Using limestone aggregates and different cements for enhancing resistance of concrete to sulphuric acid attack

A research program was undertaken to improve concrete's resistance against sulphuric acid attack. Six concretes were investigated, four using calcareous limestone aggregates and two using silicious aggregates. Cements used in these concretes included a portland cement, a binary cement containing ground granulated blast furnace slag, and two ternary cements containing slag and silica fume or fly ash and silica fume. All the concretes had the same water/cement ratio of 0.4, with compressive strengths in the range of 45 MPa and 58 MPa at the age of 28 days. In the experiment, concrete cylinders were immersed in 1% sulphuric acid solution and they were periodically examined for appearance, measured for mass change and tested in compression up to 168 days. The concrete using limestone aggregates and the ternary cement containing silica fume and fly ash performed the best.     

The effect of mechanically and chemically activated fly ashes on mortar properties

Fly ash is a waste material from coal-burning power plants that consume pulverized solid fuels. Two fly ashes from Asturias (Spain) were activated mechanically by wet milling and chemically by leaching with sulfuric acid. The activated fly ashes were characterized in terms of physico-chemical characterization, granulometry, density, blaine, BET, XRD and SEM.A comparative study was carried out of several mortars, in some cases using different additions of silica fume or activated fly ash. The influence that these additives have on the mechanical resistance of the mortars was studied. As well as the possible use of these activated fly ashes as a replacement for silica fume in producing high-strength mortar or concrete. It was found that mortars containing activated fly ash presented higher compressive strengths.A mercury intrusion porosimetry study was carried out on cement mortars made with mineral additives such as silica fume and activated fly ashes. In general, the porosities values of these mortars showed that mineral admixtures improved mechanical resistance due to the decrease in pore size.     

Effects of mineral and chemical admixtures on high-strength concrete in seawater

The effects of mineral and chemical admixtures namely fly ash, ground granulated blast furnace slag, silica fume and superplasticizers on the porosity, pore size distribution and compressive strength development of high-strength concrete in seawater curing condition exposed to tidal zone were investigated. In this study, three levels of cement replacement (0%, 30% and 70% by weight) were used. The total cementitious content used was 420 kg/m³. A water/binder ratio of 0.4 was used to produce concrete having a target compressive strength ranging between 54 and 63 MPa at the age of 28 days. At the age of 364 days, the compressive strength of the specimens produced ranged between 59 and 74 MPa. The pore size distribution of both high-strength concrete (MSS-0 and MSS-40) was significantly finer and the mean volume pore radii (MVPR) at the age of 6 months were reduced about three times compared to NPC concrete. Results of this study indicate that both concrete mixes (30% and 70%) exhibited better performance than the NPC concrete in seawater exposed to tidal zone. Hence, it is believed that both high-strength concrete produced would withstand severe seawater exposure without serious deterioration.     

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.     

Pozzolanic activity of various siliceous materials

The accelerated pozzolanic activity of various siliceous materials, like silica fume, fly ash (as received and fine ground), quartz, precipitated silica, metakaolin and rice husk ash (RHA; various fineness and carbon content), has been determined. The compressive strength of accelerated tests has been compared with cubes cured in water at 7 and 28 days. Maximum activity has been observed in case of RHA (<45 μ), followed by quartz and silica fume. The 10% replacement of cement by sand has shown accelerated pozzolanic index of 92% compared with 85% required in ASTM for silica fume as mineral admixture.     

粉煤灰/硅灰复合掺合料对水泥净浆性能的影响

研究了水泥标准稠度用水量、粉煤灰掺量、硅灰掺量、粉煤灰与硅灰双掺对水泥净浆性能的影响.结果表明,硅灰使水泥净浆需水量明显增加,粉煤灰、硅灰双掺可克服单掺粉煤灰早期强度低的缺点,短期内能提高水泥净浆的抗压强度.     

海水环境下矿物掺合料对硫铝酸盐水泥的水化影响

研究了海水环境下掺入硅灰、粉煤灰、矿渣对硫铝酸盐水泥抗压强度、化学收缩和水化产物的影响规律.结果表明:当硅灰的掺量为2.5%时,水泥浆体的抗压强度比空白组高.矿渣掺量为10%的水泥浆体28 d抗压强度明显超过掺入硅灰和粉煤灰时的强度,60 d强度高于空白组.掺入2.5%硅灰后,水泥浆体的化学收缩增大;在水化早期,粉煤灰和矿渣的火山灰活性很低,导致水泥浆体的化学收缩降低.掺入10%硅灰加快了硫铝酸盐水泥3 d水化反应,钙矾石生成量增多,水泥浆体早期强度比掺其它掺合料有所提高,但体积过快膨胀会破坏其内部结构,对水泥浆体的强度发展不利.     

