Primary particle size and agglomerate size effects of amorphous silica in ultra-high performance concrete

Silica fume is widely used in ultra-high performance concrete (UHPC). However, it is a by-product in the industrial silicon production and therefore far from an optimized additive. Silica fume improves the compressive strength, but its detailed reaction mechanisms in concretes with low water/cement ratios are not yet fully understood. This study focuses on the influence of primary particle sizes and sizes of agglomerates of different amorphous silicas in UHPC. As a reference system, wet-chemically synthesized silica was used with very high purity, defined particle sizes, narrow primary particle size distributions and controllable agglomerate sizes. The obtained data were compared to silica fume. The results indicate that non-agglomerated silica particles produce the highest strength after 7 d, but a clear dependence on primary particle sizes, as suggested by calculations of packing density, was not confirmed. UHPC may be improved by incorporating an ameliorated dispersion of silica e.g. through commercial silica sols.     

Colloidal drug probe: Method development and validation for adhesion force measurement using Atomic Force Microscopy

This study demonstrates a novel technique of preparing drug colloid probes to determine the adhesion force between a model drug salbutamol sulphate (SS) and the surfaces of polymer microparticles to be used as carriers for the dispersion of drug particles from dry powder inhaler (DPI) formulations. Model silica probes of approximately 4 μm size, similar to a drug particle used in DPI formulations, were coated with a saturated SS solution with the aid of capillary forces acting between the silica probe and the drug solution. The developed method of ensuring a smooth and uniform layer of SS on the silica probe was validated using X-ray Photoelectron Spectroscopy (XPS) and Scanning Electron Microscopy (SEM). Using the same technique, silica microspheres pre-attached on the AFM cantilever were coated with SS. The adhesion forces between the silica probe and drug coated silica (drug probe) and polymer surfaces (hydrophilic and hydrophobic) were determined. Our experimental results showed that the technique for preparing the drug probe was robust and can be used to determine the adhesion force between hydrophilic/hydrophobic drug probe and carrier surfaces to gain a better understanding on drug carrier adhesion forces in DPI formulations.     

PROBLEMS ENCOUNTERED IN ACCURATE DETERMINATION OF INTERPARTICLE FORCES IN ORDERED MIXES

The resistance of ordered mixtures to component segregation during processing is dependent upon the adhesion forces between the fine adherent particles and the carrier component within ordered units. A method is described that allows the adhesion force acting on individual adherent particles to be determined, using ultracentrifugation to apply a wide range of separating forces. Individual ordered units are located at the end of fine holes drilled through a thin brass plate and fixed in position with transparent adhesive tape.The plate is mounted in a specially constructed holder during ultracentrifugation, the axis of the holes being parallel to the centripetal acceleration, photomierographs, using a modified microscope with an enhanced depth of field of view, of the distribution of adherent fines were made after each period of ultracentrifugation. Comparison of photomicrographs enabled the determination of the numbers of fine adherent particles dislodged after each period of ultracentrifugation. Determination of the numbers of dislodged adherent fines was difficult due to problems with changes in the orientation of ordered units during ultracentrifugation and the resolution and focus of the modified microscope. Examination of scanning electron micrographs showed that changes in orientation of ordered units were due to flattening of the contact areas between the brass plate and ordered units.     

A controlled formation of cage-like nanoporous hollow silica microspheres

Cage-like hollow silica microspheres composed of mesoporous silica nanoparticles and macroporous interparticle voids were fabricated via the latex-surfactant dual templates route, simply by controlling the surfactant additions below its critical micelle concentration. The surface area, pore volume increase, and both the mesopore and macropore sizes decrease with the increase in surfactant amount. The surfactant cations preferentially assemble with negatively charged silica species generated by the hydrolysis and condensation of tetraethyl orthosilicate to form composite silica-surfactant nanoparticles. The electrostatic repulsion between the silica-surfactant composite nanoparticles and negatively charged polystyrene (PS) beads is smaller than that between surfactant-free silica and PS, favoring the deposition of composite nanoparticles on the surface of PS template. In the meantime, the deposited nanoparticles also have reduced repulsion from their neighbors, favoring their bridging to form silica shells. The more the surfactant is used, the less the repulsion exists among the composite particles and the smaller the interparticle macroporous voids are.     

