Effect of Anti-freezing Admixtures on Alkali-silica Reaction in Mortars

The influence of anti-freezing admixture on the alkali aggregate reaction in mortar was analyzed with accelerated methods. It is confirmed that the addition of sodium salt ingredients of anti-freezing admixture accelerates the alkali silica reaction to some extent, whereas calcium salt ingredient of anti-freezing admixture reduces the expansion of alkali silica reaction caused by high alkali cement. It is found that the addition of the fly ash considerably suppresses the expansion of alkali silica reaction induced by the anti-freezing admixtures.

Alkali-Silica Reaction Inhibited by LiOH and Its Mechanism

A high alkali reactive aggregate zeolitization perlite was used to test the long term effectiveness of LiOH in inhibiting alkali silica reaction.In this paper,the rigorous conditions were designed that the mortar bars had been cured at 80℃ for 3 years after autoclaved 24 hours at 150℃.Under this condition,LiOH was able to inhibit the alkali silica reaction long term effectiveness.Not only the relationship between the molar ratio of n(Li)/(Na) and the alkali contents in systems was established, but also the governing mechanism of such effects was also studied by SEM.

Multiscale Homogenization Analysis of Alkali–Silica Reaction (ASR) Effect in Concrete

The alkali silica reaction (ASR) is one of the major long-term deterioration mechanisms occurring in con- crete structures subjected to high humidity levels, such as bridges and dams. ASR is a chemical reaction between the silica existing inside the aggregate pieces and the alkali ions from the cement paste. This chemical reaction produces ASR gel, which imbibes additional water, leading to gel swelling. Damage and cracking are subsequently generated in concrete, resulting in degradation of its mechanical proper- ties. In this study, ASR damage in concrete is considered within the lattice discrete particle model (LDPM), a mesoscale mechanical model that simulates concrete at the scale of the coarse aggregate pieces. The authors have already modeled successfully ASR within the LDPM framework and they have calibrated and validated the resulting model, entitled ASR-LDPM, against several experimental data sets. In the pre- sent work, a recently developed multiscale homogenization framework is employed to simulate the macroscale effects of ASR, while ASR-LDPM is utilized as the mesoscale model. First, the homogenized behavior of the representative volume element (RVE) of concrete simulated by ASR-LDPM is studied under both tension and compression, and the degradation of effective mechanical properties due to ASR over time is investigated. Next, the developed homogenization framework is utilized to reproduce experimental data reported on the free volumetric expansion of concrete prisms. Finally, the strength degradation of prisms in compression and four-point bending beams is evaluated by both the mesoscale model and the proposed multiscale approach in order to analyze the accuracy and computational ef - ciency of the latter. In all the numerical analyses, different RVE sizes with different inner particle realiza- tions are considered in order to explore their effects on the homogenized response.

Calculation of Alkali Silica Reaction (ASR) Induced Expansion before Cracking of Concrete

A calculation method for predicting the formation of alkali-silica gel and analyzing the relationship of ASR induced expansion and aggregate size was proposed. The complicated chemistry of alkali silica reaction was simplified to be controlled by the diffusion process of chemical ions into reactive aggregates. The transport of chemical ions was described by the Fick＇s law. The ASR induced expansion was assumed to be directly related to the volume of produced alkali-silica gel. The finally expansion of a representative volume element （RVE） of concrete was then calculated according to the ratio of volume of alkali-silica gel and RVE. The input parameters of the model contains radius of reactive aggregate, volume fraction of reactive aggregate, initial concentration of chemical ions and porosity of cement paste. The applicability of the model was validated by an experiment of ASR-affected concrete specimens containing glass aggregate. It is shown that the amount of alkali-silica gel and ASR induced expansion can be well predicted. The expansion increasing with the decreasing aggregate size can be reproduced by the proposed model.