Influence of high-temperature and low-humidity curing on chloride penetration in blended cement concrete

The influence of high-temperature and low-humidity curing on chloride penetration in concrete containing cement replacement materials was investigated. Three different mixes were studied: a control mix in which no cement replacement materials were added and two mixes where cement was partially replaced by 20% fly ash and 9% silica fume (by weight), respectively, at a constant water-to-binder ratio of 0.45. High-temperature curing was employed to simulate concrete temperature in hot climate. The results show that at early periods of exposure, initial curing has a substantial influence on chloride penetration in concrete. The effect of initial curing is much reduced after a long period of exposure. The chloride penetration at early ages of exposure is directly related to the porosity of the binder phase and the absorption of concrete. Higher chloride penetration resistance was observed when cement is partially replaced with either fly ash or silica fume.     

Use of recycled copper slag for blended cements

Copper slag is a by‐product generated during smelting to extract copper metal from the ore. The copper slag obtained may exhibit pozzolanic activity and may therefore be used in the manufacture of addition‐containing cements. In this paper the effect of the incorporation of the copper slag in cement is measured. Blends of copper slag with Portland cement generally possess properties equivalent to Portland cement containing fly ash, but very different to the silica fume incorporation. Copper slag and fly ash reduce the heat of hydration more effectively than silica fume in mortars. The replacement of 30% cement by copper slag reduces the flexural and compressive strength in a similar way to fly ash; however, after 28 days, the reduction is less than the percentage of substitution. Hydrated calcium aluminate phases were analysed using scanning electron microscopy (SEM) and X‐ray diffraction (XRD) techniques. The pozzolanic activity of copper slag is similar to that of fly ash and higher than silica fume. In the presence of low water/cement ratios, certain pozzolanic materials produce a very compact cement paste that limits the space available for hydration products, a determining factor in the formation of hydrated calcium aluminates. SEM was found to be a useful analytical technique when aluminates are formed and can be clearly detected by XRD. Copyright © 2008 Society of Chemical Industry     

粉煤灰掺量对再生混凝土力学性能和抗冻性的影响研究

研究粉煤灰掺量、再生粗骨料取代率对再生混凝土抗压强度和抗折强度的影响,并对再生混凝土在不同冻融循环次数下的抗压强度和质量损失率进行了研究.结果表明:随着粉煤灰掺量的增加,再生混凝土抗压强度呈先增大后降低的趋势,当粉煤灰掺量为15%,再生粗骨料取代率为30%时,再生混凝土的抗压强度达到最大;粉煤灰掺量对抗折强度提高幅度较小;在冻融循环低于50次时,试块抗压强度下降速度较缓,此后下降速度加快,当冻融循环达到150次时,强度损失最大;再生粗骨料取代率对试块的抗冻性影响高于粉煤灰掺量.建立了考虑再生粗骨料取代率、粉煤灰掺量因素的冻融循环作用下再生混凝土抗压强度指数衰减规律预测模型.     

不同粉煤灰掺量高强混合骨料混凝土力学性能及本构关系试验研究

本文以内蒙古锡林浩特地区富含的天然浮石作为轻骨料,以不同体积分数(0％,15％,30％,45％)的轻骨料替代粗骨料,同时以不同掺量粉煤灰(15％,25％,35％,45％)替代水泥,制备高强混合骨料混凝土,对其力学性能及本构关系进行对比和研究.研究表明:浮石轻骨料最优替代率为15％,粉煤灰最优掺量为25％,此时混合骨料混凝土的立方体抗压强度和比强度达到最大值;由于浮石轻骨料替代率对试块的破坏形式及过程影响较大,在进行混合骨料混凝土本构方程的建立时,把单轴抗压强度和轻骨料替代率同时作为参考变量,得到更加适用于高强混合骨料混凝土的本构关系,为本地区的工程应用提供依据.     

矿物掺合料对UHPC性能的影响

传统超高性能混凝土(UHPC)的硅灰用量一般都比较高,导致其制作成本较高,而且自收缩比较大,对实际工程应用造成了一定的影响。本文用粉煤灰和矿粉部分或全部替代硅灰制备UHPC,并对其工作性能、力学性能、自收缩及孔结构特征进行了试验研究。结果表明:采用粉煤灰或矿粉替代硅灰可以改善UHPC拌合物的流动性,替代率越高,拌合物的流动度越大;当采用粉煤灰或矿粉替代50%(质量分数)硅灰时,在标准养护下,对28 d抗压强度的影响较小,而在高温蒸养下,则会导致28 d抗压强度下降,当替代率达到100%(质量分数)时,无论是标准养护还是高温蒸养,都会显著降低28 d抗压强度;采用粉煤灰或矿粉替代硅灰能降低细孔的占比,增大孔径,减少自收缩,且粉煤灰对于自收缩的抑制效果优于矿粉。     