A particle packing model for sand–silt mixtures with the effect of dual-skeleton

The study of particle packing models for binary mixtures is important in the field of granular materials, from both theoretical and practical perspectives. A number of particle packing models have been developed for predicting packing density (or void ratio) of a binary mixture. However, the measured results and the predicted values do not always agree with each other, particularly in the range of fines content between 25 and 50%. It is postulated herein that the discrepancies between the measured results and the predicted values are primarily due to the incorrect assumptions used in the existing models. In the existing models, the packing density is determined from one of the following two assumed mechanisms of particle mixing: (1) the mixed packing has a dominant large-particle skeleton and the small particles fill the voids of the large-particle skeleton, or (2) the mixed packing has a dominant small-particle skeleton and the large particles are embedded in the small-particle skeleton. It is obvious that the first assumed mechanism is only applicable for mixtures with low fines content, whereas the second assumed mechanism is only applicable to mixtures with high fines content. Therefore, the predictions from existing models are unsuitable for mixtures with medium fines content, such as a mixture of fines content between 25 and 50%. In this study, a 3-D discrete element simulation is carried out to show that, for a mixture of medium fines content, the packing structure has a dual-skeleton, which is neither dominated by a large nor small-particle skeleton. Then, we postulate that, in the mixed packing, both mechanisms can take place: filling of small particles and embedment of large particles. The concepts of “dual-skeleton index” and “index size” are proposed to account for the interactive effects of filling and embedment. Based on this postulation, we develop an analytical method, which has the capability of predicting minimum void ratio for sand–silt mixtures with various fines contents. The developed model is then validated by the experimental results obtained from 16 types of sand–silt mixtures.     

Effects of silica powder and cement type on durability of ultra high performance concrete (UHPC)

Ultra-high performance concrete (UHPC) achieves extraordinary strength characteristics through optimization of the particle packing density of the cementitious matrix. The dense matrix also promotes exceptional durability properties and is arguably the biggest benefit of the material. A durable concrete enables structures to last longer, reduces the cost of maintenance and helps achieve a significantly more sustainable infrastructure. To assess the durability of UHPC, the performance of several non-proprietary blends are investigated by assessing the materials' resistance to freeze-thaw cycles, ingress of chlorides as well as the presence and distribution of air voids. The main experimental variables are cement type and the quantity of silica powder, which varies from 0% to 25% of the cement weight. All mixes displayed negligible chloride ion penetration and high resistance to freeze-thaw with mass loss well below the limit in over 60 cycles of freeze-thaw. Analysis of the test data indicates that the silica powder content has little influence on performance.     

Eco-efficient ultra-high performance concrete development by means of response surface methodology

The research described in this paper represents a statistically based model with the help of response surface methodology (RSM) aiming to study the applicability of this method to ultra-high performance concrete (UHPC) mixture design and its optimization. Besides, the effects of silica fume, ultra-fine fly ash (UFFA) and sand as three main variable constituents of UHPC on workability and compressive strength as the main performance criteria and responses of this high-tech material were investigated. The models proposed here demonstrate a perfect correlation among variables and responses. Furthermore, through performing a multi-objective optimization, cement and silica fume, as two main constituents of UHPC affecting its eco-efficiency and cost, were substituted by UFFA and sand as much as possible. Finally, an eco-efficient UHPC with cement and silica fume content of 640 kg/m³ and 56.3 kg/m³ respectively and compressive strength and flow diameter of 160.3 MPa and 19 cm was developed.     

