Influence of fly ash and silica fume on the consistency retention and compressive strength of concrete subjected to prolonged agitating

Good workability at construction site is essential for high quality concrete since concretes of bad workability are prone to yield low strength and poor durability properties since placement and consolidation procedures cannot be performed properly. Nevertheless, this is what usually encountered in practice because slump loss is indispensable particularly when long delivery times are the case. This would be more pronounced when mineral additives are incorporated into the concretes. In this study, concretes of C25/30 class with fly ash and silica fume were produced and slumps were measured with time elapsed. At the end of each agitating period, the slumps of the mixtures were restored to the initial slumps using a superplasticizer and specimens were thereafter prepared for strength measurement. The effectiveness of using fly ash and silica fume in concrete in relation with slump loss was sought by determining the amount of superplasticizer used. Regardless of the mixture recipe, it can be concluded that as the total amount of cementitious materials increases in the concrete mix of the same initial slump, the slump loss with the elapsed time decreases.     

Effect of curing methods on the properties of plain and blended cement concretes

This paper reports result of a study conducted to investigate the effect of curing methods on the properties of plain and blended cement concretes. The concrete specimens were prepared with Type I, silica fume, and fly ash cement concretes. They were cured either by covering with wet burlap or by applying two types of curing compounds, namely water-based and acrylic-based. The effect of curing methods on the properties of plain and blended cement concretes was assessed by measuring plastic and drying shrinkage, compressive strength, and pulse velocity. Results indicated that the strength development in the concrete specimens cured by covering with wet burlap was more than that in the specimens cured by applying water – and acrylic-based curing compounds. Concrete specimens cured by applying curing compounds exhibited higher efficiency in decreasing plastic and drying shrinkage strain than specimens cured by covering with wet burlap. The performance of acrylic-based curing compound was better than that of water-based curing compound. The data developed in this study indicate that curing compounds could be utilized in situations where curing with water is difficult. Among the two curing compounds investigated, acrylic-based curing compound performed better than the water-based curing compound.     

Properties of self-compacting concretes made with binary,ternary, and quaternary cementitious blends of fly ash,blast furnace slag,and silica fume

Self-compacting concretes (SCCs) have brought a promising insight into the concrete industry to provide environmental impact and cost reduction. However, the use of ternary and especially quaternary cementitious blends of mineral admixtures have not found sufficient applications in the production of SCCs. For this purpose, an experimental study was conducted to investigate properties of SCCs with mineral admixtures. Moreover, durability based multi-objective optimization of the mixtures were performed to achieve an optimal concrete mixture proportioning. A total of 22 concrete mixtures were designed having a constant water/binder ratio of 0.44 and a total binder content of 450 kg/m³. The control mixture included only a Portland cement (PC) as the binder while the remaining mixtures incorporated binary, ternary, and quaternary cementitious blends of PC, fly ash (FA), ground granulated blast furnace slag (S), and silica fume (SF). Fresh properties of the SCCs were tested for slump flow diameter, slump flow time, L-box height ratio, and V-funnel flow time. Furthermore, the hardened properties of the concretes were tested for sorptivity, water permeability, chloride permeability, electrical resistivity, drying shrinkage, compressive strength, and ultrasonic pulse velocity. The results indicated that when the durability properties of the concretes were taken into account, the ternary use of S and SF provided the best performance.     

Mechanical and durability properties of high performance concretes containing supplementary cementitious materials

An experimental investigation was carried out to evaluate the mechanical and durability properties of high performance concretes containing supplementary cementitious materials in both binary and ternary systems. The mechanical properties were assessed from the compressive strength, whilst the durability characteristics were investigated in terms of chloride diffusion, electrical resistivity, air permeability and water absorption. The test variables included the type and the amount of supplementary cementitious materials (silica fume, fly ash and ground granulated blast-furnace slag). Portland cement was replaced with fly ash up to 40%, silica fume up to 15% and GGBS up to a level of 70%.The results confirmed that silica fume performs better than other supplementary cementitious materials for the strength development and bulk resistivity. The ternary mixes containing ground granulated blast-furnace slag/fly ash and silica fume performed the best amongst all the mixes to resist the chloride diffusion. The mix containing fly ash showed favourable permeation results. All the ternary combinations can be considered to have resulted in high performance concretes with excellent durability properties.     