纳米颗粒粉煤灰绿色混凝土的耐久性试验研究

为配制高性能绿色混凝土,用不同质量分数的粉煤灰（0%~30%）来替代水泥,并在混凝土中掺入不同质量分数纳米颗粒氧化锌（0%~3%）来提高混凝土的抗压强度、抗拉强度和抗氯离子性能。通过制备30组混凝土试块进行试验,得出：1）在粉煤灰替代率相同的情况下,随着纳米颗粒氧化锌含量的增加,纳米颗粒氧化锌粉煤灰混凝土的抗压强度、抗拉强度和抗氯离子性能均逐渐增加。2）在纳米颗粒氧化锌含量相同的情况下,随着粉煤灰替代率的增加,纳米颗粒氧化锌粉煤灰混凝土的抗压强度和抗拉强度均逐渐下降。但纳米颗粒氧化锌粉煤灰混凝土的抗氯离子性能却逐渐提高。因此,当纳米颗粒氧化锌质量分数为1%时,建议粉煤灰的替代率在10%以下;当纳米颗粒氧化锌质量分数为2%时,建议粉煤灰的替代率在20%以下;而纳米颗粒氧化锌质量分数为3%时,建议粉煤灰的替代率仍在20%以下,因此不建议纳米颗粒氧化锌的掺量超过2%。     

An experimental study on strength development of concrete containing fly ash and optimum usage of fly ash in concrete

This paper presents a laboratory study on the strength development of concrete containing fly ash and optimum use of fly ash in concrete. Fly ash was added according to the partial replacement method in mixtures. A total of 28 mixtures with different mix designs were prepared. 4 of them were prepared as control mixtures with 250, 300, 350, and 400 kg/m³ cement content in order to calculate the Bolomey and Feret coefficients (K_B, K_F). Four groups of mixtures were prepared, each group containing six mix designs and using the cement content of one of the control mixture as the base for the mix design. In each group 20% of the cement content of the control mixture was removed, resulting in starting mixtures with 200, 240, 280, and 320 kg/m³ cement content. Fly ash in the amount of approximately 15%, 25%, 33%, 42%, 50%, and 58% of the rest of the cement content was added as partial cement replacement. All specimens were moist cured for 28 and 180 days before compressive strength testing. The efficiency and the maximum content of fly ash that gives the maximum compressive strength were obtained by using Bolomey and Feret strength equations. Hence, the maximum amount of usable fly ash amount with the optimum efficiency was determined.This study showed that strength increases with increasing amount of fly ash up to an optimum value, beyond which strength starts to decrease with further addition of fly ash. The optimum value of fly ash for the four test groups is about 40% of cement. Fly ash/cement ratio is an important factor determining the efficiency of fly ash.     

海水和矿物掺合料对硫铝酸盐水泥砂浆性能的影响

本文研究了不同拌和水以及海水拌和时粉煤灰和硅灰掺量对硫铝酸盐水泥(SAC)砂浆力学性能和表观孔隙率以及净浆凝结时间、化学收缩、孔溶液pH值和氯离子结合能力等的影响,并通过XRD、SEM和EDS分析水泥水化产物和微观结构。结果表明,海水能加快SAC早期水化并提高其早期强度,但后期强度和淡水拌和时无明显差别。粉煤灰和硅灰均会延长SAC凝结时间,对早期抗压强度不利,而掺加质量分数为5.0%和7.5%的硅灰能提高SAC砂浆28 d抗压强度。硅灰掺量增加时会提高用水量和表观孔隙率,降低流动性,使水泥化学收缩增大,降低净浆pH值且减少氯离子结合量;粉煤灰能够提高砂浆流动性,减少水泥化学收缩,但掺量越大对SAC砂浆抗压强度和抗折强度越不利,掺质量分数为10%的粉煤灰可小幅提高氯离子结合量且减小表观孔隙率。     

Electrochemical studies on the corrosion performance of steel embedded in activated fly ash blended concrete

The use of fly ash to replace a portion of cement has resulted significant savings in the cost of cement production. Fly ash blended cement concretes require a longer curing time and their early strength is low when compared to ordinary Portland cement (OPC) concrete. By adopting various activation techniques such as physical, thermal and chemical methods, hydration of fly ash blended cement concrete was accelerated and thereby improved the corrosion-resistance of concrete. Concrete specimens prepared with 10-40% of activated fly ash replacement were evaluated for their open circuit potential measurements, weight loss measurements, impedance measurements, linear polarization measurements, water absorption test, rapid chloride ion penetration test and scanning electron microscopy (SEM) test and the results were compared with those for OPC concrete without fly ash. All the studies confirmed that up to a critical level of 20-30% replacement; activated fly ash cement improved the corrosion-resistance properties of concrete. It was also confirmed that the chemical activation of fly ash yielded better results than the other methods of activation investigated in this study.     

