Modified water-cement ratio law for silica fume concretes

The application of condensed silica fume as a mineral admixture in concrete is almost a routine one nowadays for the production of tailor-made high-performance concretes. Abrams' Law, which was originally formulated for conventional concrete containing cement as the only cementitious material, is not directly applicable to these new-generation concretes. In the present paper, modified relationships have been proposed to evaluate the strength of silica fume concrete. An extensive experimentation was carried out to determine the isolated effect of silica fume on concrete, and, analyzing the 28-day strength results of 32 concrete mixes performed over a wide range of water-binder ratios and silica fume replacement percentages, simplified relationships have been proposed. These simplified models might serve as useful guides for proportioning concrete mixes incorporating silica fume.     

Strength development of high strength concretes with and without silica fume under the influence of high hydration temperatures

High performance concretes of high compressive strength are finding increasing applications in many fields of construction such as core walls and columns in tall buildings, long-span bridges and marine structures. In thick cross-sections, the high binder contents of some high strength concretes can result in the development of high in-situ temperatures. The combined influence of limited moist curing and high hydration temperatures may significantly influence the progress of hydration. This can affect the long-term development of in-situ strength and other engineering properties. Knowledge of in-situ strength development under these conditions is needed to ensure safe utilisation of this new generation of construction materials.

This paper presents results of an investigation on the strength development of high strength concretes with and without silica fume subjected to high in-situ temperature conditions. A temperature match conditioning (TMC) system was developed and used to simulate the semi-adiabatic temperature development within medium sized high strength concrete columns. The results of this investigation show that in-situ temperatures of up to 70 °C significantly increased the 7-day strength of a high strength silica fume concrete. Although no strength regression was observed up to 1 year, the silica fume concrete subjected to high early temperatures showed significantly lower strengths when compared to concrete cured at standard temperature. For the silica fume concrete subjected to high early temperatures, non-evaporable water contents suggest little additional hydration beyond 3 days.     

Effect of water-to-cementitious materials ratio and silica fume on the autogenous shrinkage of concrete

This paper presents an experimental study on the autogenous shrinkage of Portland cement concrete (OPC) and concrete incorporating silica fume (SF). The results were compared with that of the total shrinkage (including drying shrinkage and part of the autogenous shrinkage) of the concrete specimens dried in 65% relative humidity after an initial moist curing of 7 days. The water-to-cementitious materials (w/c) ratio of the concrete studied was in the range of 0.26 to 0.35 and the SF content was in the range of 0% to 10% by weight of cement.The results confirmed that the autogenous shrinkage increased with decreasing w/c ratio, and with increasing SF content. The results showed that the autogenous shrinkage strains of the concrete with low w/c ratio and SF developed rapidly even at early ages. At the w/c ratio of 0.26, the autogenous shrinkage strains of the SF concrete were more than 100 micro strains at 2 days. For all the concretes studied, 60% or more of the autogenous shrinkage strain up to 98 days occurred in the first 2 weeks after concrete casting. The results indicated that most of the total shrinkage of the concrete specimens with very low w/c ratio and SF exposed to 65% relative humidity after an initial moist curing of 7 days did not seem to be due to the drying shrinkage but due to the autogenous shrinkage.     

低水泥浇注料中纯铝酸钙水泥和SiO_2微粉的“老化”试验

为了探讨纯铝酸钙水泥结合的低水泥浇注料"老化"的原因和影响因素,将SiO2微粉以及不同粒度、不同物相组成(CA相含量)、不同新鲜程度的纯铝酸钙水泥在温度为(20±1)℃、相对湿度>75%的潮湿环境中摊成厚度为2.5 cm左右的料层,放置一定时间(分别为0、3、7、14、28 d)使其老化,然后分别将它们与特级矾土(≤5、≤0.088 mm)、磷酸盐分散剂和减水剂等其他原料一起配制成浇注料,通过测定浇注料的可工作时间、初始流动性和早期(6 h)养护强度,来表征这些原料的老化速度.结果表明:(1)纯铝酸钙水泥的cA含量和细度越高,其老化速度越快.(2)"新鲜"的铝酸盐水泥比存放一段时间后的水泥的老化速度更快.(3)SiO2微粉的老化会导致浇注料的流动性变差,可工作时间变短.     