Efficacy of Aluminum Hydroxides as Inhibitors of Alkali-Silica Reactions

A comparative study of amorphous and crystalline forms of commercial aluminum hydroxides as inhibitors of alkalisilica reactions in Portland cement mortars has been performed. It was found that at dosages of 1% to 3%, amorphous aluminum hydroxide can efficiently inhibit alkali-silica expansion of Portland cement compositions. High inhibiting activity of amorphous Al(OH)3 additives may be explained by their ability to actively bind Ca(OH)2 formed by the hydration of silicate phases of cement, to form ettringite (with participation of gypsum). Crystalline Al(OH)3 additives that do not possess the ability to interact with Ca(OH)2 even after additional grinding, however, demonstrate week properties to inhibit alkali-silica expansion. This may indicate that the inhibitory effect of Al(OH)3 at least—partly, may be given by its influence on the concentration of Al3+ ions in the pore solution. Some expansion of the samples with admixtures of Al(OH)3 observed during the alkaline expansion accelerated test procedure is not associated with the formation of ettringite and is only due to alkali-silicate reactions.

The Impact of Aluminum- and Iron-Bearing Admixtures on the Resistance of Portland Cement Mortars to Alkali-Silica Reaction and Sulfate Attack

Study of sulfate resistance of mortars with aluminum- and iron-bearing admixtures (Al(OH)3, Al2(SO4)3, FeSO4, Fe2(SO4)3) in conditions close to those established in ASTM C 1012, and the study of the mitigation effect of these admixtures on alkali-silica reaction in accordance with accelerated “mortar bar” test ( GOST 8269.0, ASTM C 1260) were performed. Iron (II) and (III) sulfates show ability for mitigation alkali-silica reaction, while also, in contrast with Al-bearing substances, do not induce the drastic reducing of the initial setting time and do not promote the progress of sulfate corrosion. Compared with FeSO4, iron (III) sulfate has moderate deleterious impact on the early strength of cement paste and can be of interest alone as an inhibitor of ASR. Iron (II) sulfate may be used together with aluminum sulfate to offset the accelerating effect of the latter on the setting of cement paste and to reduce a risk of sulfate corrosion. During prolonged water storage, the mortar elongation and secondary ettringite formation do not occur, even when Al2(SO4)3 is available. Therefore, the investigated admixtures cannot act as agents of internal sulfate attack, however, Al2(SO4)3 can enhance the outer sulfate attack.

Effect of Mineral Admixtures on Alkali-Silica Reaction

The influence of silica fume, slag and fly ash on alkali-silica reaction under the condition of 70 ℃ is studied. The results show that silica, slag and fly ash may inhibit alkali-silica reaction only under suitable content. When the content is less than 10%, silica fume does not markedly influence the expansion of alkali- silica reaction. When the content is 15%-20%, silica fume only may delay the expansion of alkali-silica reaction. When the content is 30%-70%, slag may only delay the expansion of alkali-silica reaction, but cannot inhibit the expansion of alkali-silica reaction. When the content is 10%, fly ash does not markedly influence the expansion of alkali-silica reaction. When the content is 20%-30%, fly ash may only delay the expansion of alkali-silica reaction, but cannot inhibit the expansion of alkali-silica reaction. When the content is over 50%, it is possible that fly ash can inhibit effectively alkali-silica reaction.

Effect of Pozzolanic Reaction Products on Alkali-silica Reaction

The effect of fly ash on controlling alkail-silica rection （ASR） in simudated alkali solution was studied. The expausion of mortar bars and the content of Ca（ OH）2 in cement paste cured at 80 °G for 91 d were measured. Traasmission electron microscopy （TEM） and high-resolution transmission electron microscot9 （HRTEM） were employed to study the microstructure of C-S-H. TEM/ energy dispersive spectroscopy （EDS） leas then used to determine the composition of C-S-H. The pore structure of the paste was analyzed by mercury intntsion porosimetry （MIP）. The results show that the contents of fly ash of 30% and 45% can well inhibit ASR. And the content of Ca（OH） 2 decreases with the increase of fly ash. That fly ash reacted with Ca（OH）2 to produce C-S-H with a low Ca/Si molar ratio could bind more Na^＋ and K^＋ ious, and produce a reduction in the amount of soluble alkali available for ASR. At the same time, the C- S- H produced by pozzolanic reaction converted large pores to snudler ones （ gel pores smaller than 10 nm ） to deusify the pore structure. Perhaps that could inhibit alkali trausport to aggregate for ASR.