Strength and drying shrinkage properties of self-compacting concretes incorporating multi-system blended mineral admixtures

Drying shrinkage can be a major reason for the deterioration of concrete structures. The contraction of the material is normally hindered by either internal or external restraints so that tensile stresses are induced. These stresses may exceed the tensile strength and cause concrete to crack. The present study investigated compressive strength and particularly drying shrinkage properties of self-compacting concretes containing binary, ternary, and quaternary blends of Portland cement, fly ash (FA), ground granulated blast furnace slag (GGBFS), silica fume (SF), and metakaolin (MK). For this purpose, a total of 65 self-compacting concrete (SCC) mixtures were prepared at two different water to binder ratios. It was observed that drying shrinkage lessened with the use of FA, GGBFS, and MK while incorporation of SF increased the drying shrinkage.     

Properties of cement kiln dust concrete

This paper reports results of a study conducted to assess the properties of cement kiln dust (CKD) blended cement concretes. Cement concrete specimens were prepared with 0%, 5%, 10%, and 15% CKD, replacing ASTM C 150 Type I and Type V. The mechanical properties of CKD concrete specimens were evaluated by measuring compressive strength and drying shrinkage while the durability characteristics were assessed by evaluating chloride permeability and electrical resistivity. The compressive strength of concrete specimens decreased with the quantity of CKD. However, there was no significant difference in the compressive strength of 0 and 5% CKD cement concretes. A similar trend was noted in the drying shrinkage strain. The chloride permeability increased and the electrical resistivity decreased due to the incorporation of CKD. The performance of concrete with 5% CKD was almost similar to that of concrete without CKD. Therefore, it is suggested to limit the amount of CKD in concrete to 5% since the chloride permeability and electrical resistivity data indicated that the chances of reinforcement corrosion would increase with 10% and 15% CKD.     

Assessing the effectiveness of pozzolans in massive high-strength concrete

This paper investigates the effects of supplementary cementitious materials on the temperature rising profile, heat evolution and early-age strength development of medium- and high-strength concrete. A total of 13 different mixtures were prepared, with two water–cement ratios (0.3 and 0.46). Natural pozzolan, fly ash, and silica fume were included in the specimens. The results showed that natural pozzolan, particularly fly ash served to decrease the amplitude of peak temperature, delay the occurrence of the peak, and decrease the sharpness of the temperature rising profiles. In contrast, the temperature profile of silica fume specimens was similar to those without silica fume. It was found that the best mixture, the highest early-age strength and the lowest heat liberation, corresponded to the specimen containing fly ash at 15% (be cement weight). This result justifies the advantage effect of fly ash which is able to develop sufficient tensile strength to resist thermal cracking potential in massive high-strength concrete.     

大掺量粉煤灰活性粉末混凝土耐久性研究

研究了以大量粉煤灰取代水泥,并掺加硅灰、钢纤维配制的大掺量粉煤灰活性粉末混凝土(HVFRPC)的耐久性。研究结果表明,大掺量粉煤灰活性粉末混凝土具有较小的体积收缩率,抗碳化、抗氯离子渗透、耐硫酸盐浸蚀性优异。     

Characteristics of lightweight concrete containing mineral admixtures

This research investigates the properties of fresh and hardened concretes containing locally available natural lightweight aggregates, and mineral admixtures. Test results indicated that replacing cement in the structural lightweight concrete developed, with 5–15% silica fume on weight basis, caused up to 57% and 14% increase in compressive strength and modulus of elasticity, respectively, compared to mixes without silica fume. But, adding up to 10% fly ash, as partial cement replacement by weight, to the same mixes, caused about 18% decrease in compressive strength, with no change in modulus of elasticity, compared to mixes without fly ash. Adding 10% or more of silica fume, and 5% or more fly ash to lightweight concrete mixes perform better, in terms of strength and stiffness, compared to individual mixes prepared using same contents of either silica fume or fly ash.     

NaCl除冰盐作用下混凝土的抗冻能力分析

通过存质量浓度为3.5%的NaCl溶液中的冻融循环试验,测试了普通混凝土、引气混凝土、粉煤灰混凝土和硅灰混凝土的相对动弹性模量和质量损失率,分析了不同混凝土的抗盐冻的能力.结果表明:适当的引气能够明显改善混凝土的抗盐冻性能,掺加20%粉煤灰时,混凝土仍具有较高的抗盐冻能力,掺量达到40%时混凝土的抗盐冻性能明显下降,掺加10%硅灰明显提高混凝土的抗盐冻能力.     