Mixing of high performance concrete: effect of concrete composition and mixing intensity on mixing time

At present the use of ultra high performance concrete (UHPC) is limited to a small number of special applications. This is essentially due to the time needed to blend the UHPC mix which is considerably longer than required for ordinary concrete. The present investigations aimed at the determination of the composition features which result in the longer mixing time of UHPC. The individual components of the concrete mix (silica fume, quartz flour, water, superplasticizer and coarse aggregate) were systematically varied in order to investigate their effect on the mixing time needed to achieve optimal flow properties. Besides concrete composition, the effect of mixing tool speed was considered. The shortest necessary mixing time (stabilisation time) was calculated from the evolution of the power applied to the tool during mixing. It was confirmed that high w/c values resulted in short stabilisation times. In addition, the contents of silica fume and quartz flour as well as the type of cement and superplasticizer affected the stabilisation time significantly. It was possible to describe the effect of the individual variables in terms of the relative solid concentration ϕ/ϕ_max which depends on concrete composition. This is the ratio of the actual volumetric concentration of solids in the concrete mix ϕ to the maximum possible concentration of solids ϕ_max calculated from the volumes of particles (particle size distribution). The relative solid concentration is the basis of a model for the calculation of the mixing time of UHPC of a given composition and for given tool speed. High tool speeds and low actual concentrations of solids shorten the stabilisation time.     

Rice husk ash as both pozzolanic admixture and internal curing agent in ultra-high performance concrete

The present study investigated the effects of mesoporous amorphous rice husk ash (RHA) on compressive strength, portlandite content, autogenous shrinkage and internal relative humidity (RH) of ultra-high performance concretes (UHPCs) with and without ground granulated blast-furnace slag (GGBS) under different treatments. The results were compared with those of UHPCs containing silica fume (SF). Because of the mesoporous structure, RHA can absorb an amount of aqueous phase to decrease the free water content and to supply thereafter water for further hydrations of cementitious materials. Hence, compressive strength of RHA-blended samples is enhanced. The highly water absorbing RHA delays and slows down the decrease in the internal RH (self-desiccation) of UHPCs, and hence strongly mitigates autogenous shrinkage of UHPCs compared to SF. The combination of GGBS and RHA or SF improves the properties of UHPC. These results suggest that RHA acts as both highly pozzolanic admixture and internal curing agent in UHPC.     

Effects of microstructure on restrained autogenous shrinkage behavior in high strength concretes at early ages

Fluorescence microscopic examinations were conducted to identify damages induced by restraining autogenous shrinkage. Characteristics of fluorescent areas and their correspondence to autogeneous shrinkage behavior of high strength concretes were discussed. Silica fume concrete exhibited a greater creep potential when loaded at very early ages. The microstructure in sealed concretes with an extremely low water/binder ratio was porous. The vicinity of aggregate grains was more porous and weaker than the bulk matrix in sealed concretes. In addition, sealed silica fume concretes contained many unhydrated cement particles which were profiled by thin gaps between the core cement particles and the surrounding cement paste matrix. These features of microstructure were not observed in water ponded concretes. The detected fluorescent areas may be defects caused by selfdesiccation and autogenous shrinkage. The flaws had little effects on the development of strength. However, the presence of thin gaps around remnant cement particles may increase creep deformation to relieve internal stresses.     

钢纤维类型对超高性能混凝土高温爆裂性能的影响

为了探寻可以有效改善超高性能混凝土（Ultra-high-performance concrete,UHPC）抗火性能的钢纤维类型,本文试验测定了不同类型钢纤维（3种普通钢纤维和2种来自于废旧轮胎的再生钢纤维）增韧UHPC及空白组混凝土的从常温至800℃高温爆裂行为和断裂能。结果显示,未掺入任何钢纤维的空白组UHPC试件全都发生了严重高温爆裂,钢纤维可以显著减轻其高温爆裂但却不能避免爆裂的发生,而掺入端钩型普通工业钢纤维（长度为35 mm,直径为0.55 mm）的UHPC呈现出最优的抗高温爆裂性能,其次是掺入未附着橡胶颗粒（RSF）的再生钢纤维（RSFR）增韧UHPC。可见,钢纤维自身性能特征显著影响了钢纤维增韧UHPC的高温爆裂,相同掺量情况下混凝土单位体积内分布密度较大的钢纤维或者分布密度较小但可以显著增加混凝土断裂韧性（断裂能）的钢纤维比较适合应用于具有较高抗火要求的UHPC结构中。     

On the compressive strength development of high-performance concrete and paste—effect of silica fume

The compressive strength development of sealed high-performance concrete and paste specimens, with and without silica fume, have been studied from 1 day and up to 4 years. The paste and concrete specimens were prepared in such a way that segregation was avoided and the silica fume became well dispersed. Under these conditions silica fume increased the strength of paste just as much as it increased the strength of concrete. It appears that the enhancing effect of silica fume on concrete strength is due to an improved strength of the paste phase as a whole, and not due to an improved bond strength between the paste phase and the aggregate particles, as has been suggested earlier. The concretes and the pastes with 10% silica fume appeared to loose strength over a period of time before the strength increased again.     

