curing days progressed its strength improved despite clinker diminution due to cement hydration.60 days strength for below 10 wt.%BEPA cement blend produced enhanced strength compared with control which is evidence of slow pozzolanic reactions.The optimum percentage replacement with BEPA was obtained at 7.5 wt.%replacement beyond which significantly affected the cement properties especially its strength.It could be agreed that BEPA has potentials to be considered and employed as a cement replacement material.

The Pore Structure of Phosphoaluminate Cement

The effects of curing time and water-to-cement ratio (W/C) on the pore structure of phosphoaluminate cement (PAC) paste are here presented. Based on the adsorption and condensation theory, the adsorption isotherm of hardened paste was tested using the Brunauer-Emmett-Teller (BET) nitrogen adsorption method. The phase composition and morphology of hydration products cured at different times were analyzed using X-ray diffraction (XRD), and a hydration heat test instrument (HHT) was employed to determine the heat of hydration. The effects of curing time and W/C on the pore structure of PAC are significant. The adsorption isotherm is fitted to the second category based on the Brun-auer- Deming-Deming-Teller (BDDT) classification system. Adsorption volume was found to increase with W/C and then decrease with age. The hysteresis loop of PAC is fitted to the H3 type based on International Union of Pure and Applied Chemistry (IUPAC) guidelines, and the adsorption volume and area enclosed by the hysteresis loop were found to increase with W/C and then decrease with age. BET surface and saturated adsorbed volume of PAC both increase with W/C and decrease with curing time, which is attributable to the greater hydration that produces and changes the characteristics of the pore structure.

水泥固化土固化机理及抗冲刷特性

为了完善水泥固化土抗冲刷性能方面的研究,使水泥固化土在水下结构防护中得到应用,在砂土和黏土中分别掺入水泥和与液体固化剂,采用土体冲刷函数测定仪(EFA)对水泥固化土开展了抗冲刷性能试验。在分析砂土和黏土的固化作用机理基础上,研究了固化剂和固化时间对水泥固化土抗冲刷性能的影响。研究结果表明:水泥固化土的强度随固化时间的增长而增大,黏土的强度快速增长阶段要早于砂土;固化剂可以有效提高土体的起动切应力,同时可以大幅减小土体的冲刷速率,明显提高了土体的抗冲刷强度;起动切应力的增长速率随固化时间增长逐渐减缓,存在临界最大值。根据起动切应力的变化特征提出了水泥固化土起动切应力预测模型,经试验数据对比,该模型可以较好地预测水泥固化土的起动切应力随固化时间的变化规律。

基于无侧限抗压强度试验的不同养护环境对水泥土强度影响研究

以福建省宁德市连城路(疏港路-学院路段)道路工程项目为依托,采用无侧限抗压强度试验系统研究了不同养护条件、不同水泥掺量、不同养护龄期对水泥土强度的影响。结果表明:当水泥掺量和养护龄期均相同时,试验箱中水泥土的无侧限抗压强度均小于标养箱;在同一水泥掺量下,试验箱和标养箱中水泥土的无侧限抗压强度均与养护龄期成正相关,即随着养护龄期的增加而增长,但其强度增长幅度随着养护龄期的增加逐渐降低;同一养护龄期下,不同养护条件下水泥土的无侧限抗压强度均随着水泥掺量的增加而增长,且试验箱和标养箱中试样的强度差值随着水泥掺量的增加逐渐加快。

光固化工艺参数对氮化硅陶瓷成型性能的影响

光固化工艺参数是影响光固化成型质量的重要因素。本文重点讨论了光固化工艺参数中的曝光时间、脱脂参数对光固化Si3N4陶瓷成型性能的影响。实验表明:当固相含量为45vol.%、曝光时间为10 s时,成型出单层固化精度为IT7级的氮化硅坯体。低温脱脂时,当脱脂速率为0.1℃/min时,获得无裂纹的试样坯体。

