Influence of mix proportions on microstructure and gas permeability of cement pastes and mortars

This paper presents a study on the influence of mix proportions of cementitious materials on their transfer properties, namely porosity and gas permeability. These latter are known as durability indicators. The work is performed on a wide range of cement pastes and mortars (24 compositions). These compositions are defined by mix proportion parameters (water/cement ratio, limestone filler/cement ratio, and amount of superplasticizer and volume fraction of paste). To characterize these materials, an experimental campaign was carried out, including different types of test (water porosimetry, mercury intrusion porosimetry, desorption isotherms and gas permeability). The influence of the composition parameters on the studied durability indicators is highlighted and correlation between gas permeability and microstructural properties (total porosity and critical pore diameter) is established. Finally, a method to predict materials permeability from that of the cement paste is proposed.     

Characterization of basaltic tuffs and their applications for the production of ceramic and glass–ceramic materials

Alkaline basaltic tuffs, from Southern Turkey were characterized and employed to obtain ceramic and glass–ceramic materials by combined sintering and crystallization process. The chemical and mineralogical compositions were analyzed by X-ray fluorescence spectrometry and X-ray diffraction analyses, respectively. The phase formation and the sintering behaviour were investigated by DTA, differential dilatometer and hot-stage microscopy. The micro-structure and residual porosity of the sintered samples were observed by SEM and evaluated by pycnometric techniques. Ceramic material, based on 50% basaltic tuff and 50% clay, was obtained at 1150 °C with 13% total porosity and 4% water absorption. Glass–ceramic materials were synthesized directly using the milled basaltic tuff by mean of the sinter-crystallization technique, in the range 900–1150 °C. The investigation has showed that, due to the high porosity and low crystallinity, alkaline tuffs could be a suitable raw material for ceramic application.     

Potential use of natural perlite powder as a pozzolanic mineral admixture in Portland cement

This paper investigates the effect of incorporating natural perlite powder (NPP) as a cement replacement on cementitious materials properties. For this purpose, cement pastes and mortars were prepared by replacing 5, 10, 15, and 20% of NPP by mass of the Portland cement. Physicomechanical performances of pastes and mortars based NPP were inspected using normal consistency, setting times, heat of hydration, and compressive strength testing. Resistance against sodium sulfate attack and sulfuric acid attack were also assessed to investigate the durability characteristics of different mortar mixes. Experimental results show that cement pastes and mortars incorporating up to 20% of NPP demonstrate satisfactory physical and mechanical properties with very comparable results to cementitious materials without NPP. In addition, improved sulfate and acid attacks resistance with increased NPP content were demonstrated. The X-ray diffraction analyses confirm that NPP can be considered as a good pozzolanic material that can be used satisfactorily as a mineral admixture in cement production.     

Modeling of diffusive mass transport in micropores in cement based materials

In order to predict long-term leaching behavior of cement constituents for safety assessments of radioactive waste disposal, we modeled diffusive mass transport in micropores in cement based materials. Based on available knowledge on the pore structure, we developed a transport porosity model that enables us to estimate effective porosity available for diffusion (transport porosity) in cement based materials. We microscopically examined the pore structure of hardened cement pastes to partially verify the model. Effective diffusivities of tritiated water in hardened cement pastes were also obtained experimentally, and were shown to be proportional to the estimated transport porosity.     

Physical modeling of the electrical double layer effects on multispecies ions transport in cement-based materials

The transport of ions through cement-based materials is described at a microscopic scale with a pore modeled by two infinitely large flat plates. The theory of the electrical double layer (EDL) shows that (i) the overlapping between the diffuse layers occurring in the pore is more important as the pore diameter will be small (low than the Debye length) and the pore walls will be strongly charged, (ii) the fluxes of coions and counterions will be respectively increased and attenuated in such pores. The gel pores of cement based materials have similar characteristics. As the capillary pores of the cement based materials with low porosity are connected between them by the pores gel, the transport of ions at a macroscopic scale could be greatly influenced by the overlapping effect of the diffuse layers.     

Addition of cement to lime-based mortars: Effect on pore structure and vapor transport

The main focus of this work is to determine the effect of cement addition, a common practice in many restorations, on the pore structure of lime-based mortars. A second target is to establish correlations between microstructure and water vapor transport across the mortar, which is a key characteristic of building decay. In order to achieve these objectives, we prepared a set of mortars consisting of air-hardening lime with a progressively increasing cement content, as well as a mortar containing hydraulic lime. Several different techniques, most notably mercury intrusion porosimetry and scanning electron microscopy in the backscatter mode, were used to investigate the pore structure. The results from these procedures led to the conclusion that porosity and pore size are progressively reduced as cement content increases. Moreover, an excellent correlation between pore radius parameter and the vapor diffusion coefficient was established. Hydraulic lime mortar exhibited textural parameters and diffusivity values halfway between those of the different lime/cement mixes studied.     