Utilization of fly ash with silica fume and properties of Portland cement-fly ash-silica fume concrete

This paper reports the normal consistency, setting time, workability and compressive strength results of Portland cement-fly ash-silica fume systems. The results show that water requirement for normal consistency was found to increase with increasing SF content while a decrease in initial setting time was found. Workability, measured in term of slump, was found to decrease with silica fume content (compared to blends without silica fume). However, it must be noted that despite the reduction in the slump values, the workability of Portland cement-fly ash-silica fume concrete in most cases remained higher than that of the Portland cement control concrete. Furthermore, the utilization of silica fume with fly ash was found to increase the compressive strength of concrete at early ages (pre 28 days) up to 145% with the highest strength obtained when silica fume was used at 10 wt%. Moreover, scanning electron micrographs show that utilization of fly ash with silica fume resulted in a much denser microstructure, thereby leading to an increase in compressive strength.     

Relationship between resistivity, diffusivity and microstructural descriptors for mortars with silica fume

Mortars with a sand-to-cement ratio of 3 and water-to-cement ratio of 0.5 were made with 0% and 10% silica fume (SF). Resistivities were measured with alternating current impedance spectroscopy (ACIS). Diffusivities were determined with the propan-2-ol counterdiffusion method. Microstructure was investigated with mercury intrusion porosimetry. It was found that there is a relationship relating hydration time to the product of resistivity and diffusivity. Furthermore, the product of resistivity and diffusivity was related to porosity, mean and threshold pore diameters. The influence of silica fume in refining the pore structure was marked.     

Effect of silica fume on pore structure and chloride diffusivity of low parosity cement pastes

This paper presents the results of an experimental investigation on the effect of silica fume on pore structure and diffusivity of low porosity cement pastes. For cement pastes with w/c ratios in the range of 0.20 to 0.30, a 10% replacement of the cement with silica fume only reduced the total porosity to a small extent. However, a refinement of the pore size distribution took place in such a way that the content of larger pores was reduced for decreasing w/c ratio. For pure portland cement pastes the effect of a certain constant change of w/c ratio on the chloride diffusivity was substantially higher at high w/c ratios than at low w/c ratios. A 10% replacement with silica fume reduced the chloride diffusivity for all w/c ratios so much that the effect of w/c ratio became less important.     

低卤低烟阻燃HDPE电缆料的研制

对有卤和无卤阻燃体系的复配进行了探讨。研究了不同阻燃体系的燃烧性能。结果表明,十溴联苯醚与Sb2O3在一定的配比范围内具有好的协效作用,其质量比为3：1时,阻燃效果最佳。卤系与无卤体系配合使用阻燃效果最佳,其氧指数高达31．8％,水平燃烧级别为FH—1。羟基硅油为最佳表面活性剂,其最佳用量为6—8份。     

Strength estimation model for high-strength concrete incorporating metakaolin and silica fume

A mathematical model is presented for estimating compressive strength of high-strength concrete incorporating pozzolanic materials, based on the strength of a control ordinary Portland cement (OPC) concrete made with similar mixture characteristics and curing history. In this study, metakaolin (MK) and silica fume (SF) were used as cement replacement materials at 5%, 10%, and 15% by mass. Water/cementitious materials (w/cm) ratios varied from 0.27 to 0.33, and strength testing was conducted up to an age of 180 days. It was found that the strength of a pozzolanic mixture could be related to the strength of its equivalent control by a linear function. Key parameters involved in the model are the pozzolanic and dilution factors, which can be correlated to the pozzolan content in the mixture. The study concludes that the accuracy of the model increases with concrete age. At ages 28 days and above, 97% of the estimated strengths are within ±5% of the actual value.     

