Thermodynamic equilibrium calculations in cementitious systems

This review paper aims at giving an overview of the different applications of thermodynamic equilibrium calculations in cementitious systems. They can help us to understand on a chemical level the consequences of different factors such as cement composition, hydration, leaching, or temperature on the composition and the properties of a hydrated cementitious system. Equilibrium calculations have been used successfully to compute the stable phase assemblages based on the solution composition as well as to model the stable phase assemblage in completely hydrated cements and thus to asses the influence of the chemical composition on the hydrate assemblage. Thermodynamic calculations can also, in combination with a dissolution model, be used to follow the changes during hydration or, in combination with transport models, to calculate the interactions of cementitious systems with the environment. In all these quite different applications, thermodynamic equilibrium calculations have been a valuable addition to experimental studies deepening our understanding of the processes that govern cementitious systems and interpreting experimental observations. It should be carried in mind that precipitation and dissolution processes can be slow so that thermodynamic equilibrium may not be reached; an approach that couples thermodynamics and kinetics would be preferable. However, as many of the kinetic data are not (yet) available, it is important to verify the results of thermodynamic calculations with appropriate experiments. Thermodynamic equilibrium calculations in its different forms have been applied mainly to Portland cement systems. The approach, however, is equally valid for blended systems or for cementitious systems based on supplementary cementitious materials and is expected to further the development of new cementitious materials and blends.     

Effects of nano-SiO2 particles on the mechanical and microstructural properties of ultra-high performance cementitious composites

In this research the effects of nano-SiO₂ particles on the mechanical performance, hydration process and microstructure evolution of ultra-high performance cementitious composites were investigated by different methods. The results showed that the compressive and flexural strength increased with the increase of the nano-SiO₂ content up to 3% and due to agglomeration of nano-SiO₂ particles, the mechanical properties decreased slightly when the nano-SiO₂ content was more than 3%. The hydration process was accelerated by the addition of nano-SiO₂. The porosity and the average pore diameter decreased with the increase of the nano-SiO₂ content and aging. The microstructure was more homogenous and dense for nano-SiO₂ specimens as compared to the control specimen. All of these improvements could be mainly attributed to the pozzolanic and filler effects of nano-SiO₂.     

Effect of molecular architecture of polycarboxylate ethers on plasticizing performance in alkali-activated slag paste

A range of anionic and cationic polycarboxylate ether (PCE) plasticizers with different molecular architectures (molecular weights, side chain lengths, and ratios of side chain density to backbone charge) are synthesized and tested to determine their effects on the rheological properties of fresh alkali-activated slag (AAS) pastes. A higher density of long side chains in the lower molecular weight polymers can provide a noticeable yield stress reduction, indicating a mild increase in workability compared to that of an unmodified AAS paste. It is hypothesized that side chains may have two important roles, i.e., providing steric hindrance to disperse particles after PCE adsorption on a particle surface, and also providing partial protection of the backbone charges against attachment of one PCE molecule to two or more slag particles, which is called bridging. This enhances the likelihood of adsorption on single particles, and thus increases the plasticizing action. A very similar plasticizing mechanism is observed for PCEs with similar structures but differing charge signs (cationic/anionic), which indicates that both anionic and cationic adsorption sites are available on AAS particle surfaces. The measured flow curves of all pastes are well described by the Herschel–Bulkley model with shear thinning behavior.     

On the performance of molecular dynamics applications on current high-end systems

The effective exploitation of current high performance computing (HPC) platforms in molecular simulation relies on the ability of the present generation of parallel molecular dynamics code to make effective utilisation of these platforms and their components, including CPUs and memory. In this paper, we investigate the efficiency and scaling of a series of popular molecular dynamics codes on the UK's national HPC resources, an IBM p690+ cluster and an SGI Altix 3700. Focusing primarily on the AMBER, DL_POLY and NAMD simulation codes, we demonstrate the major performance and scalability advantages that arise through a distributed, rather than a replicated data approach.     

