Strength and self-desiccation of slag-cemented paste backfill at early ages: Link to initial sulphate concentration

This paper presents the results of an experimental study on the early age strength and changes in the self-desiccation process of cemented paste backfill (CPB) that contains blast furnace Slag (Slag-CPB) and sulphate. Slag-CPB samples with different initial sulphate concentrations (0, 5000, 15,000 and 25,000 ppm) are prepared and cured in well-sealed molds at early ages (1, 3, 7 and 28 days) in room temperature (∼23 °C). Mechanical and hydraulic conductivity tests are performed on the samples, in which the suction and electrical conductivity of the Slag-CPB specimens are monitored. Furthermore, microstructural analyses are conducted on the Slag-CPB and slag-cement paste samples. The results show that sulphate can have a positive or negative effect on the early age strength and self-desiccation of Slag-CPB, i.e., cause an increase or decrease in strength and acceleration or reduction in the amount and rate of self-desiccation, depending on the initial sulphate content and curing time. This study also shows that an understanding of the positive or negative effect of sulphate on the early strength and self-desiccation of Slag-CPB is critical for designing cost-effective CPB structures with high early strength, speeding up mining cycles or assessing the deterioration of CPB caused by high sulphate attack to ensure safety in mining.     

Initial temperature-dependence of strength development and self-desiccation in cemented paste backfill that contains sodium silicate

Advanced knowledge of the effect of the initial temperatures of cemented paste backfill (CPB) on its strength development and self-desiccation ability is needed to provide a rational basis for mixture proportioning as well as the cost-effective design of CPB structures and speeding up of the mining cycle. An experimental testing and monitoring program has been undertaken to determine the influence of various initial temperatures (2 °C, 20 °C, 35 °C and 50 °C) on the strength development and evolution of the self-desiccation (measured by the volumetric water content and suction) of the CPB that contains sodium silicate as the admixture (S-CPB) at early ages (up 28 days). The evolution of the temperature, electrical conductivity (gives information about the cement hydration progress), volumetric water content and suction of S-CPB samples with the specified initial temperatures have been monitored for 28 days. Moreover, mechanical tests and microstructural analyses are performed on these S-CPB specimens after specific curing times (6 h, 1 day, 3 days, 7 days and 28 days). The results obtained show that the initial backfill temperature has a significant influence on the curing temperature of S-CPB, its strength development and self-desiccation. Furthermore, the results reveal that the temperature-time history, cement hydration, self-desiccation and strength development of S-CPB are strongly coupled. The initial S-CPB and these couplings should be taken into account for a safe and economical design of S-CPB structures and the speeding up of mining cycles.     

Experimental investigation into the compressive strength development of cemented paste backfill containing Nano-silica

This paper investigates the influence of Nano-silica (NS) addition on the consistency and compressive strength development of cemented paste backfill (CPB). Tetraethyl-Orthosilicate (TEOS) was used as the precursor of Nano-silica along with ether-based Polycarboxylate superplasticizer (PCS). Two binder types (Portland cement and Slag-cement) and different amounts of TEOS (0.7–14% by mass of binder) with and without PCS are examined for 3, 7, 14, and 28 days curing time. Uniaxial compression tests for unconfined compressive strength (UCS) determination, slump height measurement, changes in gravimetric water content, and differential thermogravimetric analysis (DTG) were used to assess the influence of NS and admixtures (TEOS-PCS) on CPB performance. The results of this experimental study indicate that the addition of approximately 5% TEOS along with 0.5% PCS (by mass of binder) provide the best compressive strengths that can also be anticipated through the higher amount of calcium silicate hydrate (CSH) on DTG curves. It is also noticeable that the positive influence of NS is more evident when the amount of binder was decreased. The addition of PCS to CPB containing NS improved both the consistency of the mixture and the compressive strength development of CPB.     