Potassium geopolymer foams made with silica fume pore forming agent for thermal insulation

Porous potassium based geopolymers with a mutli-scale porosity were synthesized. Silica fume is introduced as an additive to the geopolymer formulation. The free silicon contained inside this silica fume is oxidized in alkaline solution, releasing molecular hydrogen which generates the porosity. Previous work has shown how the porosity can be controlled with temperature, repeated temperature cycles and the mass introduced. Using this protocol, homogeneous foams were made and then studied with scanning electron microscopy. In particular the foam expansion has been followed with time in relation to the microstructure. The thermal conductivity values of the foams were evaluated using a fluxmeter method. The effective thermal conductivities are comprised between 0.12 and 0.35 W m^?1 K^?1 for apparent densities ranging from 0.40 to 0.85 g cm^?3. The corresponding calculated pore volume fractions are in the range of 65–85%. The interest of this material is that it combines the advantages of low bulk density and insulating properties with the characteristics of a geopolymer skeleton. Literature reports a very good fire and acid/base resistance, a low cost of production and the possibility of recycling industrial waste in the form of silica fume.     

Cement pastes of low water to solid ratio: An investigation of the polymerization of silicate anions in the presence of a superplasticizer and silica fume

Polymerization of silicate anions was investigated by the molybdate method in cement pastes with water to solid ratio of 0.28. Two series of samples - with and without superplasticizer admixture - were prepared using ordinary portland cement (OPC) in which 0, 5, 10, and 15 percent by weight was replaced by condensed silica fume (CSF). During the hydration ranging from 4 hours up to 50 days, the free lime content and insoluble residue were also determined. The 28 day compressive strengths of the hardened composites were between 79 and 108 MPa. The results from the molybdate complex confirm that the presence of CSF in hydrating blends and its pozzolanic activity influences the size dispersity of silica anions by increasing the proportion of polymers. It is also suggested that in the cement pastes of w/s ratio of 0.28, the conversion level of CSF by pozzolanic reaction decreased. Finally, a comparison is made between the polymerization characteristics of the 0.28 w/s-ratio pastes and pastes of w/s = 0.68 which have been hydrated for 6.3 years.     

Rheological characterisation of cement pastes with nanosilica,silica fume and superplasticiser additions

Abstract

Abstract

This paper reports the effect of silica fume (SF), nanosilica (NS) and superplasticiser (SP) addition on the rheological behaviour of Portland cement pastes with different amounts of SF (0-10%), NS (1%) and SP (0-0·4%) and water to binder (W/B) ratio of 0·4. A rheometer with concentric cylinders geometry was used and the maximum shear rate of 100?s^?1 was applied. SP decreased the hysteresis area, while SF and NS increased it. Samples with NS showed high slopes in descending curves, while samples with SF and high dosages of SP showed lower values. In both cases, the Bingham model resulted in negative values for the yield stress, so that the Herschel-Bulkley model was used instead. The spread values variation on flow table did not follow a linear regression model. For this investigation, commercially available SF and NS were used.     

Efficiency of calcined kaolin and silica fume as cement replacement material for strength performance

The concept of efficiency can be used for comparing the relative performance of various pozzolans when incorporated into concrete. In this paper, an alternative approach for the evaluation of efficiency factor k of a pozzolanic material has been introduced. The method, developed following Abram's strength-W/C ratio rule, calculates efficiency in terms of relative strength and cementitious materials content. The advantage of this method is that only two mixtures are required to determine the k factor of a specific mixture. A laboratory investigation on silica fume (SF) and metakaolin (MK) concrete found that the computed efficiency factors varied with pozzolan type, replacement level and age. At 28 days, the k values ranged from 1.6 to 2.3 for MK and 2.1 to 3.1 for SF mixtures, while at 180 days, the k values varied from 1.8 to 4.0 for MK and 2.4 to 3.3 for SF mixtures. Generally, the k factors increased with age but declined with higher pozzolanic content. It was also observed that change in W/CM ratio from 0.33 to 0.27 did not significantly affect the resultant efficiency factors.     

Interaction between sulfate and chloride solution attack of concretes with and without fly ash