Self-healing performance of aged cementitious composites

This study investigates the autogenous self-healing capability of one-year-old engineered cementitious composites (ECC) with different mineral admixtures to understand whether self-healing performance in late ages is similar to that of early ages. Sound and severely pre-cracked specimens were subjected to different environmental conditions including water, air, “CO₂-water,” and “CO₂-air” for one year plus 90 days of initial curing. Self-healing performance of ECC mixtures was assessed in terms of crack characteristics, electrical impedance testing, rapid chloride permeability testing and microstructural analysis. Laboratory findings showed that the presence of water is crucial for enhanced autogenous self-healing effectiveness, regardless of mixture composition. “CO₂-water” curing resulted in the best self-healing performance of all curing conditions, which was confirmed with results from different performance tests throughout the experimental study. By further curing specimens under “CO₂-water” (depending on the ECC mixture composition), cracks as wide as half a millimeter (458 μm) were easily closed by autogenous self-healing within only 30 days of further curing, and all cracks closed completely after 90 days. Because high levels of CO₂ emission are a global problem, the effectiveness of “CO₂-water” curing in closing microcracks of aged cementitious composites specimens through autogenous self-healing can help reduce the increasing pace of CO₂ release. The results of this study clearly suggest that late-age autogenous self-healing rates of ECC specimens can be significantly enhanced with proper further environmental conditioning and mixture design.     

RAPID for high-performance computing systems: architecture and performance evaluation

The limited bandwidth and the increase in power dissipation at longer communication distances and higher bit rates will create a major communication bottleneck in high-performance computing systems (HPCS), affecting not only their performance, but also their scalability. As a solution, we propose an optical-interconnect-based architecture for HPCS called reconfigurable all-photonic interconnect for parallel and distributed systems (RAPID) that alleviates the bandwidth density, optimizes power consumption, and enhances scalability. We also present two cost-effective design alternatives of the architecture, a modified version called M-RAPID and an extended version called E-RAPID that minimizes the cost of the interconnect based on the number of transmitters required. We perform a detailed simulation of the proposed RAPID architecture and compare it to several electrical HPCS interconnects. Based on the performance study, RAPID architecture shows 30%-50% increased throughput and 50%-75% reduced network latency as compared to HPCS electrical networks.     

Effects of deicers on the performance of concrete pavements containing air-cooled blast furnace slag and supplementary cementitious materials

This study investigates the effects of continuous deicer exposure on the performance of pavement concretes. For this purpose, the differences in the compressive strength, the changes in the dynamic modulus of elasticity (DME) and the depth of chloride ingress were evaluated during and after the exposure period. Eight different concrete mixtures containing two types of coarse aggregates (i.e. air-cooled blast furnace slag (ACBFS) and natural dolomite) and four types of binder systems (i.e. plain Type I ordinary portland cement (OPC) and three combinations of OPC with fly ash (FA) and/or slag cement (SC)) were examined. These mixtures were exposed to three types of deicers (i.e. MgCl₂, CaCl₂, and NaCl) combined with two different exposure conditions (i.e. freezing-thawing (FT) and wetting-drying (WD)). In cold climates, these exposure conditions are the primary durability challenges that promote the physical deterioration of concrete pavements. The results indicated that among the studied deicers, CaCl₂ had the most destructive effect on the tested concretes while NaCl was found to promote the deepest level of chloride ingress yet was shown to have the least damaging impact on concretes. The microstructure evaluation revealed that the mechanism of concrete deterioration due to the deicer exposure involved chemical reactions between the deicers and concrete hydration products. The use of FA or SC as partial replacements for OPC can offset the detrimental effects of both deicers and FT/WD cycles.     

The effect of superplasticizers on the mechanical performance of concrete made with fine recycled concrete aggregates

It is considered that using crushed recycled concrete as aggregate for concrete production is a viable alternative to dumping and would help to conserve abiotic resources. This use has fundamentally been based on the coarse fraction because the fine fraction is likely to degrade the performance of the resulting concrete. This paper presents results from a research work undertaken at Instituto Superior Técnico (IST), Lisbon, Portugal, in which the effects of incorporating two types of superplasticizer on the mechanical performance of concrete containing fine recycled aggregate were evaluated. The purpose was to see if the addition of superplasticizer would offset the detrimental effects associated with the use of fine recycled concrete aggregate.The experimental programme is described and the results of tests for splitting tensile strength, modulus of elasticity and abrasion resistance are presented. The relative performance of concrete made with recycled aggregate was found to decrease. However, the same concrete with admixtures in general exhibited a better mechanical performance than the reference mixes without admixtures or with a less active superplasticizer. Therefore, it is argued that the mechanical performance of concrete made with fine recycled concrete aggregates can be as good as that of conventional concrete, if superplasticizers are used to reduce the water–cement ratio of the former concrete.     

