Enhanced modeling of moisture equilibrium and transport in cementitious materials under arbitrary temperature and relative humidity history

This paper focuses on behaviors of moisture dispersed in nano‐macro scale pores under various temperature and relative humidity conditions. The authors formulated an equilibrium relationship between liquid and vapor phases and a moisture flux driven by pore pressure, vapor pressure and temperature gradients. In addition, liquid and interlayer water were measured separately by ethanol in order to reveal each temperature sensitivity in saturation‐humidity paths. Based on the experiments, a modified hysteresis model for moisture isotherm was proposed. Verifications with experimental data showed that the proposed method can simulate moisture behaviors under various temperature conditions.     

Dielectric properties of densified hardened cementitious materials

Relative permittivity (K′) and loss factor (tan D) of densified hardened cementitious materials with w/c=0.20 and heat treated to 500°C prepared with type I, type III, and a special microfine high silica content (MC500) cement have been compared as a function of temperature and frequency. These K′ and tan D values were found to be considerably lower for materials with microfine silicate cement than those with portland cements.

Activation energies (Q) and relaxation times for the loss mechanism values were related to an ionic jump relaxation process. Lower K′ and tan D values have been measured for cement/polymer composites prepared with a methyl cellulose polymer and a lower w/c ratio of 0.17. The relative permittivity and loss factor of materials with a 60 vol. % of silica microspheres prepared with the silicate cement ranged from 5.4 to 5.1 and from 0.035 to 0.012 respectively.     

可随机成型的透波性胶凝材料的微波加热特性

研究了硬化铝酸钙材料在不同煅烧温度（700,800,900,1000,1100,1200,1300,1400和1500℃）下的吸波能力、抗压强度、体积收缩率和质量损失率,采用XRF和XRD对不同煅烧温度（800,1000,1300和1500℃）下的试样进行表征,利用铝酸钙材料制备一种简易杯状容器并对其进行微波冲击特性测试。结果表明：硬化铝酸钙材料的吸波能力随煅烧温度的升高而降低,超过1000℃后,吸波能力趋于稳定;材料抗压强度随煅烧温度的升高先升高,在800℃左右达到最高,然后逐渐降低,并在1300℃达到最低,再经过小幅地升高后趋于稳定;同时,材料煅烧后体积收缩率和质量损失率都随着温度的升高而增加;微波冲击特性测试表明硬化铝酸钙材料在长时间微波辐照下强度基本不发生变化,微波-金属放电试验证明该胶凝材料硬化后的高温稳定性好,具有优异的抗高温冲击能力。综上所述,1000℃煅烧后的硬化铝酸钙材料具有透波性能好、力学性能优异、高温稳定性好的特点,非常适用于微波加热领域。     

Rheological properties of cementitious materials containing mineral admixtures

The rheological properties of cementitious materials containing fine particles, such as mineral admixtures (MA), were investigated using a Rotovisco RT 20 rheometer (Haake) with a cylindrical spindle. The mineral admixtures were finely ground blast furnace slag, fly ash and silica fume. The cementitious materials were designed as one, two and three components systems by replacement of ordinary portland cement (OPC) with these mineral admixtures. The rheological properties of one-component system (OPC) were improved with increasing the dosage of PNS-based superplasticizer. For two-components systems, yield stress and plastic viscosity decreased with replacing OPC with blast furnace slag (BFS) and fly ash (FA). In the case of OPC-silica fume (SF) system, yield stress and plastic viscosity steeply increased with increasing SF. For three components systems, both OPC-BFS-SF and OPC-FA-SF systems, the rheological properties improved, compared with the sample with SF. In the two and three components systems, the rheological properties of samples containing BFS improved much more than with FA replacement alone.     