Deterioration and Recovery of Metakaolin Blended Concrete Subjected to High Temperature

The deterioration and spalling frequency of metakaolin (MK) blended concrete subjected to high temperature is analyzed and compared with the equivalent silica fume (SF), fly ash (FA) and pure OPC concretes. Normal and high strength concrete mixes incorporating 0–20% MK were prepared and exposed to a series of high temperatures till 800°C. The residual compressive strength, porosity and pore size distribution were determined. It was found that after an increase in compressive strength at 200°C, the MK concrete suffered a more severe loss of compressive strength and permeability-related durability than the corresponding SF, FA, and pure OPC concretes at higher temperatures. Explosive spalling was observed in both normal and high strength MK concretes, and the frequency increased with higher MK contents. It is concluded that the dense micro-structure and low porosity are the main reasons of the poor performance of MK concrete at high temperature. The effect of post-fire curing on the strength and durability recovery of fire-damaged concrete was also investigated. The test results indicated that the post-fire curing, results in substantial strength and durability recovery, and its extent depends upon the types of concrete, exposure temperature, method, and duration of re-curing.     

硅粉、粉煤灰作掺合料的C80高性能混凝土研究

研究了硅粉、粉煤灰掺量对混凝土强度与流动性的影响和硅粉、粉煤灰混凝土28d强度规律及混凝土的后期强度增长规律。采用P.O.42.5级普通硅酸盐水泥、中砂、5~25mm碎石及适量NF2-6缓凝高效减水剂,水胶比0.28,硅粉掺量5%,粉煤灰掺量5%~15%或硅粉掺量10%~15%,粉煤灰掺量5%~20%及水胶比0.31,硅粉掺量15%,粉煤灰掺量5%~10%时,可配制出C80高性能混凝土并且给出混凝土配合比参考公式。     

Strength and corrosion properties of Portland cement mortar and concrete with mineral admixtures

This work aims to validate the design assumptions by the California Department of Transportation in order to better define the strategies used to design concrete structures with adequate corrosion mitigation and thus a “maintenance-free” service life. To this end, various laboratory tests were conducted to investigate the compressive strength of and chloride diffusivity in mortar and concrete samples with cement partially replaced by various minerals (class F and class N fly ash, ultra-fine fly ash, silica fume, metakaolin, and ground granulated blast-furnace slag), the porosity of mineral concretes, the freeze–thaw resistance of mineral mortars in the presence of deicers, and the effect of supplementary cementitious materials on the chloride binding and chemistry of the pore solution in mortar.     

A study on the hydration rate of natural zeolite blended cement pastes

Natural zeolite is a type of mineralogical material containing large quantities of reactive SiO₂ and Al₂O_3. It is widely used in the cement industry in China as a cement blending material. Like other pozzolanic materials such as silica fume and fly ash, zeolite contributes to concrete strength mainly through the pozzolanic reaction with Ca(OH)₂, Thus, the pozzolanic reactivity of this type of material in comparison with other pozzolans is of much interest. This paper presents experimental results on the compressive strength, degree of pozzolanic reaction, and porosity of zeolite modified cement pastes. These results are compared with those obtained from similar blended cement pastes prepared with silica fume and fly ash replacements. Based on the experimental results, it can be concluded that natural zeolite is a pozzolanic material, with a reactivity between that of silica fume and fly ash. Generally, in blended cement pastes with a lower water-to-cementitious materials ratio, the natural zeolite contributes more to the strength of the pastes. But in the pastes with a higher water to cementitious ratio and a lower cement replacement level it undergoes a higher degree of reaction.     

Chloride ion diffusivity of fly ash and silica fume concretes exposed to freeze–thaw cycles

This research focuses on investigating the durability of concretes containing fly ash and silica fume exposed to combined mode of deterioration. For this purpose, the chloride ion diffusivity of concrete was evaluated before and after 300 freeze–thaw (F–T) cycles. It was found that the coefficient of chloride ion diffusivity (CCID) increased as water to cementitious material ratio (w/cm) and air content increased. Test results clearly showed that CCID for all concretes increased after F–T cycles. In addition, concrete incorporating silica fume showed the lowest CCID and highest durability factor (DF), regardless of curing regime, air content, and w/cm. However, fly ash concrete showed good resistance to chloride ion diffusivity before and after F–T cycles when low w/cm as well as a proper curing and air content were provided.     