制备工艺对微生物水泥基材料性能的影响

采用相同方向和不同方向两种不同制备工艺制备出高为50 cm,直径为5 cm的微生物水泥基材料,然后分别对两种制备工艺下相同位置处砂柱进行抗压强度、方解石含量、渗透系数、超声波速和微观结构进行研究分析。实验结果表明,相同方向工艺下胶结的砂柱抗压强度从7.80 MPa降低到0.88 MPa,方解石含量从394.62 kg/m3降低至103.03 kg/m3,渗透系数从0.18×10-2cm/s增加到0.10×10-2cm/s,超声波速从3.33 km/s降低到2.08 km/s;采用不同方向工艺下胶结的砂柱抗压强度从6.23 MPa降低到0.23 MPa,方解石含量从382.39 kg/m3降低至76.20 kg/m3,渗透系数从0.45×10-2cm/s增加到1.79×10-2cm/s,超声波速从2.46 km/s降低到1.89 km/s;采用相同方向制备工艺制备的微生物水泥基材料均匀性相比于采用不同方向制备工艺制备的微生物水泥基材料差。     

Evaluation of early age cracking characteristics in cementitious systems

Early age cracking has re-emerged as an important issue in modern concretes, and it has its impact in developing new formulations for high strength and repair. Adequate performance with respect to cracking cannot be properly assessed on the basis of free shrinkage tests. More advanced methodologies need to be developed and applied, to consider in addition to the strains obtained also the stresses and stress relaxation. The present paper presents an overview of the testing techniques and methodologies of early age cracking. It classifies the tests into four categories: ring, plate, longitudinal and substrate restraint and assesses their significance and limitations.     

Error sources and their impact on the performance of dual-wedge beam steering systems

This paper presents analytical and numerical results that elucidate the impact of error sources on the performance of dual-wedge beam steering systems. Different types of error sources are considered. Specifically, we investigate optical distortions in the pattern scanned out by a single ray through a pair of rotatable wedge elements with slightly different parameters. Case examples are given to reveal the difference between the distorted patterns and the patterns produced by a pair of perfectly equal wedge elements. Furthermore, nonparaxial ray tracing is performed to investigate the impact of assembly errors on the accuracy of steering a laser beam to a remote target. We found that a misalignment in a bearing axis of rotation with respect to the system optical axis will result in a change of beam deflection off-axis that gives rise to a severe decrease of pointing accuracy to a level well below the level that a tilted wedge prism may attain.     

Effects of target structure on the performance of laser time-of-flight velocimeter systems

The consequences of partially developed speckle and the effects giving rise to bias errors in velocity determination are discussed with respect to robustness of a classical laser time-of-flight velocimetry (LTV) system. It is demonstrated that surface regimes exist that define the degree of partially developed speckle. These regimes are explored both theoretically and experimentally; surface models are developed to predict the resulting cross covariance from which velocity estimations can be obtained. The surface models describe the behavior of the cross covariance caused by reflection structures and with disparate lateral-roughness scales. In particular, it is shown that it is possible to obtain a twin-Gaussian cross covariance as a result of the presence of partially developed speckle. All models described are compared with experimental observations of the cross covariance for differing surface regimes. The objects are solid targets having lateral spatial correlations in reflection amplitude, height, or both, generally giving rise to partially developed speckle. In almost all cases good agreement with the corresponding theoretical predictions are found. Decorrelation caused by velocity misalignment is shown to shift the peak of the cross covariance significantly, giving a velocity bias. A corresponding theoretical model is developed and verified experimentally. Cross-talk measurements have been performed and compared with a theory developed herein. Both measurements and theory indicate that only spot sizes comparable with or larger than their corresponding separation will lead to a measurable peak shift of the time lag for the maximum of the cross covariance. We conclude that LTV systems will provide accurate velocity estimates under a wide variety of practical conditions.     