Ion-cement hydrate interactions govern multi-ionic transport model for cementitious materials

The main objective of this investigation is to describe the interaction between cement hydrates and electrolyte solution to understand multi-ionic transport in cementitious materials. A surface complexation model in PHREEQC including an electrostatic term is used to simulate the ionic adsorption on the calcium silicate hydrate (C–S–H) surface. The equilibrium constants for the adsorption of ions on C–S–H surfaces are obtained by fitting experimental data to the model. The adsorption of both divalent and mono-valent cations, and also anions significantly changes the surface charges of hydrated paste. Chloride is being held in a chemical binding as Friedel's salt and bound mainly by the adsorptive action of C–S–H. An integrated modelling approach employing a phase-equilibrium model, a surface complexation model, and a multi-component diffusion model has been developed in PHREEQC to simulate the multi-ionic transport through hydrated cement paste. It was found that the physical adsorption of ions on C–S–H, the size of pores, and the surface site density of C–S–H govern the rate of penetration of ionic species. Finally, the proposed model has been validated against chloride profiles measured in this study as well as with data available in the literature for hydrated cement paste.     

Modeling the effect of temperature on ionic transport in cementitious materials

This paper deals with the effect of temperature on ionic transport in cementitious materials. The necessary steps to modify a model designed to work in isothermal conditions to take temperature variations of the environment into account are described. The following parts of the model were modified: transport properties, ionic transport and chemical reactions. Simulations of laboratory tests and field concrete measurements are presented.     

Investigation of capillary flow in discrete cracks in cementitious materials

A series of experimental studies are presented that simulate capillary flow of water in discrete cracks in cementitious materials. A number of amendments to existing capillary flow theory are adopted which take the form of correction parameters for stick–slip behaviour of the meniscus, frictional dissipation at the meniscus wall boundary and slip between the fluid and solid wall. A benchmark study to examine capillary flow in small diameter glass capillaries is reported and this provides data to validate the amended theoretical model. Predictions made using the amended model closely match the experimental results of capillary rise in discrete cracks in cementitious materials allowing the correction parameters for capillary flow in planar cracks to be determined. Finally, capillary rise in a discrete natural crack of known aperture is considered and a relationship is proposed which predicts the capillary rise response in a natural crack in terms of an equivalent planar crack.     

Supplementary cementitious materials

The use of silica rich SCMs influences the amount and kind of hydrates formed and thus the volume, the porosity and finally the durability of these materials. At the levels of substitution normally used, major changes are the lower Ca/Si ratio in the C–S–H phase and consumption of portlandite. Alumina-rich SCMs increase the Al-uptake in C–S–H and the amounts of aluminate containing hydrates. In general the changes in phase assemblages are well captured by thermodynamic modelling, although better knowledge of the C–S–H is needed.At early ages, “filler” effects lead to an increased reaction of the clinker phases. Reaction of SCMs starts later and is enhanced with pH and temperature. Composition, fineness and the amount of glassy phase play also an important role. Due to the diverse range of SCM used, generic relations between composition, particle size, exposure conditions as temperature or relative humidity become increasingly crucial.     

球磨时间对磷石膏基胶凝材料性能影响研究

以原状磷石膏为研究对象,在用热重分析与相组成分析技术探究磷石膏脱水温度与时间的基础上,研究了球磨时间对原状磷石膏粒径大小及分布、磷石膏硬化体以及磷石膏-水泥胶结料性能的影响。原状磷石膏脱水温度为130 ℃、脱水时间为60 min。在0~20 min,延长球磨时间可以有效降低磷石膏-水泥胶结料的流动度,缩短凝结时间,磷石膏硬化体以及磷石膏-水泥胶结料的力学强度先提高后降低。最优球磨时间为15 min,此时原状磷石膏粒径约为29 μm;所得的磷石膏-水泥胶结料具有较好的力学性能和耐水性能。     