Effect of the Composite of Natural Zeolite and Fly Ash on Alkali-Silica Reaction

The effect of the composite of natural zeolite and fly ash on alkali-silica reaction (ASR) was studied with natural alkali-reactive aggregate and quartz glass aggregate respectively.The expansive experiment of mortar bar and concrete prism was completed.The results show that ASR can be suppressed effectively by the composite of natural zeolite and fly ash.

The Mechanism of the Eeffect of Mineral Admixtures on the Expansion of Aalkali-silica Reaction

On the base of the influence rule of silica fume, slag and fly ash on alkali-silica reaction under the condition of 70 ℃, the mechanism of the effect of mineral admixtures on alkali-silica reaction is studied further in the paper. The results show that the effects of mineral admixtures on alkali-silica reaction are mainly chemistry effect and surface physichemistry effect. Under suitable condition, the chemistry effect may make alkali-silica reaction to be inhibited effectively, but the physichemistry effect only make alkali-silica reaction to be delayed. The chemistry effect and the physichemistry effect of minerals admixture are relative to the content of Ca（OH）2 in system. Under the condition that there is a large quantity of Ca（OH）2, mineral admixture cannot inhibit alkali-silica reaction effectively. Only when Ca（OH）2 in the system is very less, it is possible that mineral admixture inhibits alkali-silica reaction effectively.

Calcined Clay Pozzolan as an Admixture to Mitigate the Alkali-Silica Reaction in Concrete

Calcined clay pozzolan has been used to replace varying portions of high alkali Portland limestone cement in order to study its effect on the alkali-silica reaction (ASR). Portland limestone cement used for the study had a total Na2Oeq of 4.32. Mortar-bar expansion decreased as pozzolan content in the cement increased. The highest expansion was recorded for reference bars with no pozzolan, reaching a maximum of 0.35% at 42 days whilst the expansion was reduced by between 42.5% and 107.8% at 14 days and between 9.4% and 16.4% at 84 days with increasing calcined clay pozzolan content. Mortar bars with 25% pozzolan were the least expansive recording expansion less than 0.1% at all test ages. X-ray diffractometry of the hydrated blended cement paste powders showed the formation of stable calcium silicates in increasing quantities whilst the presence of expansive alkali-silica gel, responsible for ASR expansion, decreased as pozzolan content increased. The study confirms that calcined clay pozzolan has an influence on ASR in mortar bars and causes a significant reduction in expansion at a replacement level of 25%.

Ultrafine Silica Additives Behavior during Alkali-Silica Reaction Long-Term Expansion Test

A silica fume, precipitated silica, metakaolin and siliceous fly ash behavior as constituents of mortars was studied, while mortar samples have been tested for long-term alkali-silica reaction expansion in accordance to the GOST 8269.0 specification. Solid-state 29Si-MAS NMR spectroscopy and thermogravimetric analysis were used to describe Portland cement hydration, supplementary cementitious material pozzolanic reaction and to establish a structure of products of those processes. It was found that long-term test conditions, in contrast to the accelerated test, do not affect the composition of products formed too much, compared to normal conditions. This allows results obtained with long-term test to be expected as more relevant in terms of predicting of supplementary cementitious materials inhibiting properties.