水分对分子筛结构及吸附性能的影响

分子筛是变压吸附制氧过程中最重要的组分,使用中该组分常出现吸水失效问题,极大地影响了氧产量及浓度.针对此问题,本文以HD-1型和PU-8型分子筛为原料,通过在不同温度(100、200、300、400℃)下活化得到不同含水量的分子筛,并通过对不同材料的结构形貌、热性能及吸附性能的表征分析了水分对分子筛结构及吸附性能的影响.研究结果表明:水分不会破坏分子筛的骨架结构,但会增大晶胞参数,并导致HD-1型分子筛中阳离子迁移;分子筛中的物理吸附水和化学结晶水会影响其吸附性能,其中物理吸附水的影响最大;活化有效提高了分子筛的吸附性能,且温度越高,含水量越少,吸附性能越好;在150~180℃下活化,将物理吸附水脱附后,分子筛的吸附能力可恢复70%,可在此温度下进行吸附器内低温活化;在400℃下活化,脱除化学结晶水,分子筛吸附性能可100%恢复,400℃为两种分子筛的最佳活化温度.     

Effects of fabric parameters on the tensile behaviour of sustainable cementitious composites

The mechanical behaviour of fabric-reinforced composites can be affected by several parameters, such as the properties of fabrics and matrix, the fibre content, the bond interphase and the anchorage ability of fabrics. In this study, the effects of the fibre type, the fabric geometry, the physical and mechanical properties of fabrics and the volume fraction of fibres on the tensile stress–strain response and crack propagation of cementitious composites reinforced with natural fabrics were studied. To further examine the properties of the fibres, mineral fibres (glass) were also used to study the tensile behaviour of glass fabric-reinforced composites and contrast the results with those obtained for the natural fabric-reinforced composites. Composite samples were manufactured by the hand lay-up moulding technique using one, two and three layers of flax and sisal fabric strips and a natural hydraulic lime (NHL) grouting mix. Considering fabric geometry and physical properties such as the mass per unit area and the linear density, the flax fabric provided better anchorage development than the sisal and glass fabrics in the cement-based composites. The fabric geometry and the volume fraction of fibres were the parameters that had the greatest effects on the tensile behaviour of these composite systems.     

Effect of superplasticizers on the rheological properties of cements

In this paper, the rheological properties of cement pastes made with different types of cement and superplasticizers are discussed. As a tool for the discussion, experiments involving dispersion of cement particles, fluidity, viscosity, yield stress and zeta potential have been conducted. The results obtained show that the chemical compositions of the cements such as C₃A and sulfate content, alkali and ground lime content are important features controlling the rheology of cement pastes. Three types of sulfonated superplasticizers (lignosulonate-based, melamine formaldehyde sulfonic acid, naphthalene formaldehyde sulfonic acid) and one type of polycarboxylic acid polymer were used. The results revealed that the mechanism by which these polymers disperse cement particles differs fundamentally. Sulfonated superplasticizers induced a negative charge on cement particles dispersing them by electrostatic repulsion, whereas with the polycarboxylate-based polymer the dispersion mechanism is mainly controlled by steric hindrance. A model for the adsorption of superplasticizer on a cement particle is proposed.     

Synthesis,characterisation and performance of piezo-resistive cementitious nanocomposites

Smart material reinforced non-destructive structural health monitoring technique has been evolving as the most predominated route for assessing the performance of the civil structures. In the present study, multiwalled nanotubes (MWNT) were suitably incorporated into the cement matrix, which act as actively embedded sensor for monitoring real-time flaws in structures. Initially, the stable homogenous MWNT dispersion was prepared by using ionic surfactant technique with high-intensity ultrasonic agitation process. Since, a suitable and adequate synthesis procedure to incorporate MWNT in cement matrix is essential, but complicated, the role of amplitude and frequency of sonication on dispersion of nanotubes was categorically evaluated. Further, this paper focuses to find out the effect of surfactant on MWNT dispersion by using the UV Visible spectroscopy and by evaluating the effective hydro-dynamic diameter. Based on micromechanics based analytical model, the influence of the interface layer thickness and geometrical configuration of nanotubes on the electrical conductivity of cement nano-composite are also analyzed. Further, the electrical conductivity of MWNT incorporated cement system, as developed in the present study, is measured using four probe method. Piezo-resistivity of the oven dried samples is measured to evaluate the change in potential drop under cyclic loading regime. It is found that the efficiency of the piezo-resistive strain sensors greatly depends on synthesis process and the circuit system. Appropriately proportioned and properly synthesized MWNTs incorporated in cement matrix were capable of providing consistent and steady response under the variable external stress. Thus, the material can be used as embedded sensor for health monitoring and identifying initiation of any damage in reinforced concrete structure.     