考虑渗流阈值时早龄期混凝土水渗透系数的试验研究

水渗透系数是评价混凝土耐久性的重要指标,为了明确各试验变量对混凝土水渗透系数的影响,通过稳定渗流法系统探究了水灰比、骨料含量和龄期对水渗透系数的影响。研究发现:随着骨料含量的增加,混凝土水渗透系数逐渐减小,一旦骨料含量超过50%,在渗流阈值的影响下渗透系数开始出现增大的趋势;当水灰比在0.3~0.5变化时,混凝土水渗透系数会随着水灰比的增大而显著增加;在养护龄期28 d内,混凝土水渗透系数随龄期的增长而显著减小。

水泥改良土抗剪强度试验研究

为获得濒水环境下水泥改良土抗剪强度设计参数,采用室内试验方法,研究水泥与土的质量比、压实度、含水量及养护龄期对水泥改良土抗剪强度的影响。结果表明:含水量、压实度、养护龄期相同时,水泥改良土的黏聚力、内摩擦角及抗剪强度随水泥与土的质量比的增大而增大;水泥与土的质量比、含水量、养护龄期相同时,随压实度的增大,水泥改良土的黏聚力、内摩擦角及抗剪强度线性增大,水泥与土的质量比为8.0%时,黏聚力平均增大3.4%,水泥与土的质量比为10%时,黏聚力平均增大2.1%;水泥与土的质量比、压实度和养护龄期相同时,水泥改良土的黏聚力、内摩擦角及抗剪强度随含水量的增大而减小,黏聚力变化较小,内摩擦角、抗剪强度变化较大;水泥与土的质量比、压实度和含水量相同时,水泥改良土的黏聚力、内摩擦角及抗剪强度随养护龄期的增大而增大,但增速放缓,养护28 d后内摩擦角基本不变,抗剪强度因黏聚力继续增大而增大。

高含水量水泥土工程特性试验研究

为研究高含水量水泥土工程特性,对不同配比的高含水量水泥土试块进行了无侧限抗压强度试验以及直剪试验。结果表明:可以采用水泥土含水量与水泥掺入量的比值作为评价水泥土性质的参数,且采用幂函数能够较好地表述水泥土强度和含水量与水泥掺入量比值的关系。将龄期为28 d时的水泥土强度作为基准,利用对数函数估算不同龄期下的水泥土强度值。本次试验观测到水泥土的变形模量约为强度的86.5倍。水泥土的内摩擦角随着强度的增加而增大,本次试验中水泥土的无侧限抗压强度从85.4 kPa增加到1811 kPa时,对应的内摩擦角从30.4°增加到45.0°。水泥土的黏聚力随强度的增加呈线性增大,本次试验中黏聚力约为强度的0.181倍。

废旧沥青再生混合料水泥稳定基层特性研究

对老旧公路改建过程中产生的废旧沥青混合料进行回收处理,并作为新修公路路面水泥稳定基层的填料,不仅节约了工程成本,还降低了环境污染。因此,通过室内试验,对不同工况下的废旧沥青混合料水泥稳定基层的力学特性进行了研究。研究结果表明:(1)再生沥青水泥稳定混合料的最优水泥掺量为5%,当沥青再生结合料含量在10%~20%之间时,抗压强度和劈裂强度出现峰值;(2)随着养护时间的增加,混合料的强度先增加、后趋于稳定,在养护前28 d内,混合料的无侧限抗压强度基本形成。

有机质含量对水泥土强度影响的室内定量研究

采用对盐城泻湖相软土掺入不同含量的有机质作为试料土,进行了大量的水泥加固室内试验。对试验结果进行了分析,提出了不同有机质含量软土水泥加固后的室内无侧限抗压强度的预测公式,同时建立了不同有机质含量的试料土经水泥加固后的室内强度比与有机质含量之间的定量关系,该定量关系式可以有效地对有机质含量在5.5%～17.5%的水泥加固后的无侧限抗压强度进行定量的预测。