利用煅烧硅藻土制备高稳态超高性能混凝土基体研究

超高性能混凝土(UHPC)早期自收缩大,使结构存在收缩开裂风险,掺入天然多孔活性粉料以激发其火山灰和内养护协同效应可有效降低UHPC自收缩,改善基体体积稳定性。本文利用煅烧硅藻土以一定体积分数(3%、6%、9%)置换水泥制备UHPC基体,并对其新拌性能、力学性能、自收缩、耐久性能及孔结构与微观结构进行了系统评价。结果表明:掺入煅烧硅藻土能显著改善UHPC力学性能和体积稳定性,并进一步提升其耐久性能;掺入煅烧硅藻土UHPC基体孔结构得到优化,孔隙率降低,孔径细化;掺入煅烧硅藻土UHPC基体中C-S-H凝胶平均Ca/Si比下降,C-S-H(I)占比提高,水化产物结构及致密性改善。优化设计条件下,UHPC基体56 d抗压强度提高9%,56 d抗折强度提高18%,7 d自收缩下降29%,28 d快速氯离子迁移系数下降35%,28 d电通量下降27%。     

Characterization of Pore Space of Cement-Based Materials by Combined Mercury and Wood's Metal Intrusion

Analysis of the pore space is crucial for a profound understanding of the transport and mechanical properties of porous materials. Mercury intrusion porosimetry (MIP) is an easy and widely applied method to determine the pore size distribution of mesoporous materials, but a principal problem makes data interpretation difficult. Large ink-bottle pores may be accessed by the intruding mercury through smaller, so-called neck pores only. This leads to significant under estimaion of pore sizes and to hysteresis effects between intrusion and extrusion in materials with a broad pore size distribution such as cement-based materials. More accurate pore space information is obtained when ink-bottle pores in the measurement are excluded from analysis. This may be achieved by repeated intrusion cycles or by impregnating the ink-bottle pore space with Wood's metal. The combination of Wood's metal impregnation (WMI) and mercury intrusion in mortars and cement pastes as presented allows a characterization of the pore space independent of accessibility considerations. Different special pore types are defined, analyzed, and quantified. In a cement paste, 50% of all pores are found to be ink-bottle type, of which 60% are accessible through neck entrances larger than 20 nm in diameter. A further 30% of all pores are nonink-bottle type but are connected to the surface through such ink-bottle pores only. Furthermore, hysteresis and contact angle alternation effects between intrusion and extrusion were studied. A contact angle shift of 26° between intrusion and extrusion is proposed.     

Strength, durability and micro-structural aspects of high performance volcanic ash concrete

This paper presents the results of investigations to assess the suitability of using volcanic ash (VA) as a cement replacement material to produce high performance concrete. Tests were conducted on concrete mixtures replacing 0 to 20% by mass of ordinary Portland cement (OPC) by VA. The performance of high performance volcanic ash concrete (HPVAC) mixtures was evaluated by conducting comprehensive series of tests on fresh and hardened properties as well as durability. The mechanical properties were assessed by compressive strength, while durability characteristics were investigated by rapid chloride permeability (RCP), drying shrinkage (DS), mercury intrusion porosimetry (MIP), differential scanning calorimetry (DSC) and microhardness tests. HPVACs showed better durability properties compared to control concrete with 0% VA. The improved performance of HPVACs was attributed to the refinement of pore structure, and pozzolanic action of VA. HPVAC having a minimum 28-day compressive strength of 60 MPa can be obtained by replacing up to 20% (by mass) of cement by VA. Development of non-expensive and environmentally friendly HPVAC with acceptable strength and durability characteristics (as illustrated in this study) is extremely helpful for the sustainable development and rehabilitation of volcanic disaster areas around the world.     

聚合物改性水泥基材料的机理研究进展

综述了近年来聚合物改性水泥基材料(polymer modified cement based materials,PMCBM)改性机理方面的国内外研究进展.从理论角度对PMCBC进行了归纳与总结,重点从4个方面探讨了聚合物改性水泥基材料的改性机理:聚合物对水泥水化过程的影响存在物理作用和化学作用;聚合物对微观结构的主要影响是乳胶粒子的分散和聚合物薄膜的形成;从孔洞结构看,聚合物改变了水泥基材料的孔径分布、特征孔径、平均孔径、最可几孔径、孔隙率等,提高了材料的内聚强度;从聚合物自身结构来看,聚合物的链结构和聚集态结构直接影响水泥基材料的性能.     