Effects of an amphoteric copolymer on the properties of cementitious materials

The effects of an amphoteric copolymer, poly(acrylamide‐co‐(2‐(3‐carboxyacryloyloxy)‐N‐(carboxymethyl)‐N,N‐dimethyl‐ethanaminium) (PAC) on the properties of cement pastes or mortars were investigated. PAC was prepared from acrylamide and 2‐(3‐carboxy‐acryloyloxy)‐N‐(carboxymethyl)‐N,N‐dimethylethanaminium (CAC) through free radical polymerization. The results indicate that the presence of the polymer delayed the setting of cement pastes; it also reduced the water demand so that the resulting mortars showed improved compressive strength and reduced length change. PAC with about 16.7% CAC and a weight‐average molecular weight of about 5 × 10⁴ was effective in promoting the material properties of mortars. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Alteration of fractured cementitious materials

The alteration of cement materials in a fractured repository was investigated by experimental and modelling techniques to predict the long-term evolution of a cementitious repository for the safety assessment of radioactive waste disposal. A flow-through experiment with an artificially fractured cement column sample was carried out, and the evolution of a chemical composition in discharged water and the distribution of mineral components in a solid matrix, which was dominated by the dissolution of portlandite and calcium-silicate hydrate (C-S-H), were observed. A coupling transport and chemical equilibrium calculation code, which includes a thermodynamic incongruent dissolution model of C-S-H, was developed to predict the alteration of the fractured cement materials. The advection transport of a component in a solution within a crack and the diffusion of a component in a solid matrix were modelled in the calculations. With the proposed model, the possible alteration of cement materials along a crack was described.     

Rheology of fiber-reinforced cementitious materials

An improved understanding of the influence of fibers on the rheology of cementitious systems is needed so that fiber reinforcement can be used effectively. However, conventional rheometers are not suitable for testing stiff fiber-reinforced materials. In this study, a parallel plate rheometer that is capable of evaluating the rheology of stiff fiber-reinforced cement paste and mortar systems was designed and built. The governing equations for the rheometer were derived and experimental procedures were developed that yielded reproducible results. A comparative analysis of the custom-built parallel plate rheometer, a commercial rheometer and the values reported in the literature, indicated that the measurements obtained using the rheometer were reasonable. The rheometer was then used to evaluate the rheology of a variety of cement paste systems, including stiff steel fiber-reinforced cement pastes.     

Investigations on the cementitious grouts containing supplementary cementitious materials

Concrete structures often exhibit distress during their service life due to one or more of the following causes: faulty design, use of substandard materials, poor construction, misuse or overloading. Of these, poor construction practices result in porous concrete which necessitates remedial and strengthening measures, if a structure is to meet the strength, serviceability and durability requirements, for which it has been originally designed. However, before strengthening the structure, the integrity of the concrete should be restored, which is often carried out through grouting using cement slurry, followed by grouting with epoxy or low viscous monomer. Grouting using cement slurry to fill up the voids in porous matrix of the concrete is the most simple and economical method, requiring less capital investment and skills. However, there are problems associated with cement grouting, such as shrinkage, stability of the grouts, etc. These problems associated with cement grouting can be solved by using cementitious grouts, i.e., grouts containing supplementary cementitious materials (SCMs) such as fly ash, ground granulated blast furnace slag (GGBS) and silica fume (SF) as admixtures in cement grout. This paper gives the results of the investigation undertaken to evaluate the flow, strength and durability characteristics of the cementitious grouts.     

固废基路用胶凝材料配制及性能研究

以粉煤灰、电石渣、脱硫石膏、钢渣、矿渣等工业固体废弃物为主要原料,配制道路水稳层路用胶凝材料,全部或部分替代路用水泥,开展原料预处理加工和配比优化实验,考察原料细度和原料配方对胶凝试块强度的影响。结果表明：通过粉磨机械力活化,可明显增强固废的胶凝活性,其中,适宜的粉煤灰、电石渣、脱硫石膏、矿渣粉的中位径D50范围为8～12μm,而适宜的钢渣微粉中位径D50为5～8μm之间;通过固废超微粉原料间配方优化,可获得7 d和28 d强度分别为29.3 MPa和37.5 MPa的70%固废掺加量的无机胶凝粉体材料,该固废优化配比为粉煤灰：电石渣：脱硫石膏：钢渣：矿渣=31.8∶13.6∶9.1∶27.3∶18.2,按比例加入30%P·S42.5水泥,在此配方体系下,胶砂试块强度可以达到或超过纯路用32.5水泥强度指标。     