Coupled thermo-hydro-chemical effect on rheological behavior of fresh cemented tailings backfill

Cemented tailings backfill (CTB) is a mixture of tailings, binder and water. The freshly prepared CTB slurry is commonly transported into underground mined-out areas via pipeline. The flowability of fresh CTB slurry is significantly affected by the coupled thermal (T), hydraulic (H), rheological (R) and chemical (C) processes. Therefore, this paper develops a THRC coupling model, which considers the evolutions of thermal conduction, fluid flow, rheology, and binder hydration to predict the fluidity of fresh CTB slurry. Rheological testing experiments are conducted to verify the availability of the developed model. The validated model is used to further investigate the effect of time and temperature factors on the hydraulic behavior of the hydrating CTB slurry. The obtained outcomes can make contributions to a better regulation of the time and temperature factors in the preparation and transportation of fresh CTB slurry.     

Effect of aggregate size distribution and confining pressure on mechanical property and microstructure of cemented gangue backfill materials

The use of gangue, cementitious materials, and water mixed to make cemented gangue backfill material (CGBM) can achieve solid waste recycling while reducing environmental problems caused by its accumulation. In this paper, the fractal dimension of particle size distribution (PSD) and the confining pressure were investigated on the compressive strength, elastic modulus, stress–strain behavior, dilatancy deformation, and failure mode of CGBM using uniaxial compression, conventional triaxial compression, and microscopic scanning tests. The mechanism of the PSD fractal dimension on the mechanical properties of CGBM was revealed from a microscopic perspective. The results demonstrate that the compressive strength and elastic modulus of CGBM are quadratic polynomial and positively linearly related to the PSD fractal dimension and confining pressure respectively, with the PSD fractal dimension characterizing the maximum compressive strength of CGBM ranging between 2.4150 and 2.6084. The volume strain variation of CGBM diminishes as the PSD fractal dimension grows and increases dramatically when the confining pressure rises. The PSD fractal dimension has a quadratic polynomial relationship with both the cohesive force and the internal friction angle of CGBM. A reasonable PSD fractal dimension can optimize the microstructure of CGBM, reduce the distribution of defects such as microcracks and micropores, and enable hydration products to effectively fill the defects, guaranteeing that the CGBM has sufficient load-bearing capacity.     

Damage mechanism of composite cemented backfill based on complex defects influence

In order to reduce mining waste discharge and realize efficient application of tailing material in underground backfilling, the characteristics and mechanism of backfill damage is analyzed, and also the composed behavior and failure characteristic of cemented backfill. Five types of backfills were tested under uniaxial compression loading in SHT4206 electro‐hydraulic servo testing system. Based on the experimental results, the damage constitutive equations of cemented backfills with five cement?tailing ratios were proposed on the basis of damage mechanics and then validated. Research shows that the damage growth rate of backfills with lower cement?tailing is lower before peak value of stress, but it grows more rapidly after that. In addition, four backfill combination schemes were designed for mechanical test. Combining with research results on fracture characteristic of different backfills, a defects coupled constitutive model based on continuum damage mechanics was established. Research shows that the piecewise nonlinear model can well embodies effect of complex defects in backfills on the stress‐strain curve. Accuracy of the model is well verified by measured curve of backfill composite members. The results of this work provide a scientific basis for failure process prediction of backfills and reasonable matching design of framed filling.     

碳纳米管对微胶囊铜复合浆料导电性能的影响

为提高铜浆料的导电性能,利用微胶囊技术对铜粉表面做改性处理,添加碳纳米管为导电增强相,制备碳纳米管-微胶囊铜复合浆料。利用四探针测试仪、扫描电子显微镜(SEM) 等研究了微胶囊铜粉的抗氧化性能及碳纳米管的参数、添加量对铜浆料导电性能的影响,分析其导电机理并建立导电相连接模型。研究结果表明:微胶囊化的铜粉具有较好的抗氧化性和导电性。当碳纳米管与铜粉的质量比为4∶[KG-2mm]96时,采用管径1~2 nm,长度5~30 nm的碳纳米管制备的复合浆料的电阻率达到最小值6.05 mΩ·cm,与纯铜浆料相比降低了89.39%。以碳纳米管-铜复合浆料与铜浆料分别制得导电膜,两者相比,前者更平坦、更致密,导电相间的接触更紧密,大量的碳纳米管覆盖在铜粉颗粒表面或填充铜粉颗粒间隙,同时碳纳米管之间相互“吸引”,形成致密的网状结构,在铜粉颗粒之间建立起大量的导电“桥梁”,从而改善了复合浆料的导电性能。     