粉煤灰-水泥浆体的孔体积分形维数及其与孔结构和强度的关系

采用压汞法对不同龄期粉煤灰-水泥浆体的孔分形结构进行了实验研究，测定了复合浆体孔体积分形维数，探讨了孔体积分形维数与孔隙率，孔表面积、平均孔径、孔分布及宏观力学性能的关系。实验结果表明：粉煤灰-水泥浆体的孔结构具有明显的分形特征，孔体积分形维数在3．3～3．5之间；孔体积分形维数越大，浆体的孔径越小、孔隙率越低，孔表面积越大，小于20nm的微孔越多，大于100nm的大孔越少，而且复合体系的抗压及抗折强度也越高。     

Pore structure and strength development of cement composites

Two stages are to be distinguished in the development of cement composite pore structure. The first stage is determined by the composition of the mixture as well as by its mixing efficiency and compaction degree. During this stage the initial pore structure, characterized by the initial porosity - P, is being formed. The second stage of the pore structure deveopment is a consequence of the hydration products creation in hardening cement composite. In dependence on the volume as well as on the kind, morphology and other properties of the created hydration products the total porosity diminishes, the hydration micropores arise and the final pore structure -with a characteristic pore size distribution and micropores shape- is formed.

A close relationship between the pore structure and strength of cement based composites follows from the fact that both these properties depend decisively on the volume, kind and other parameters of hydration products created.     

Recycling of polyurethane foam waste in the production of lightweight cement pastes and its irradiated polymer impregnated composites

This study aimed at using polyurethane foam waste in the production of lightweight white cement pastes by a partial replacement of white cement with different ratios of polyurethane foam waste (10%, 20%, 30%, and 40%) based on the weight of cement. The lightweight white cement pastes specimens in addition to conventional white cement paste were cured under tap water for 7 and 28 days. The physical, mechanical, and thermal properties were evaluated. The results showed that the specimens cured for 28 days achieved better properties as compared to the specimens cured for 7 days. Furthermore, at each curing age the specimens of lightweight white cement pastes showed relatively lower properties as compared to the conventional white cement paste and as the polyurethane waste content increased, the properties in terms of compressive strength and bulk density decreased while the total porosity percentage increased especially at higher ratios. On the other hand, the effects of styrene–butadiene rubber latex and irradiation dose on the properties of irradiated polymer impregnated lightweight white cement composites have been investigated. The results confirmed that the impregnation of the hardened lightweight white cement pastes with styrene–butadiene rubber latex and their exposure to different doses of gamma rays (50, 100, 150, and 200 kGy) showed a gradual improvement in the mechanical and physical properties up to 150 kGy and then started to decrease at 200 kGy. Characterization of some selected specimens was carried out by the studying of thermogravimetric analysis, scanning electron microscopy, and X‐ray diffraction. J. VINYL ADDIT. TECHNOL., 25:328–338, 2019. © 2019 Society of Plastics Engineers     

纳米颗粒对道路粉煤灰混凝土耐磨性能的影响及作用机理

为探讨纳米颗粒对道路粉煤灰混凝土耐磨性能的影响,采用纳米材料和超细粉煤灰等量取代水泥制备了纳米改性粉煤灰水泥砂浆,在此基础上制备出三种纳米改性道路粉煤灰混凝土:纳米SiO2 (NS)混凝土、纳米SiC(NC)混凝土和纳米复掺混凝土,并研究了两种纳米材料对粉煤灰水泥砂浆力学性能和道路粉煤灰混凝土耐磨性能的影响及作用机理.结果表明:混凝土中掺入纳米材料能显著提高其耐磨性能.单掺NS混凝土中NS最优掺量为2％,单掺NC混凝土中NC最优掺量为3％;而复掺2％ NS、2％ NC时,纳米复掺混凝土耐磨性最佳,与基准混凝土相比,磨损量降低了75％.分析认为:纳米材料的表面效应、活性效应和微集料填充效应使其具有较大的表面能,在水泥浆体中与Ca(OH)2晶体发生二次水化反应,改善了Ca(OH)2的取向程度,强化了水泥石微细观结构,优化了水泥基体内孔径分布与孔结构,使其更加密实,从而提高了道路粉煤灰混凝土的耐磨性能.     

Correlations between pore structure parameters and gas permeability of corundum porous materials

Corundum porous materials with different contents of calcium hexaluminate formed in situ were prepared using pure calcium aluminate cement as the calcium source. The surface fractal dimensions of the porous materials were calculated based on the experimental data of mercury intrusion. Correlations between pore structural parameters and the permeability coefficients k₁ and k₂ of the porous materials were then studied based on the grey system theory. The results showed that pores in the corundum porous materials have great fractal characteristics. The surface fractal dimension was a significant pore structural parameter that reflected the complexity of pore shape, pore surface, and pore-size distribution, which had the maximum correlation coefficient with the permeability of this type of porous materials. The apparent porosity and pore-size distribution had relatively high correlation coefficients to the permeability as well. Increasing the apparent porosity and the volume percentage of larger pores, and decreasing the volume percentage of smaller pores all benefited the permeability of the porous materials. In addition, the mean pore size and median pore size showed lower correlation coefficients to the permeability—especially for porous materials with a wide pore-size distribution.     

