粉煤灰对混凝土性能的影响

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1、粉煤灰对混凝土性能的影响摘要： 粉煤灰对混凝土性能的改变分为三个阶段，在这三个阶段以后，会逐渐 对混凝土的下列性能产生影响，如强度，和易性，收缩，徐变，碳化性能，钢筋 锈蚀，水化热等。粉煤灰混凝土的性能 粉煤灰对混凝土性能的改变可分为三个阶段： 1 .新拌混凝土阶段: 影响混凝土 的凝结时间，改善和易性，改变流变性质，提高可泵性等； 2 .硬化中的混凝土 阶段: 调节硬化过程，降低水化热； 3 .硬化后的混凝土阶段: 提高后期强度， 提高各项耐久性，如抗渗性、抗硫酸盐侵蚀性，抑制碱集料反应等。1 强度 粉煤灰对混凝土强度有三种影响：减少用水量、增大胶结材含量和通过长期火山 灰反应提高其强度。低

2、钙粉煤灰中的微粒为硅氧四面体结构，自身的活性很低。 在水泥的最终产物中，高碱性水化硅酸钙和Ca (OH) 2胶体的结晶强度很低，特 别是 Ca(OH) 2 仅是托勃莫来石强度的 1-2%，而 Ca(OH) 2 的体积占整个水泥 石体积的25%。粉煤灰中含有的大量的硅、铝氧化物，能逐步与Ca (OH)2及高 碱性水化硅酸钙发生二次反应，生成强度较高的低碱性水化硅酸钙，这样，不但 使水泥石中水化胶凝物质的数量增加，而且也使其质量得到大幅度提高，有利于 混凝土强度的提高。同时，粉煤灰的掺入可分散水泥颗粒，使水泥水化更充分， 提高水泥浆的密实度，使混凝土中骨料与水泥浆的界面强度提高。粉煤灰对抗拉 强度

3、和抗弯强度的贡献比抗压强度还要大，这对混凝土的抗裂性能有利。粉煤灰 混凝土的弹性模量与抗压强度相类似，早期偏低，后期逐步提高，到28d时可比 基准混凝土提咼5-10%。与钢筋的握裹力，粉煤灰混凝土的28d粘结强度基本与 等标号的基准混凝土相同，但粉煤灰混凝土的均匀性好，粘结强度试验值的离散 性比基准混凝土好粉煤灰的二次水化反应一般在混凝土浇筑14d以后才开始进行，在温度低时，该 反应所需的时间更长。如果对混凝土的早期强度有严格要求，粉煤灰的掺量不宜 超过 30%，冬季施工非大体积混凝土时，粉煤灰的掺量不宜超过 20%。由于现代 混凝土中外加剂的使用，一方面，可减少混凝土拌和用水量，减小水灰比，

4、提高 混凝土中水泥的浓度；另一方面，减水剂能使水泥中硅酸钙水化所产生的 Ca(OH) 2 增多，有利于粉煤灰与 Ca(OH) 2 的二次水化反应，激发粉煤灰的活性，这对 于改善粉煤灰的早期强度是有效的，另外，使用粉煤灰活性激发剂或在非大体积 混凝土中使用早强型水泥，也可以补偿粉煤灰的掺入对混凝土早期强度的影响。2 和易性粉煤灰对混凝土和易性的改善作用有以下几点：优质粉煤灰中含有70%以上 的球状玻璃体，这些球状玻璃体表面光滑无棱角，性能稳定，在混凝土的泵送、 振捣过程中起着一种类似于轴承的润滑作用；新拌混凝土中水泥颗粒易聚集 成团，粉煤灰的掺入可有效分散水泥颗粒，释放更多的浆体来润滑骨料，有利

5、于 混凝土工作性能的提高；.掺入粉煤灰可以补偿细骨料中细屑的不足，中断砂 浆基体中泌水渠道的连续性，同时品质良好的粉煤灰在同样的稠度下能减少混凝 土的拌和用水量，使混凝土中的水灰比降低到更小水平，减少泌水和离析现象3 收缩 混凝土的收缩与混凝土的拌和用水量和浆体体积有关，用水量越少，收缩也越小。 优质的粉煤灰需水量比小于 100%，拌和水量的减少使掺粉煤灰混凝土 28d 后的 自干燥收缩和干燥收缩都小。粉煤灰混凝土的干缩也随粉煤灰掺量的提高而降 低。但由于粉煤灰混凝土的水化反应慢，水分蒸发快，所以粉煤灰对混凝土的早 期干缩影响很大。为防止粉煤灰混凝土的早期收缩开裂，对其更应加强早期养护。4 徐