Coupled effects of sulphate and temperature on the strength development of cemented tailings backfills: Portland cement-paste backfill

Cemented paste backfill (CPB), which is a mix of tailings, water and cement, is subjected to the combined actions of temperature and sulphate during its service life. There is a need to acquire solid knowledge on the coupled effects of temperature and sulphate on the strength of CPBs for a safe, durable and cost-effective design of CPB structures. Hence, the main objective of this paper is to use an experimental approach to study the combined effect of temperature and sulphate on the strength development and microstructure (mineralogical composition of the hardened cement paste) of CPBs. About 200 CPB specimens with various initial sulphate contents (0, 5000, 15,000, and 25,000 ppm) and cured at different temperatures (0 °C, 25 °C, 20 °C, 35 °C, and 50 °C) are tested at different curing times (28, 90, and 150 days). The results show that the coupled effect of temperature and sulphate has a significant impact on the strength and mineralogical composition of the CPB. Depending on the curing time, temperature and initial sulphate content, the sulphate can have a positive or negative impact, i.e., leads to an increase or decrease of CPB strength. The obtained results show a strong indication that the absorption of sulphate by calcium–silicate–hydrate (C–S–H) could lead to the formation of lower quality C–S–H, thereby decreasing the strength of the CPB. This study has demonstrated that the coupled effect of sulphate and temperature on CPBs is an important factor for consideration in the designing of cost-effective, safe and durable CPB structures.     

氧化石墨烯对水泥净浆流动度及水泥石结构和性能的影响

采用Hummers法及超声破碎分散法制备了氧化石墨烯(GO)纳米相分散液,研究了GO对水泥净浆流动度和水泥石微观结构的影响,用FT-IR、AFM、XRD及SEM对GO及水泥石结构进行了表征。结果表明GO的掺入降低了净浆流动性,每增加0.01%的GO需要增加0.02%的聚羧酸系减水剂(PCs)以保持水泥净浆流动度在3 h内在200 mm以上,GO的掺入能够使水泥石的微观结构发生明显的变化,当GO/PCs掺量为0.01%/0.24%~0.03%/0.28%时,水化龄期7 d的水泥石出现了大量分散均匀的花状微晶体,当GO/PCs掺量为0.05%/0.32%~0.07%/0.36%时,同龄期水泥石中出现大量的片状晶体,在水化龄期延长到28 d时有转化为密实结构的趋势,结果说明GO具有调控水泥水化产物形貌的能力及增强增韧的作用,此研究结果对于提高水泥基材料的力学性能具有重要意义。     

Influence of temperature on the electrical conductivity of leachate from municipal solid waste

A bioreactor landfill is designed to manage municipal solid waste, through accelerated waste biodegradation, and stabilisation of the process by means of the controlled addition of liquid, i.e. leachate recirculation. The measurement of electrical resistivity by Electrical Resistivity Tomography (ERT) allows to monitor water content present in the bioreactors. Variations in electrical resistivity are linked to variations in moisture content and temperature. In order to overcome this ambiguity, two laboratory experiments were carried out to establish a relationship between temperature and electrical conductivity: the first set of measurements was made for leachate alone, whereas the second set was made with two different granular media saturated with leachate. Both experiments confirm a well known increase in conductivity of about 2% degrees C(-1). However, higher suspended matter concentrations lead to a lower dependence of electrical conductivity on temperature. Furthermore, for various porous media saturated with an identical leachate, the higher the specific surface of the granular matrix, the lower the effective bulk electrical conductivity. These observations show that a correct understanding of the electrical properties of liquids requires the nature and (in particular) the size of the electrical charge carriers to be taken into account.     