The Mitigation of Alkali-Silica Reactions by Aluminum-Bearing Substances

An ability of aluminum-bearing substances-amorphous aluminum hydroxide, aluminum sulphate and basic aluminum sulphate to mitigate alkali-silica reactions in Portland cement mortars has been studied. At equivalent dosages in terms of Al2O3, these substances are ranged in the following order in respect of inhibiting effect: Al(OH)1.78(SO4)0.61 ≥ Al2(SO4)3 > Al(OH)3. It is found that the plasticizing agents of the main types used in cement compositions have no influence on the inhibiting effect of aluminum-bearing admixtures. To control the setting time of cement paste, iron(II) sulphate may be used for partial substitution of Al2SO4·18H2O, and this operation is not influence on the results of ASR expansion test.

Effect of Glass Powder on Chloride Ion Transport and Alkali-aggregate Reaction Expansion of Lightweight Aggregate Concrete

The effects of glass powder on the strength development, chloride permeability and potential alkali-aggregate reaction expansion of lightweight aggregate concrete were investigated. Ground blast furnace slag, coal fly ash and silica fume were used as reference materials. The re- placement of cement with 25% glass powder slightly decreases the strengthes at ？ and 28 d, but shows no effect on 90 d＇s. Silica fume is very effective in improving both the strength and chloride penetration resistance, while ground glass powder is much more effective than blast furnace slag and fly ash in improving chloride penetration resistance of the concrete. When expanded shale or clay is used as coarse aggregate, the concrete containing glass powder does not exhibit deleterious expansion even if alkali-reactive sand is used as fine aggregate of the concrete.

Alkali-Silica Reactivity and Strength of Mortars with Expanded Slate, Expanded Glass or Perlite

Lightweight aggregates are increasingly used in concrete construction. They reduce concrete selfweight furnishing a structural advantage. In contrast, the mechanical properties and durability of lightweight concrete can become the governing factor on lightweight aggregate replacement ratios. Alkali-Silica Reactison (ASR) and compressive strength of mortar samples with expanded slate, expanded glass or perlite, covering the spectrum of internal porosity and weight of lightweight aggregates, were evaluated. Scanning electron microscopy was utilized to evaluate the contribution of the aggregates’ porosity and chemical composition in inhibiting ASR. Perlite, owing to its highly porous microstructure and lower matter excelled in ASR expansion while chemical composition and denser microstructure of the heavier expanded slate resulted in more signified late ASR expansion and higher compressive strength. An attempt in visual inspection of ASR attack of alkali metal ions on silica-rich expanded glass using an ultra-accelerated exposure to sodium hydroxide solution was made.

稻壳灰粒径对水泥砂浆的碱硅酸反应风险影响

固废资源化是实现可持续发展的重要内容.稻壳灰的火山灰活性高,有望作为辅助性胶凝材料用于水泥砂浆或混凝土,但其中K 2O和Na 2O含量较大,需探明不同粒径稻壳灰的掺入可能带来的碱硅酸反应风险.现有研究表明,粉煤灰的火山灰活性能抑制碱硅酸反应,已得到广泛应用.对比起见,设置了基准组、掺4种粒径粉煤灰组和稻壳灰组,在分别测试其抗压、抗折强度的基础上,测试膨胀率,通过SEM和EDS测试了掺粒径为5μm的稻壳灰试件和未掺稻壳灰试件的微观形貌和元素组成.试验结果表明:稻壳灰或粉煤灰的掺入均可以提高砂浆的力学性能,稻壳灰粒径越小提高的程度越大,中值粒径为5μm的稻壳灰可以使砂浆28 d抗压强度提高50.6%,抗压强度提高64.7%,效果均好于粉煤灰;稻壳灰的粒径不超15μm时,对砂浆的膨胀率可起到显著的抑制作用,且抑制效果优于粉煤灰,5μm的稻壳灰能使14 d膨胀率减小90%,并使骨料表现为无害,而8~20μm粉煤灰均仅减小60%左右的14 d膨胀率,骨料仍表现为有害;稻壳灰的火山灰活性不仅能生成低Ca/Si比的C-S-H凝胶,降低碱含量,还能使砂浆致密性提高,抑制碱离子和水的渗透,从而表现为对碱硅酸反应的抑制效果.