Physico-chemical deformations of solidifying cementitious systems: multiscale modelling

At early stages of hydration and in autogenous conditions (no mass transfer with the outside), solidifying cementitious systems exhibit dimensional variations following two main processes: Le Chatelier contraction (also called chemical shrinkage) and self-desiccation shrinkage causing autogenous shrinkage. Chemical shrinkage results from the difference between the specific volumes of reactants (anhydrous cement and water) and hydration products. Early-age autogenous shrinkage is generally attributed to the development of a negative capillary pressure in the porous network related to the water consumption by the hydration reactions. If restrained, deformations associated to these shrinkages can induce the development of internal stresses high enough to generate cracking of the hardening material. The purpose of this study is to propose a multiscale approach to model the rate of self-desiccation shrinkage of cementitious materials at very early-age, between 0 and 48 h. Within the first hours, Le Chatelier contraction is computed from a formulation suggested in a later work which is based on the chemical equations of hydration and the specific volume of each phase. Then, when the setting of the cement paste takes place, the autogenous shrinkage is calculated according to the evolution of the capillary pressure and the stiffness of the cement paste. The stiffness is calculated by applying a classical homogenization method. Computed results are discussed and analyzed. Good agreements between experiments and simulations are achieved and a sensitivity study is performed to assess the influence of the cement fineness and the aggregate volume fraction on early-age autogenous strain.     

Multi-response optimization of post-fire performance of strain hardening cementitious composite

This paper presents results of an experimental program conducted to optimize the post-fire performance of Strain Hardening Cementititous Composites (SHCC) using Taguchi approach with utility concept. The experiments were first undertaken by determining nine SHCC mixes using a standard L9 (3⁴) orthogonal array of four parameters and each parameter with three levels. The four parameters of SHCC mixes included fly-ash/binder ratio, sand/binder ratio, water/binder ratio and fiber proportions. The responses of SHCC to be optimized were tensile strain capacity, compressive strength and post-fire compressive strength after subjected to 200 °C, 400 °C, 600 °C and 800 °C of isothermal heating. Utility concept was introduced to simplify the multi-response problem into mono-response question together with Taguchi method. The role of different parameters on the composite responses of SHCC was examined. Furthermore, an optimal SHCC mix to maximize multi-responses was determined based on statistical analysis and validated by additional confirmation tests.     

Durability of proprietary cementitious materials for use in wastewater transport systems

Several methods and materials, such as high performance coatings, fiber glass reinforced linings, special mortars, brick or ceramic linings, etc., are used to protect concrete from sulfuric acid attack in a sewage environment. Two proprietary high alumina cementitious lining materials, CC and SC, are recent additions to the list of protective materials used in the Arabian Gulf. This paper documents the findings of a laboratory study under accelerated conditions as well as a two-year field study of CC and SC in a wastewater lift station in Jubail, Saudi Arabia. In the laboratory investigations, 50 mm cube mortar specimens prepared using: (1) SC, (2) CC, (3) Type I+8% silica fume cement, (4) Type I+20% fly ash cement and (5) Type I cement were exposed to 2% sulfuric acid for 150 days. The laboratory specimens were tested for weight reduction, compressive strength, sulfate content, and alkalinity. In the field, the walls and ceiling of a wastewater manhole were coated using the proprietary lining materials, SC and CC, and were exposed to a normal sewage service environment. Performance of the liner materials was monitored for sulfate content and alkalinity after 6, 12 and 24 months of exposure. The analysis and evaluation test data generated from the accelerated laboratory study and the field study, which lasted for 24 months, showed that SC performed better than other materials tested in this investigation.