硬化水泥净浆基于纳米压痕的相态识别与水化程度计算

采用纳米原位压痕手段测量硬化水泥净浆中单一相态的代表性微观力学性能,并采用纳米点阵压痕研究各相态的含量。研究对象囊括水灰比为0.3、0.4、0.5的纯水泥净浆和水灰比为0.3情况下含50%、70%矿渣掺量复合体系,共5种配比,以表征它们的相态分布和微观力学性质的异同点。掺矿渣的试件中含有明显多的复合相,因此提出三相模型测算复合相中未水化物的体积分数。此外,提出基于纳米压痕技术计算纯水泥和掺矿渣水泥试件水化程度的方法,结果吻合于热重分析的结果,其中纯水泥净浆中复合相较少,计算得到的水化程度优于对掺矿渣水泥试件的计算。     

Effect of NaOH activation on sulphate resistance of GGBFS and binary blend pastes

This paper presents an investigation into the observed enhanced performance that alkali activated ground granulated blast furnace slag (GGBFS) and binary blends offer against sulphate attack. X-ray diffraction (XRD) was carried out to identify and quantify the crystalline phases formed in a wide range of GGBFS and GGBFS-OPC (ordinary Portland cement) blends. Furthermore, specimens were exposed to a sulphate solution to examine the evolution of compressive strength along with identification of activation and/or hydration products. XRD demonstrated that ettringite was completely decomposed into its constituents in the presence of NaOH while quantification ascertained the formation of considerable amounts of hydrotalcite in the activated GGBFS and binary blends. Alkali activated GGBFS and binary blends specimens with higher GGBFS content offered enhanced resistance against aggressive sulphate ions and no significant degradation products were observed in these specimens after 6 months of exposure to sulphate solution. The results demonstrated that hydrotalcite formation may be a major reason for the improved sulphate resistance of alkali activated GGBFS and binary blend pastes with higher GGBFS content.     

An analytical solution for hydraulic conductivity of concrete considering properties of the Interfacial Transition Zone (ITZ)

As a composite material, hydraulic conductivity of concrete depends on conductivity of its components that are the mortar, aggregates and the Interfacial Transition Zone (ITZ). Since hydraulic conduction is analogous to heat and electrical conduction, analytical models from these analogous areas relating effective conductivity of composite to conductivity of its components can be used to find the effective hydraulic conductivity of concrete as a function of properties of its components, i.e., aggregate, mortar and the ITZ. However, effect of the conduction in the ITZ has not been considered in these models. This paper presents an analytical solution for the hydraulic conductivity of concrete as a three-phase composite material. The solution is an extension to the model originally proposed for conduction of composite media with randomly suspended spheres. Results of the proposed model compare well against the experimental results and those obtained from rigorous numerical analysis using the Finite Element (FE) method. The principal significance of this study lies in the development of a versatile analytical model that can be employed as a quick tool for assessment of hydraulic conductivity of concrete without the need for sophisticated FE models at the meso-scale level. It offers more insight into effect of different components of concrete on its overall conductivity.     

压电陶瓷振子专用高温银浆的制备及工艺研究

研究了一种用于压电陶瓷振子的银电极制备的浆料。这种浆料主要由银粉、玻璃料、有机载体组成。本文对银浆各组成部分的属性以及银浆制备工艺和加工工艺进行讨论。     

粉磨工艺对赤泥胶结充填料性能的影响

为提高胶结充填材料的强度性能,试验研究了分别粉磨和梯级粉磨工艺对赤泥全尾砂胶结充填材料强度性能、粉体粒径以及水化过程的影响.结果表明：梯级粉磨工艺下试块的1、3、28 d最大抗压强比分别粉磨工艺下试块1、3、28 d最大抗压强度分别高出21%、16.6%、3.7%;梯级粉磨工艺使物料粉体的比表面积均高于分别粉磨工艺的粉...     