Effects of cold-bonded fly ash aggregate properties on the shrinkage cracking of lightweight concretes

This paper presents an experimental study on the restrained shrinkage cracking of the lightweight concretes made with cold-bonded fly ash lightweight aggregates. Two types of fly ash having different physical and chemical properties were utilized in the production of lightweight aggregates with different strengths. Afterwards, lower strength aggregates were also surface treated by water glass and cement–silica fume slurry to improve physical and mechanical properties of the particles. Therefore, a total of eight concrete mixtures were designed and cast at 0.35 and 0.55 water–cement ratios using four types of lightweight coarse aggregates differing in their surface texture, density, water absorption, and strength. Ring type specimens were used for restrained shrinkage cracking test. Free shrinkage, creep, weight loss, compressive and splitting tensile strengths, and modulus of elasticity of the concretes were also investigated. Results indicated that improvement in the lightweight aggregate properties extended the cracking time of the concretes resulting in finer cracks associated with the lower free shrinkage. Moreover, there was a marked increase in the compressive and splitting tensile strengths, and the modulus of elasticity.     

Adhesive interaction of elastically deformable spherical particles

Two spherical particles that attract each other by van der Waals volume forces and can undergo deformation as a result of the attraction are considered. Small deformations of such particles can be described by the solution of the Hertz problem. The deformation of particles, in turn, alters the force of attraction between them. It has been established that the relationship between the adhesion and elasticity of the indicated particles is determined by the degree to which these particles deform and that the adhesion force acting between the particles depends on their elasticity, size, and the Hamaker constants.     

Cellulosic fines: Properties and effects

Any cellulosic pulp consists of particles of different dimensions. When trying to understand and control its properties, it is important to consider not only the bulk amount of long fibres, but also the material known as ‘fines’, which may comprise between 1 and 40% of a pulp. These fines have a great impact on the behaviour of pulp, on its processing, and on the characteristics of the resulting products. We compiled a review of research efforts to characterise the fines fraction by origin, morphology, and chemical composition, and to evaluate the fines’ effects especially in papermaking. The main feature of fines is the large specific surface area associated with their size. Their chemical constitution, particularly their charge, and the magnitude of their surface are the basis for their interactions with other pulp components such as extractives, fillers, and retention aids. The surface of fines affects drainage, as well as sheet density and strength. Several optical paper properties are influenced by the morphology of fines and by their chemical composition, which deviates from that of the long fibre fractions. The targeted utilisation of fines is a potential control variable in papermaking applications in order to obtain desirable paper properties.     

再生钢纤维增韧超高性能混凝土的力学性能

采用来自于废旧轮胎的两种再生钢纤维制备含粗骨料的超高性能混凝土,并测定其抗压强度、劈裂抗拉强度、断裂能和静弹性模量等力学性能,空白组及普通钢纤维增韧超高性能混凝土作对比性能试验。结果显示,未附着橡胶颗粒的再生钢纤维使超高性能混凝土的抗压强度略微下降,降低幅度为3.91%,其余各类型钢纤维均有利于提高超高性能混凝土的力学性能;而附着橡胶颗粒的再生钢纤维显著提高了超高性能混凝土的断裂能,约为普通钢纤维增韧超高性能混凝土的4倍。此外,再生钢纤维对超高性能混凝土的劈裂抗拉强度和静弹性模量的提高效果均优于普通钢纤维。再生钢纤维,尤其是附着橡胶颗粒的再生钢纤维,可以作为一种增韧材料替代普通钢纤维应用到超高性能混凝土工程结构中。      

FTIR spectroscopic study on liquid silica solutions and nanoscale particle size determination