矿物掺合料对C100～C120超高强混凝土基本性能的影响

通过正交试验考察了胶凝材料总量、水泥用量及超细粉煤灰用量对超高强混凝土抗压强度的影响,研究了硅灰、超细粉煤灰与复合硅材等矿物掺合料对C100~C120超高强混凝土基本性能的影响。结果表明,对超高强混凝土强度影响最主要的因素是水泥在胶凝材料中所占的比例;超细粉煤灰与硅灰复掺对超高强混凝土工作性能与强度改善效果较好;复合硅材对超高强混凝土的工作性能与强度均有一定的提高。     

双掺粉煤灰和硅粉混凝土抗冻性能试验研究

在深入分析混凝土冻融破坏机理及其影响因素的基础上,提出在混凝土中双掺硅粉和粉煤灰的方法,并按有关规范要求进行普通混凝土与双掺混凝土抗冻性能的对比试验。试验结果表明:双掺混凝土因基于"等量替换法"用硅粉和粉煤灰替换了一部分水泥,减小了水泥用量和增大了水灰比,故双掺混凝土的强度和弹性模量均明显低于普通混凝土,但减水剂的使用降低了水胶比,使双掺混凝土的抗冻性能优于普通混凝土。     

早期高温养护对预制混凝土构件长期抗压强度的影响

混凝土构件的早期养护条件是决定其长期抗压强度的重要因素,为研究早期养护温度对预制混凝土构件长期抗压强度的影响,制作了单掺粉煤灰(SPC)、双掺粉煤灰、矿渣(DPC)和三掺粉煤灰、矿渣、硅灰(TPC)以及对比普通水泥混凝土(OPC)试件,按照有、无预养分别进行了40℃、60℃和80℃的水养护,达到设计强度后取出放置在室内大气环境,再分别测试其28 d、100 d、200 d和300 d龄期的抗压强度。研究结果表明:无论普通混凝土还是矿物掺合料混凝土,早期养护温度越高,混凝土达到设计强度所需的养护时间越短,但长期的抗压强度越低。掺加矿物掺合料和20℃的预养有助于增强混凝土抵御高温养护导致的热应力问题,有利于预制混凝土构件长期强度的提高。     

纳米SiO2超高强高流态混凝土及改性机理

通过正交试验提出纳米超高强高流态混凝土的胶凝材料配合比设计参数,并研究了纳米SiO_2的掺入对传统掺硅灰、粉煤灰超高强水泥基胶凝材料强度及工作性能的影响。在保证水胶比不变的条件下,开展了混凝土配合比试验,并研究了纳米SiO_2对混凝土抗压强度的影响及其微观机理。结果表明:超高强高流态混凝土中胶凝材料最优比例为:纳米SiO_2:硅灰:粉煤灰:水泥=1:8:20:71;在胶凝材料用量为600～1 000kg/m~3范围内,随着其掺量的增加,混凝土流动度不断增加,抗压强度先增大后减小,当其掺量为800kg/m~3时,抗压强度最大。分析认为,纳米SiO_2、硅灰与粉煤灰形成的三元多尺度堆积体系能优化粉体材料在混凝土中的微集料密实填充效应,纳米SiO_2的二次水化反应也有效改善了硬化水泥石的微观结构,并优化其形态分布,进一步增大其强度。     

Effect of environmental conditions on the properties of concretes with different cement types

The paper reports on the changes in properties of concretes with different cement types associated with environmental conditions. Three strength classes with three different cement types (ordinary portland cement PC 42.5 (CEM I 42.5), portland composite cements PKC-A 42.5 (CEM II/A-M 42.5) and PKC-B 32.5R (CEM II/B-M 32.5R)) were used in the study. Also, a mixture was prepared with PC 42.5 and silica fume (SF). The effects of variable ambient conditions on plastic shrinkage of fresh concrete and cement paste, compressive strength, modulus of elasticity, capillary absorption and drying shrinkage of hardened concrete were investigated. In contrast to PC 42.5 cement paste, plastic shrinkage cracks were observed in PKC-B 32.5 and PKC-A 42.5 pastes. Water absorption coefficients of all concretes stored in natural environment were higher at all ages as compared to coefficients of concretes kept in laboratory. Drying shrinkage values of concrete with SF, except the first week, were significantly lower than those of others. Although different behaviors for different cement types were observed, water–cement ratio was one of the dominating factors determining the behavior of concrete. This ratio should be lowered to improve the durability of concrete.