天然-人工混合骨料砂浆棒的碱-硅酸反应研究

为了研究混凝土中粗细骨料搭配比例对碱活性粗骨料碱-硅酸反应(ASR)的影响,根据工程中混凝土常见的粗细骨料比例设计了四种混合骨料比例,使用与粗骨料同源的细骨料替代粗骨料,采用砂浆棒快速法(AMBT)对四种不同比例混合骨料的碱活性进行评价;探究单掺25%粉煤灰、复掺25%粉煤灰和5%硅灰对混合骨料碱-硅酸反应的抑制效果,并对反应产物进行XRD分析。结果表明:四种不同比例的混合骨料仍具有潜在碱活性且不存在明显的“最劣比”现象;砂浆棒试件中掺加粉煤灰与硅灰后,碱活性骨料比例较低的混合骨料所受到的ASR膨胀抑制效果更明显;粉煤灰与硅灰主要通过减少水泥水化产物中的CH,增加C-S-H以及改性C-S-H以增强其结合碱离子的能力来减弱ASR。

碱金属离子对废玻璃砂浆碱骨料反应的影响

以废玻璃为骨料,NaOH和NaCl为碱金属离子来源,通过玻璃溶解及玻璃砂浆碱骨料反应(ASR),研究了碱金属离子对玻璃砂浆ASR的影响.结果表明:玻璃溶解速率随溶液中OH—浓度增加而升高,当OH—浓度为1.000 mol/L时达到峰值,随后降低;在NaOH环境中,ASR膨胀率随NaOH浓度升高而增加,膨胀速率随时间延长而递减;在NaCl环境中,膨胀主要发生在侵蚀初期;在NaOH与NaCl共存环境中,膨胀率与膨胀速率均明显增大,侵蚀后期膨胀速率增加显著;NaOH浓度的增加及NaCl的加入,均导致ASR凝胶钠硅比升高,增加了凝胶的吸水肿胀性,最终加剧膨胀.

不同离子对混凝土碱硅酸反应影响的研究进展

混凝土的碱硅酸反应(Alkali-silica reaction,ASR)本质上是孔溶液中的离子、水分子与骨料中活性二氧化硅的反应。根据不同离子对ASR的影响效果,可将离子分为碱离子(Na^(+)、K^(+)和OH-)、锂离子和铝离子、钙离子。碱离子(Na^(+)、K^(+)和OH^(-))促进混凝土的碱硅酸反应,导致混凝土发生更严重的膨胀性破坏。Al^(3+)和Li^(+)减缓混凝土碱硅酸反应造成的膨胀性破坏;Ca^(2+)起到的作用与n(Ca)/n(Si)(物质的量比)密切相关,当n(Ca)/n(Si)<0.2时,Ca^(2+)对ASR起促进作用;当n(Ca)/n(Si)≥0.2时,Ca^(2+)对ASR起抑制作用。本文首先介绍了ASR反应产物的最新研究进展,包括ASR产物的种类、微观形貌、原子结构及水稳定性,综述了这些离子对ASR反应过程、ASR产物组成、ASR产物结晶性能及膨胀性的影响,展望了不同离子对混凝土ASR影响的未来研究方向。

混凝土碱-硅酸反应膨胀率预测模型修正研究

为建立准确、可行的混凝土棱柱体法碱-硅酸反应膨胀率预测模型,基于幂函数模型、双曲线函数模型、复合指数函数模型预测值与实测值偏差情况,分别采用拟合回归方法确定引入模型的修正参数,进而实现对幂函数、双曲线函数模型的修正,评价了修正方法的准确性和可行性。结果表明:通过2组混凝土配合比180 d龄期膨胀率拟合回归分析,复合指数函数模型预测值与实测值相符性好;修正后幂函数、双曲线函数模型的拟合回归相关系数得到提高,模型所用修正方法具有一定的准确性和可行性;幂函数修正模型整体上符合程度优于其它模型,因此采用预测模型时建议优先采用幂函数修正模型。