Silica gel is a very important material in technology. Usually tetraethyl orthosilicate (TEOS) is used as precursor in the sol-gel science. But silica gel can also be formed by liquid silica solutions, like alkali silica solutions and silica sols. Due to their importance in paint technology and as a constituent in building material, we investigated alkali silica solutions (especially potassium water glass) and the silica sol Levasil 300. The gel formation process of inorganic silica solutions is quite different to gel formation from TEOS. Gel formation by TEOS is due to polymerization of monomers, whereas the gel formation process of inorganic silica solutions is due to condensation of dense SiO₂ particles with particle diameters of a few nanometers. Fourier transformed infrared (FTIR) spectroscopy proved to be a powerful tool to obtain information on the structure of these liquid silica solutions and their sol-gel processes. Most information could be derived from the main silica peak at ~1070 cm^-1. This peak can be assigned to the TO₃ vibration mode of silica. In silica solutions and in silica gels this peak is composed of different peaks. Compared to earlier studies an additional peak can be found at ~1040 cm^-1 in potassium silica solutions. By comparison of FTIR spectra of related silica glasses and their liquid solutions the peak at ~1027 cm^-1 can be assigned to vibration modes of SiO₂ on the surface of silica particles. The intensities of these individual peaks contain information on the degree of polymerization and the particle size of the silica particles in the liquid solutions. The information about particle size is limited to nanosized particles between 2 and 6 nm.     

Comparison of particle packing models for proportioning concrete constitutents for minimum voids ratio

This paper reports the findings of a study of four particle packing models used to proportion the mix constituents (solid particles) of concrete to produce a minimum voids ratio (or maximum packing density). The models have been compared using laboratory tests and published data. The basic mathematics of the models is discussed, particularly how each model defines the particle size distribution of the solid particles. The models have been applied to both the aggregate (sand and gravel) and the cement phase (PC, PFA, GGBS and limestone fines) and the estimated voids ratio compared with that measured in the laboratory. It was found that the models give broadly the same output and suggest similar combinations of materials to give the minimum voids ratio. Using the materials considered it was found that the largest improvement in voids ratio was achieved with the aggregate phase. The particle sizes of the cements considered here were similar and, as a result, only small improvements in voids ratio could be achieved. It was noted that proportioning concrete mix constituents to minimise voids ratio did tend to produce a harsher mix than normal. However, using the mix suitability factor, proposed by Day (1999), reduced this problem. There are some detail differences between the models suggesting further refinements could be carried out and a modification to one of the models is provided.     

The effect of nanosilica addition on flowability,strength and transport properties of ultra high performance concrete

The experimental study herein presented was conducted aiming to evaluate the influence of nanosilica (nS) addition on properties of ultra-high performance concrete (UHPC). Thermo gravimetric analysis results indicated that nS consumes much more Ca(OH)₂ as compared to silica fume, specifically at the early ages. Mercury intrusion porosimetry measurements proved that the addition of nS particles leads to reduction of capillary pores. Scanning electron microscope observation revealed that the inclusion of nS can also efficiently improve the interfacial transition zone between the aggregates and the binding paste. The addition of nS also resulted in an enhancement in compressive strength as well as in transport properties of UHPC. The optimum amount of cement replacement by nS in cement paste to achieve the best performance was 3 wt.%. However, the improper dispersion of nS was found as a deterrent factor to introduce higher percentage of nS into the cement paste.     

Adding limestone fines as cementitious paste replacement to improve tensile strength,stiffness and durability of concrete

The addition of a filler such as limestone fines (LF) to fill into the voids between aggregate particles can reduce the cementitious paste volume needed to produce concrete. In previous studies, it has been found that the addition of LF to reduce the cementitious paste volume would substantially increase the cube strength, and reduce the heat generation and shrinkage of the concrete produced. In this study, the authors aimed to evaluate the effects of adding LF as cementitious paste replacement on the tensile strength, stiffness and durability of concrete. For the evaluation, a series of concrete mixes with LF added to replace an equal volume of cementitious paste were tested for their workability, cube strength, tensile splitting strength, modulus of elasticity, water penetration depth and chloride permeability. The results showed that the addition of LF as cementitious paste replacement would at the same water/cement ratio, and even at the same cube strength, improve the tensile strength, stiffness and durability of concrete.