污水环境对水泥土渗透性能影响的试验研究

现多数水泥土防渗工程都间接或直接的与地下腐蚀性介质环境接触,会导致水泥土防渗性能的降低甚至失效。因此,通过室内模拟试验,设计了不同的水泥掺入比,进行了在污水和清水环境下水泥土的渗透性能试验,研究了在不同龄期下水泥土的渗透系数变化规律,并同时分析了渗透后水泥土中离子浓度的变化规律。试验结果表明,在清水环境下,随着龄期的增长,水泥土的渗透系数逐步减小,60 d后随着龄期的增加,水泥土渗透系数减小的程度越来越缓慢;在污水养护条件下,龄期在60 d之前,水泥土渗透系数逐步减小,龄期在60 d之后,由于水泥土被侵蚀,致使水泥土随着龄期的增加,水泥土的渗透系数逐步增大;无论是清水还是污水环境,随着水泥掺量的增大,其渗透系数均逐步降低;在污水环境中,随着龄期的增加,其水泥土中的Ca2+,Mg2+,Cl?,24SO?浓度均逐步增大,而清水环境中Mg2+,Cl?,24SO?浓度均逐步减小。其研究成果可为水泥土防渗工程的抗渗性和耐久性设计和应用提供技术参数,具有重要的工程意义。

膨胀土水泥改性试验研究

膨胀土是一种特殊的黏性土,具有遇水膨胀、崩解、软化,失水收缩、干裂、硬化等工程特性,膨胀土掺入水泥后其工程特性会发生改良。选取南水北调中线工程南阳段中膨胀土,进行水泥改性研究。通过改性后的自由膨胀率试验,确定水泥的最佳掺量;通过胀缩特性试验,研究水泥改性对降低膨胀潜势的改良效果;并通过无侧限抗压强度、压缩等力学性质试验,研究水泥改性对抑制膨胀土强度软化、模量减小的效果。为膨胀土现场水泥改性的设计施工提供科学的、有价值的依据。

地下工程动水注浆速凝浆液黏度时变特性研究

注浆法是地下水害治理的主要方法,而浆液的黏度时变特性是注浆治理的关键因素,因此浆液黏度时变特性研究十分必要。针对地下工程动水注浆中常用的2种速凝浆液,水泥-水玻璃浆液和高聚物改性水泥浆液,开展室内试验并对试验数据进行拟合分析。采用振动式黏度计测试其黏度时变性和反应温度变化,分析2种材料在不同水灰比和不同混合体积比下的黏度和温度时变特性,根据黏度时变特性将浆液反应过程划分为3个阶段,并分析不同阶段的黏度特性,对数据进行分析得到不同水泥浆水灰比和不同浆液混合体积比下的黏度时变方程,在此基础上得出速凝注浆材料的适用性。

乳化沥青冷再生混合料高温稳定性研究

采用50℃车辙试验,分析了温度、基质沥青针入度大小、乳化沥青用量、水泥掺量、含水率和养生时间对乳化沥青冷再生混合料高温稳定性的影响。结果表明:乳化沥青冷再生混合料在后期强度形成后具有较好的高温稳定性,且混合料高温稳定性受温度的影响没有热拌沥青混合料敏感;采用低标号沥青或增加水泥用量均能提高冷再生混合料的高温稳定性,但二者的变化对冷再生混合料高温稳定性影响不大;随着养生时间增加乳化沥青冷再生混合料的抗变形能力增强且早期含水率变化对抗变形能力影响显著,而后期含水率变化对高温稳定性影响不大。

水泥改良桂林红粘土的试验研究

为了研究水泥掺量和养护时间对桂林红粘土的改良效果,通过直剪试验、液塑限联合测定法等室内定量试验进行分析。经试验测定,在相同水泥掺量时,桂林红粘土粘聚力增长呈先快后慢势态。在相同养护龄期时,内摩擦角随水泥掺量增加而增大,而粘聚力增大则更明显。液塑限和界限含水率提高。究其原因,掺入水泥并养护使得桂林红粘土与水泥发生凝结核硬化,构成团粒、镶嵌、胶结的空间结构,表现出较高的假粉、砂性特征。