磁铁矿防辐射超高性能混凝土制备及性能研究

核技术在造福人类的同时,也产生了无处不在的核辐射,而当前的普通防辐射混凝土并不能完全满足安全防护的需要。本文基于最紧密堆积理论,采用不同比例的磁铁矿替换河砂,制备了防辐射超高性能混凝土(UHPC),并对其工作性能、力学性能、微观结构、孔结构,以及γ射线屏蔽性能进行了研究。结果表明,磁铁矿的加入使得UHPC的流动性以及抗压强度略有降低,但降幅较小。随着磁铁矿替换比例的增加,UHPC对γ射线的屏蔽性能明显提高。当磁铁矿替换率为100%(体积分数)时,UHPC的线性衰减系数增大了31.3%,而半值层及十值层均下降了23.8%。与此同时,磁铁矿的加入并未改变水化产物的类型,但可改善UHPC的孔结构,有效降低其孔隙率。     

利用黄金尾矿制备发泡陶瓷的研究

中国黄金尾矿资源量大,作为二次资源在建筑材料领域的综合利用有着重要的经济价值和环境意义。以黄金尾矿为主要原料,SiC为发泡剂,通过高温制备发泡陶瓷,用激光共聚焦显微镜、XRD等手段,研究了烧结温度、黄金尾矿掺入量、原料粒度对材料的容重、真气孔率和孔径等性能的影响。研究表明:随着烧结温度的升高,发泡陶瓷材料的真气孔率和孔径增大,容重减小;材料的真气孔率和容重随着黄金尾矿掺入量的增大分别降低和提高,随着原料粒度的减小分别提高和降低,气孔孔径随着黄金尾矿掺量的增大和原料粒度的减小均呈下降趋势。优化后,在烧成温度1 050 ℃,黄金尾矿掺入量50%(质量分数),黄金尾矿平均粒度D(50)=5.6 μm,SiC平均粒度D(50)=3.0 μm的条件下可制备出性能良好的发泡陶瓷。     

磷渣对水泥浆体强度和孔结构的影响

研究了磷渣对水泥浆体孔结构和抗压强度的影响。结果表明,与纯水泥浆体的孔结构相比,磷渣的掺入使浆体早期的孔隙率增大,大孔所占的比例增加,但降低了浆体后期的孔隙率和孔径尺寸。浆体早期的抗压强度随磷渣掺量的增加而减少,90d时,其强度超过了纯水泥浆体。     

Preparation and characterization of eco-friendly and low-cost mullite-corundum foamed ceramics with low thermal conductivity

Mullite-corundum foamed ceramics were prepared by direct-foaming method using white clay and industrial alumina as raw materials, and calcium aluminate cement (CAC) as the binder. Effects of the calcium aluminate cement content on the phase compositions, microstructures and properties of the foamed ceramics were investigated through X-ray diffractometer (XRD), scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS), etc. The results showed that with increasing the calcium aluminate cement content from 0 wt% to 8 wt%, the viscosities of the initial slurries decreased, the porosities of the samples improved, the thermal conductivities decreased, and the compressive strengths increased first and then decreased. The optimized sample with 4.0 wt% CAC content had a high porosity of 80.8%, a low bulk density of 0.56 g/cm³ and a low thermal conductivity of 0.232 W/(m·K) (800 °C). On the foundation of research results, the Gong equation (GE model) and a modified GE model were used to predict the thermal conductivity at 200 °C and 800 °C of the mullite-corundum foamed ceramics with different pore characteristics and phase compositions, respectively.     

煤矸石对泡沫混凝土孔结构的影响

本文将粉磨之后的煤矸石替代部分水泥掺入到混凝土中,通过物理发泡工艺制备煤矸石泡沫混凝土。采用FiJi-imageJ图像分析技术等对泡沫混凝土的孔径分布、平均孔径、气孔直径等进行表征,分析了煤矸石改善泡沫混凝土成孔的原因以及煤矸石粉磨粒径对泡沫混凝土抗压强度的影响。结果表明:当掺入粒径为45 μm的煤矸石粉时,泡沫混凝土的强度随着掺量的增加逐步下降,而掺入粒径为15 μm的煤矸石粉时,强度则随掺量的增加呈先上升后下降的趋势;在调节孔结构方面,粒径为45 μm的煤矸石粉优于粒径为15 μm的煤矸石粉,并在50%(质量分数)掺量下达到最优效果。