6、变28 天龄期以前，混凝土的强度较低，其相应龄期的徐变应变也较普通混凝土的 大，然而与普通混凝土等强度的粉煤灰混凝土在此后所有龄期的徐变均小于普通 混凝土。5 碳化性能 粉煤灰混凝土的抗碳化性能较差。粉煤灰混凝土中的水泥用量减少，水泥水化析 出的Ca (OH) 2数量也相应减少，而且，火山灰反应也消耗了一定量的Ca (OH) 2 ，使混凝土的 PH 值降低，会增加混凝土的碳化速度。特别在水化早期，粉煤 灰火山灰反应程度低，粉煤灰-水泥体系孔结构疏松，CO2、02、水分等入侵阻力 小，因此碳化深度较大。随着龄期的增长和粉煤灰火山灰效应的逐渐发挥，碳化 速度将逐渐降低。粉煤灰混凝土的碳化深度随水灰

7、比及粉煤灰掺量的增加而有所 增加。在水灰比为 0.5-0.55，粉煤灰掺量不大于30%和一般施工水平的情况下， 15-17 年混凝土的碳化深度可达 20mm 左右。 碳化反应在一定的相对湿度范围内进行最快，否则，反应较慢。当相对湿度在 25%以下或者接近 100%，即混凝土在充分干燥或水饱和的场合，混凝土都不易产 生碳化收缩。在基础工程等不与大气接触的混凝土工程中，由于与C02隔绝，不 会发生碳化反应，因此可较多地掺加粉煤灰，以充分降低混凝土的水化热，提高 混凝土的耐久性。采用超量取代法，较低的水胶比，同时掺加以减水剂为主的外 加剂进行配合比设计，可使粉煤灰混凝土的抗碳化性能有所改善6 钢筋锈

8、蚀混凝土中的钢筋能够防锈是由于混凝土的碱性(PH212.5)在金属表面形成一层 致密的钝化膜。在混凝土中掺加粉煤灰，一方面会消耗 Ca(OH) 2 ，降低混凝 土的碱环境；另一方面，粉煤灰又与Ca (0H) 2反应生成水化物，提高混凝土的 密实度，增加混凝土的不透水性和对氯离子扩散的阻力，阻碍和防止C02的侵入， 可对钢筋起保护作用，所以粉煤灰的掺入，在防止钢筋锈蚀方面，可以抵消因碱 度降低带来的不利影响。粉煤灰在一定的掺量范围(FAW24%),对钢筋锈蚀基本 无影响，甚至优于空白混凝土。但是若粉煤灰的掺量大于 30%，混凝土的 碳化 可使混凝土的PH值由12.5降至8.5左右，在这样低的PH

9、值条件下，钢筋不再 钝化。当碳化深度到达钢筋位置，保护层被完全碳化，在水与氧气渗入的条件下， 钢筋就会发生锈蚀而导致混凝土的开裂甚至破坏。7 水化热 粉煤灰对降低混凝土水化热的作用十分明显。低钙粉煤灰在头几天的水化程度并 不明显，所产生的水化热仅及水泥的一半。在混凝土中用粉煤灰取代 20%的水泥， 可使混凝土 7d的水化热下降11%。l-28d龄期内，大致为掺入粉煤灰的百分数， 就是温升和水化热降低的百分数。在大体积混凝土中粉煤灰的掺入一般可使水化 热峰出现的时间延缓至3d以后才出现，可以有效防止混凝土产生温度裂缝。关键词：强度，和易性，收缩，徐变，碳化性能，钢筋锈蚀，水化热。Fly ash

10、on properties of concreteAbstract：Fly ash to concrete changes in performance is divided into three stages, in the three stage, the concrete will gradually have an impact on the performance of the following， For example ，Intensity ，and easy，shrinking，creeps，carbonization performance，reinforcing bar c

11、orrosion，heat of hydration and so on.Performance of the fly ash concrete The change of the fly ash to concrete performance can be divided into three stages: 1 . Mix concrete stage newly: Influence the coagulation time of the concrete , is improving and easy , change rheology nature , can improve pum