Enhanced mechanical properties of wood ash and fly ash as supplementary cementitious materials

The incorporation of fly ash (FA) and wood ash (WA) in concrete as supplementary cementitious materials (SCM) was studied. The chemical composition of ordinary Portland cement, FA and WA was determined according to ASTM C-114. SEM and optical microscopy were used for the analysis of concrete. Setting time, compressive strength, water absorption and acid resistance of the concrete with different percentages of SCM ranging from 0 to 60% were evaluated. The results obtained showed that setting time and rate of water absorption increased with the increase in percentage of SCM. After 7 and 28 days, the compressive strength of concrete with 20% FA as SCM was higher than that with substitution with 20% WA. Resistance of concrete against sulphate attack increased with an increase in the percentage of FA. It was found that incorporating more than 20% WA resulted in a decrease in sulphate attack resistance.     

矿渣-粉煤灰水泥基复合材料的流变性能及适用流变模式的研究

测试了不同矿渣-粉煤灰比例的水泥浆的流变性,并用普通硅酸盐水泥进行对比。对水泥浆在布氏粘度计不同转速的黏度进行测试,并对测量的剪切应力和剪切速率进行三种流变模式的拟合,对比分析水泥浆适用的流变模式。结论为:在相同水灰比情况下,矿渣粉煤灰比例为80/20的矿渣粉煤灰水泥,其表观粘度和屈服应力均小于普通硅酸盐水泥。相对于宾汉塑性模式和幂率模式,赫切尔-巴尔克流变模式对两种类型水泥浆体的流变曲线拟合的效果均更好。     

Investigation of the carbonation front shape on cementitious materials: Effects of the chemical kinetics

Carbonation depth-profiles have been determined by thermogravimetric analysis and by gammadensitometry after accelerated carbonation tests on ordinary Portland cement (OPC) pastes and concretes. These methods support the idea that carbonation does not exhibit a sharp reaction front. From analytical modelling, this feature is explained by the fact that the kinetics of the chemical reactions become the rate-controlling processes, rather than the diffusion of CO₂. Furthermore, conclusions are drawn as to the mechanism by which carbonation of Ca(OH)₂ and C-S-H takes place. Carbonation gives rise to almost complete disappearance of C-S-H gel, while Ca(OH)₂ remains in appreciable amount. This may be associated with the CaCO₃ precipitation, forming a dense coating around partially reacted Ca(OH)₂ crystals. The way in which CO₂ is fixed in carbonated samples is studied. The results indicate that CO₂ is chemically bound as CaCO₃, which precipitates in various forms, namely: stable, metastable, and amorphous. It seems that the thermal stability of the produced CaCO₃ is lower when the carbonation level is high. It is also shown that the poorly crystallized and thermally unstable forms of CaCO₃ are preferentially associated with C-S-H carbonation.     

Interaction of silylated superplasticizers with cementitious materials

Incorporation of silane groups into polycarboxylate superplasticizer (PCE) opens a new technical approach to improve properties of PCE, such as to enhance the adsorption of PCE on cement and hence the dispersing performance, to minimize the retardation effect, and potentially to increase mechanical strength of cement mortars. Silylated PCEs were synthesized using silane monomer via radical copolymerization. Dispersing effect, adsorption behaviors, and influences on mortar strength of these polymers were systematically investigated. Results show that increasing the incorporation of sliane groups in PCE promotes the adsorption of polymer on cement surface and hence leads to good fluidity retention capability. Furthermore, sulfate resistance ability of silylated PCE is superior due to stronger chemical adsorption of polymer on cement surface. The retardation effect of PCE is minimized by the introduction of silane groups. The addition of silylated PCE significantly increases 3 days compressive strength without notably affecting the long‐term strength. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44161.