In this paper, the two sets of concretes under attack of erosion solution of sulfate and chloride salt were investigated. The one set is the plain concrete without fly ash addition. The other set is the concrete with 20% and 30% of fly ash addition, respectively. The corrosion solution includes three types: 3.5%NaCl, 5% Na₂SO₄, and a composite solution of 3.5%NaCl and 5% Na₂SO₄. In addition, two corrosion regimes were employed in this study: naturally immersion (stored in corrosion solution for long duration), drying-immersion cycles. The damage process of the two sets of concretes was systematically investigated under the above three types of corrosion solutions and two corrosion regimes. The interaction between sulfate and chloride salt was also quantitatively determined. The experimental results shown that a presence of sulfate in the composite solution increased the resistance to chloride ingress into concretes at early exposure period, but the opposition was observed at latter exposure period. For the damage of concretes, a presence of chloride in the composite solution reduces the damage of concrete caused by sulfate. Addition of fly ash may significantly improve the resistance to chloride ingress into concretes and the resistance to sulfate erosion when a suitable amount of fly ash addition and low water-to-binder (W/B) was employed. Studies of the different corrosion regimes indicate that concretes stored in corrosion solution for about 850 d, the changes in relatively dynamic modulus of elastically (RDME) could be described by three stages: linearly increasing period, steady period, and declining period. Whereas for drying-immersion cycles, an accelerated trend could be found. The changes in RDME included an accelerated decreased stage, linearly increased stage, and then a slowly decreased stage, finally accelerating failure stage. In order to elucidate the above experimental results in a microscopic scale, the mechanism was also investigated by the modern microanalysis techniques.     

Properties of plain and latex modified Portland cement pastes and concretes with and without superplasticizer

This paper deals with the effects of latex concentration on the workability and strength characteristics of Portland cement pastes with and without superplasticizer. Durability assessments are made by immersing these pastes in 5% Na₂SO₄ and 2.5% NaCl solutions. From the results obtained, it is found that the superplasticizer and superplasticizer-latex combinations may improve the workability of the Portland cement pastes. The Portland cement pastes with superplasticizer have much higher strengths than the latex modified Portland cement pastes with and without superplasticizer. In general, curing in lime-saturated water adversely affect the strength of the pastes containing latex from about 28 days onwards. In the durability test, the resistance of latex modified Portland cement pastes with and without superplasticizer to NaCl is decreased. Degradation mechanism depends on the characteristics of the corrosive medium as well as the resistance of the material itself to the resulting chemical action. The character of strain-stress data of latex modified concretes becomes more prominent as the latex concentration increases. These data are anomalous when compared with the data normally observed for concretes without admixture. The proposed equations are found adequate to describe the stress-strain behaviour of latex modified concretes in compression. These equations can also be applied in calculating the initial modulus of elasticity and proportional limit in the case of polymer modified concretes, which exhibit non-linear behaviour at high stress.     

A multiscale model for effective moduli of concrete incorporating ITZ water-cement ratio gradients, aggregate size distributions, and entrapped voids

This paper develops a model for the effective elastic properties of concrete, which is a function of the volume fractions, size distributions, and elastic properties of fine aggregate (FA) and coarse aggregate (FA) and entrapped voids. Furthermore, the model is a function of the overall water-cement ratio and specific gravity of cement. Explicitly modeled are the water-cement ratio gradients through the interfacial transition zone (ITZ), which, in turn, affect the variation of the cement paste elastic properties through the ITZ, while maintaining the total fractions of cement and water consistent with the overall water-cement ratio. The ITZ volume is also conserved.     

Cement pastes and mortars with low water-to-cement ratio I

Clinkers of nominal Czechoslovak production were ground in presence of grinding aid (derivative of lignin) to specific surfaces 400 – 2700 m²/kg and were processed in presence of sodium lignosulfonate and alkali carbonate (resp. bicarbonate) into pastes and mortars with low water-to-cement ratio. The clinker lignosulfonate-carbonate-H₂O system (further under “A”) is qualitatively different to the clinker-gypsum-H₂O system in rheological properties and in reaching short and long-term strengths. The volume changes (A) are similar to volume changes in Portland cements. The properties of system A are affected by concentration and type of carbonate and lignosulfonate.     

GS-HA低水比乙苯脱氢催化剂的开发

通过组合铈源和引入固定钾显著提高了催化剂的活性和耐低水比的能力,开发成功了新一代GS-HA低水比催化剂,该催化剂具有低水比条件下失活速率慢、活性高的特点。在二段负压绝热评价装置上,一、二反入口温度分别为615℃和620℃,乙苯液体空速为0.5 h-1,水比为1.15(wt),二反出口压力-50 kPa条件下,乙苯转化率为67.0%,苯乙烯选择性大于96.8%。催速老化试验结果表明,该催化剂的失活速率为-0.007 974%/h,优于最新引进催化剂。