12、p ,etc.; 2 .The concrete stage while hardenning: Regulate the course of hardenning, reduce the heat of hydration; 3 . The concrete stage after hardenning: Raise the intensity on later stage , raise every durability, such as anti-permeability, resist the sulphate corroding, suppress the alkali - Coll

13、ect the material and react etc.1 IntensityThe fly ash has three kinds of influence on the intensity of concrete: Reduce water consumption , increase glued material content and reflect through the long-term volcanic ash that raises his intensity. The particle in the fly ash of low calcium is the sili

14、con oxygen tetrahedron structure, ones own activation is very low. In the final result of the cement, high alkaline water silicic acid calcium and crystallization of 2 colloid of Ca (OH ) are very low in intensity, especially Ca (OH) 2 ask vigorous to come 1-2% , stone of intensity only, but Ca (OH)

15、 2 volume account for whole cement 25% , stone of volume. A large number of silicon , aluminium oxide that contain in the fly ash, can and high alkaline water silicic acid calcium 2 responses take place with Ca (OH ) 2 progressively, produce intensity than high low alkaline water silicic acid calciu

16、m , so , make cement stone water glue congeal quantity of material increase, and make its quality improved by a large margin , is favorable to the improvement of the intensity of concrete. Meanwhile, to is it can disperse cement particle to mix fly ash, make cement to be ink sufficient, improve clos

17、ely knit degree of the grout , make the intensity of the interfaces of the aggregate and grout in the concrete improve. Fly ash is it draw intensity compare with contribution of the bending strength compression strength to be heavy to confront with each other, this is useful to resisting the perform

18、ance of splitting of concrete. The elastic mould amount of the fly ash concrete is similar to compression strength, it is early and on the low side, improve progressively later stage , get 28d the constant basic concrete is raised by 5-10%. With to hold strength of wrapping up in reinforcing bar, 28

19、d , fly ash of concrete is it form intensity the same with ,etc. basic concrete of grade basically to glue, but homogeneity, fly ash of concrete fine, is it form intensity person who testgetting dispersed and kind than basic concrete to glue.Water two times of the fly ash reacts and generally starts

20、 after the concrete builds 14d, when temperature is low , will reflect necessary time is longer. If strict with early intensity of the concrete, the mixing amount of the fly ash is unsuitable to exceed 30%, winter construction large volume at the concrete, the mixing amount of the fly ash is unsuita

21、ble to exceed 20% non-. Because modern concrete China and foreign countries add use of pharmaceutical, on one hand, reducible concrete mix and stir water consumption, it reduce by dust water than,of not lasting concrete density of cement; On the other hand, water-reducing agent can make cement silic

22、ic acid calcium ink Ca (OH ) produced 2 increase, help fly ash and water two times of 2 of Ca (OH ) to reflect , excites the activation of the fly ash, this effectual for early intensity to improve fly ash, in addition use fly ash activation excite pharmaceutical or in large volume use strong cement

23、 early among the concrete non-, can compensate mixing the impact on early intensity of concrete of fly ash too.2 and easyWhether fly ash there are the following several points to concrete and getting easy improvement function. (l).The high-quality fly ash contains more than 70% of the spherical vitr

24、eous bodies, too smooth to have raised angle the body surface of these spherical glass, performance is steady, play a kind of lubricated function which is similar to the bearing in the pump sending , shaking the course of smashing of the concrete; (2).Mix particle apt to gather group cement in the c

25、oncrete newly, to is it can disperse cement particle effectively to mix fly ash, release more thick liquid body is it lubricate aggregate to come , help concrete work the improvement of performance ; (3).Mix fly ash can compensate detailed deficiency of bits in the detailed aggregate , cut off morta

26、r base body secrete continuity , water of channel, the fly ash that at the same time quality is good can reduce the mixing water consumption of the concrete under the same consistency, make water dust of concrete than reduce light level to , is it secrete ink and emanate the phenomenon to reduce.3 s

27、hrinkingThe shrink of the concrete relates to the fact that the mixing water consumption and thick liquid body body of the concrete are accumulated, the less water consumption is, shrink the smaller. High-quality fly ash water requirement than smaller than 100% , mix and stir reduction , water of qu