Strength properties of high-volume fly ash roller compacted and workable concrete, and influence of curing condition

A laboratory investigation was carried out to evaluate the strength properties of high-volume fly ash (HVFA) roller compacted and superplasticised workable concrete cured at moist and dry curing conditions. Concrete mixtures made with 0%, 50% and 70% replacement of normal Portland cement (NPC) with two different low-lime Class F fly ashes, good and low quality, were prepared. Water-cementitious material ratios ranged from 0.28 to 0.43. The compressive, flexural tensile and cylinder splitting tensile strengths were measured and presented. The relationship between the flexural tensile and compressive strengths was discussed. The influence of loss on ignition (LOI) content of fly ash on water demand and the strength of concrete was also discussed. The influence of moist and dry curing conditions on the high-volume fly ash (HVFA) concrete system was assessed through a proposed simple efficiency factor. The study showed that producing high-strength concrete was possible with high-volume fly ash content. LOI content increased the water demand of fresh concrete. HVFA concrete was found to be more vulnerable to dry curing conditions than was NPC concrete. It was concluded that HVFA concrete was an adequate material for both structural and pavement applications.     

The effects of expanded perlite aggregate, silica fume and fly ash on the thermal conductivity of lightweight concrete

Thermal conductivity coefficients of concretes made up of mixtures of expanded perlite and pumice aggregates (PA) were measured. To determine the effect of silica fume (SF) and class C fly ash (FA) on the thermal conductivity of lightweight aggregate concrete (LWAC), SF and FA were added as replacement for cement by decreasing the cement weights in the ratios of 10%, 20% and 30% by weight.The highest thermal conductivity of 0.3178 W/mK was observed with the samples containing only PA and plain cement. It decreased with the increase of SF and FA as replacement for cement. The lowest value of thermal conductivity, which is 0.1472 W/mK, was obtained with the samples prepared with expanded perlite aggregate (EPA) replacement of PA and 70% cement+30% FA replacement of cement. Both SF and FA had a decreasing effect on thermal conductivity. EPA (used in place of PA) also induced a decrease of 43.5% in thermal conductivity of concrete.     

低温养护下矿物掺合料湿喷混凝土力学性能及配比优化研究

为系统研究粉煤灰掺量、硅灰掺量及养护温度对湿喷混凝土力学性能的影响规律,通过设计正交试验对湿喷混凝土抗压强度进行极差和方差分析。结果表明:湿喷混凝土抗压强度随养护龄期增加而增大,但抗压强度增幅随养护龄期延长而减弱;增加硅灰和粉煤掺量均能有效提高湿喷混凝土抗压强度,但粉煤灰掺量超过10%(质量分数,下同)后,粉煤灰掺量的增加对混凝土后期抗压强度没有显著的影响;三因素对湿喷混凝土抗压强度影响程度顺序为硅灰掺量>养护温度>粉煤灰掺量;湿喷混凝土抗压强度对矿物掺合料的敏感性与养护温度呈正相关,增大养护温度能够提高矿物掺合料对湿喷混凝土抗压强度的改善效果;随着养护温度的提高,团絮状胶凝物质大量生成,水化产物黏结得更为密实,混凝土的抗压强度和承载性能得到进一步增强;构建多元非线性回归模型能够对混凝土抗压强度进行预测,并且湿喷混凝土在硅灰掺量、粉煤灰掺量及养护温度分别为15%、15%和10 ℃时具有最佳的抗压强度。     

Alkali release characteristics of blended cements

This paper presents results of an experimental program conducted to investigate the capacity of hydration products of different cementing materials to retain “bound” alkalis when the alkalinity of the surrounding solution drops. The study covered paste samples containing high-alkali Portland cement and various levels of silica fume and/or fly ash. The results showed that the ability of the hydration products of cement-fly ash systems to bind alkalis is a function of the CaO content of the fly ash, the binding increasing as the calcium content decreases. High-alkali fly ashes (Na₂O_e > 5.0% and CaO in the range of 15% to 20%) showed considerable amounts of alkali contributed to the test solutions. Silica fume does not have a high capacity to retain alkalis in its hydration products; however, ternary blends containing silica fume and fly ash have excellent capacity to bind and retain alkalis.