常用早强剂对砂浆早期收缩和开裂性能的影响

发布时间：2018-03-10 05:06

本文选题：砂浆　切入点：早期开裂　出处：《哈尔滨工业大学》2014年硕士论文　论文类型：学位论文

【摘要】：对混凝土结构而言，早期开裂一直是一个十分重要的问题。而现如今，为了应对一些低温施工的工况，人们通常使用一些含盐外加剂以改变混凝土的早期性能。常用的含盐外加剂虽在多方面改善了混凝土及砂浆的性能，但也很大程度地影响了其早期开裂。因此，研究常用含盐外加剂对砂浆早期开裂的影响，对增强其耐久性具有实际意义。本文选择甲酸钙、乙酸钙和硫氰化钠三种无机盐外加剂，研究了它们在0.5%、1.0%、1.5%和2.0%四种不同掺量情况下，对水灰比为0.30、0.35和0.40的砂浆的早期开裂和强度的影响。同时，本文还研究了它们在以上三种掺量下对水泥凝结时间及化学结合水含量、水化热的影响。通过研究发现，甲酸钙和乙酸钙在一定的掺量范围内都对水泥砂浆的早期开裂具有较明显促进作用，而当两者掺量的提高一定程度时，对砂浆的早期开裂的促进作用又会有所减弱，而且裂缝宽度分布逐渐集中；硫氰化钠对砂浆早期开裂具有促进作用，且随着硫氰化钠掺量的增加，这种促进作用越明显，且平均裂缝宽度则呈逐步递增趋势。在水泥水化方面，甲酸钙对水泥0~3h的水化过程有促进作用，而后则会减缓水泥水化速率；乙酸钙在水泥水化最初阶段对水化速率影响较小，在90min后，会起到抑制水化的作用；硫氰化钠对水泥水化有着明显的抑制作用。在强度方面，适量的掺入甲酸钙会起到明显的早强作用，但在水灰比并不高的情况下，甲酸钙掺量过大时反而会起到抑制砂浆强度增长的作用；乙酸钙对砂浆强度的作用于甲酸钙类似，少量掺入会提高强度，掺量过高则会减缓砂浆强度增长速率；硫氰化钠的掺入则对砂浆强度的影响很小。在凝结时间方面，甲酸钙起到极为显著的促凝作用，过高掺量甚至会导致速凝现象，，且甲酸钙具有一定的减水作用；乙酸钙对水泥凝结时间的影响整体表现为促凝；硫氰化钠则起到明显的混凝效果，且凝结时间随着硫氰化钠掺量的增加而延长。甲酸钙和乙酸钙都可以在混凝土工程之中作为促凝剂和早强剂使用，但是掺入这两种盐同时也存在促进早期开裂的风险；硫氰化钠可作为缓凝剂使用在工程中，硫氰化钠的掺入对强度影响较小，但会使后期强度增长速率加快，但也会对混凝土构件的早期开裂造成不利影响。因此，在实际的工程中，需注意对外加剂掺量的控制，以确保其在最佳使用范围内。
[Abstract]:Early cracking has always been a very important problem for concrete structures. Now, in order to deal with some low-temperature construction conditions, Some salt-containing admixtures are usually used to change the early performance of concrete. Although the commonly used salt-containing admixtures improve the properties of concrete and mortar in many ways, they also greatly affect the early cracking of concrete and mortar. It is of practical significance to study the effect of common salt-containing admixture on early cracking of mortar. In this paper, three inorganic salt admixtures, calcium formate, calcium acetate and sodium thiocyanate, are selected, and their effects on the early cracking and strength of mortar with water-cement ratio of 0.30 ~ 0.35 and 0.40 are studied under four different dosages of 0.5% and 2.0%. The effects of the above three contents on the setting time of cement, the content of chemically bound water and the hydration heat of cement were also studied. It is found that both calcium formate and calcium acetate can obviously promote the early cracking of cement mortar in a certain range, but when the content of both is increased to a certain extent, The promoting effect on the early cracking of mortar will be weakened, and the distribution of crack width will be concentrated gradually, and sodium thiocyanate can promote the early cracking of mortar, and with the increase of the content of sodium thiocyanate, the more obvious the promoting effect is. In the aspect of cement hydration, calcium formate can promote the hydration process of cement for 3 h, then slow down the hydration rate of cement, and calcium acetate has little effect on the hydration rate in the initial stage of cement hydration. After 90 minutes, the hydration of cement was inhibited by sodium thiocyanate, and the hydration of cement was inhibited by sodium thiocyanate. In terms of strength, proper addition of calcium formate played a significant role in early strength, but the ratio of water to cement was not high. The effect of calcium acetate on the strength of mortar is similar to that of calcium formate, a small amount of calcium formate can increase the strength of mortar, and the increase rate of mortar strength will be slowed down if the content of calcium formate is too high. The addition of sodium thiocyanate has little effect on the strength of mortar. In setting time, calcium formate plays an extremely significant role in promoting coagulation, too much content of calcium formate even leads to rapid coagulation, and calcium formate has a certain water reducing effect. The effect of calcium acetate on the setting time of cement is as a whole to promote coagulation, while sodium thiocyanate plays an obvious role in coagulation, and the setting time is prolonged with the increase of sodium thiocyanate content. Both calcium formate and calcium acetate can be used as coagulants and early strength agents in concrete projects, but there is a risk of early cracking in the presence of both salts; sodium thiocyanate can be used as a retarder in engineering. The addition of sodium thiocyanate has little effect on the strength, but it will accelerate the growth rate of strength in the later stage, but it will also have a negative effect on the early cracking of concrete members. Therefore, in practical engineering, attention should be paid to the control of admixture content. To ensure that it is in the best range of use.
【学位授予单位】：哈尔滨工业大学
【学位级别】：硕士
【学位授予年份】：2014
【分类号】：TU528

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