28、antity make mix fly ash concrete after the 28d since dry to shrink and not dry to shrink little. Fly ash concrete is it is it mix with fly ash improvement of quantity reduce too to contract to do. But because water , fly ash of concrete reponse slowly, moisture evaporate fast, so between fly ash and

29、 to do epitome to be loud and very much heavy in early days concrete. For prevent early shrink , fly ash of concrete from fracture , should strengthen in early days raising to it4 creepsIssue in the past will it be age one day, intensity of concrete relatively low, issue is it it meets an emergency

30、to be heavy than ordinary concrete too to creep corresponding age, but with ordinary concrete fly ash in one age one creep and was smaller than the ordinary concrete after this concrete of intensity.5 carbonization performanceFly ash concrete resist carbonization to be performance relatively poor. C

31、ement consumption , fly ash of concrete reduce, cement water Ca (OH ) 2 too corresponding to reduce quantity that appear, and volcanic ash is it consume Ca (OH ) 2 of a certain amount too to react, make hydrogen ion index of the concrete reduce, will increase the speed of carbonization of the concre

32、te. Unless especially at water in early days, fly ash the low in degree in response in volcanic ash,fly ash- there is the loose in structure in hole in system cement, CO2 , O2 , moisture ,etc. invade obstruction small, so carbonization is relatively large in depth. With the growth of one and gradual

33、 full play of the fly ash volcanic ash effect of age, the speed of carbonization will be reduced gradually. Carbonization , fly ash of concrete as water for dust in depth than and fly ash last increase of quantity and not increased to some extent. Than 0.5-0.55 in water dust, fly ash person who mix

34、greater than 30% and under the general construction situation of competence, the depth of carbonization of the concrete can reach about 20mm in 15-17. Carbonization reacts and goes on within the range of certain relative humidity fastest, otherwise, the response is relatively slow. Under or is close

35、 to 100% 25% in the relative humidity, namely the occasion in the abundant dryness or water saturation of concrete, have more difficult to produce carbonization shrink concretes.In such concrete projects not contacted with the atmosphere as the foundation works ,etc., because of isolating with CO2,

36、can take place carbonization react , so can mix and add the fly ash more , in order to fully reduce the heat of hydration of the concrete, improve the durability of the concrete. It is ultra to be adopt law quantity replace,water glue not lower than,mix carry on match ratio design water-reducing age

37、nt for admixture of main fact at the same time, can make fly ash concrete resist carbonization performance improve to some extent.6 reinforcing bar corrosionReinforcing bar of concrete can it antirusts to be because concrete getting alkaline to form one pre-pressing passivation membrane in metal sur

38、face. Is it add fly ash to mix among concrete, on one hand it consume Ca (OH ) 2, reduce the alkali environment of the concrete; On the other hand, fly ash 2 responses turn into the hyrate with Ca (OH), raise closely knit degree of the concrete, increase the impermeability of the concrete and obstru

39、ction spread to the chlorine ion, hinder and prevent the invasion of CO2, can shield the reinforcing bar , so fly ash is mixed, in preventing the reinforcing bar corrosion, can offset because alkali degree reduce adverse effect that bring. The fly ash is on certain range (FA 24% ) of mixing amount,

40、have not influenced the reinforcing bar corrosion basically, even superior to the blank concrete. But if the mixing amount of the fly ash is greater than 30%, the carbonization of the concrete can make hydrogen ion index of the concrete dropped to about 8.5 from 12.5 , under so low a hydrogen ion in

41、dex condition, the reinforcing bar is no longer passivated. Reach reinforcing bar position by depth as carbonization, complete carbonization , on terms that water and oxygen are permeated antiabrasion layer, reinforcing bar can take place but corrosion lead to the fact fracture of the concrete even

42、destroy.7 heat of hydrationThe fly ash is very obvious to reducing the function of the concrete heat of hydration.Low calcium fly ash water in the first several days obvious degree have, heat of hydration produced only and half of cement. Replace 20% of the cement with fly ash in the concrete , can

43、make the heat of hydration of concrete 7d drop by 11% . 1-28d in the issue, roughly for mix percentage of fly ash, Sheng Wen and percentage that heat of hydration reduce age. Fly ash is it can make heat of hydration time that peak appear is it appear to 3d to delay generally to mix among large volume concrete, can prevent the concrete from producing the temperature crack effectivelyKeyword: Intensity ，and easy，shrinking，creeps，carbonization performance， reinforcing bar corrosion，heat of hydration