基于土体isotache模型的硬化水泥净浆微观徐变预测

水泥基材料是一种应变率敏感性材料,充分掌握水泥基材料的应变率效应对其力学性能和变形的影响可保障混凝土结构的安全设计。采用连续刚度测试（CSM）研究不同应变率下硬化水泥净浆微观徐变特征,试样的水灰比为0.3、0.4和0.5,CSM的最大压入深度为30 μm,应变率分别为0.01 s^-1、0.05 s^-1、0.1 s^-1和0.5 s^-1,并讨论土体的isotache模型表征硬化水泥净浆微观徐变行为的适用性。结果表明,在0.01~0.5 s^-1应变率范围内,微观尺度上的硬化水泥净浆存在唯一的h_vp-P-ε关系,与土体isotache模型的ε_vp-σ-ε关系类似,该关系可由归一化后的h_vp-P/（P_N/P_N0.05）曲线和P_N/P_N0.05-ε曲线来表征。采用h_vp-P/（P_N/P_N0.05）曲线和P_N/P_N0.05-ε曲线可预测不同应变率下硬化水泥净浆加载阶段的h_vp-P曲线,预测结果与实测结果基本吻合,表明可以采用土体的isotache模型来预测硬化水泥净浆微观尺度上的徐变行为。     

Thermal conductivity estimation of the organic and inorganic refrigerants in the saturated liquid state

The use of a general correlation, proposed for organic liquid thermal conductivity λ prediction and checked in the case of organic refrigerant fluids, is extended to inorganic refrigerant fluids whose λ decreases when the reduced temperature, T_r, increases. Suggestions are given for the refrigerants fluids whose λ increases with the reduced temperature.     

Effect of pore shape on the thermal conductivity of partially saturated cement-based porous composites

Increasing demands of new cement-based porous composites for thermal insulation required by sustainable development are crying out for clarifying the key factors affecting their thermal properties. In this study, we focused on the effect of pore shape, a factor that was often slighted or ignored before, on the thermal conductivity of composite materials. We established a predicting model for a simple two-phase inclusion-matrix system based on effective medium and mean-field theories. The model can be easily extended to assess the effective thermal conductivity of multi-phase and multi-scale composites. The generalized model involves both homogenization schemes and material structures including the shapes and orientations of inclusions. We showed that the shapes of inclusions affect the effective thermal conductivity of composites significantly. The established model were used to predict the thermal conductivities of cracked ordinary concrete and partially saturated autoclaved aerated concrete reported in the literature with in-depth discussion.     

超声振动对石墨烯微片/聚丙烯复合材料导电导热性能的影响机制

在挤出过程加入超声振动作用,研究超声振动对高石墨烯微片(GNP)含量的聚丙烯基(GNP/PP)复合材料微观形态、结晶、导电性和导热性的影响。结果表明:由于超声振动提供强烈的冲击波与微射流,挤出过程中加入超声振动可有效地减薄GNP片层厚度,减少GNP团聚,增强GNP在PP中的分散均匀性,有利于构建导电导热网络,从而提高GNP/PP纳米复合材料的导电导热性能。相较于无超声振动,加入100 W超声振动后,GNP含量越高,GNP/PP电导率和热导率提升幅度越大,在GNP含量为15wt%时,电导率升幅为85%,热导率升幅为9.7%。而在GNP含量同为12wt%时,随着超声振动功率的增加,电导率和热导率呈现先增大后减少的规律。当超声功率为200 W时,电导率升幅为214%,热导率升幅为17.2%。而超声功率达到300 W时,较高功率的超声振动使部分石墨烯微片的片径减少,导致片层间更难以搭建完整的导电导热网络,使GNP/PP性能均略有下降。     

温度对高压烧结氮化铝陶瓷热导率的影响

以直接氮化法合成的AlN微米粉为原料,添加3%(质量分数)的CaC2为烧结助剂,在5GPa的压力下烧结30min,考察不同烧结温度对AlN陶瓷热导率的影响。用阿基米德排水法、XRD、SEM等技术手段对AlN烧结体进行性能检测。研究表明,在1500～1800℃范围内,温度的升高能促使AlN陶瓷内部晶粒长大,晶型饱满,尺寸均一,晶界相减少,实现烧结致密化,利于热导率的提高。