Study On Influence Of Compositions On Hydration And Shrinkage Cracking In Cement-based Materials At Early Ages

Shrinkage deformation of cement-based materials is a hot topic that has attracted a great attention for a long time. With the development of concrete technology, many techniques, such as material compositions, and testing methods, have been advanced. However, cracking problem in concrete structures has not been solved, especially the cracking sensitivity of cement-based materials at early ages. Under the urgent need of the sustainable development and durable design of concrete structures, it's important to understand the influence of different factors on early hydration and deformation of cement-based materials, the mechanism of early age cracking and their influence on mechanical properties of cement-based materials, and the relationship between microstrucutre and global performance of cement-based materials.In this thesis, the experiments were designed with different cement mix proportions. Some contains additional soluble alkali, some cheimcal admitures, and some fly ash and silica fume. The initial cracking time was meastured using a new ellipse-ring shrinkage cracking testing apparatus with multi-channel. Meanwhile, the rate of cracking, mechanical strength, free shrinkage and corresponding weight changes were measured to investigate the effect of different compositions. Besides, the influence of the different compositions on the early hydration and hardening of cement pastes was studied using a new non contact resistivity meter together with the measruement of hydration heat. Other techniques such as thermogravimetry, differential thermal analysis, scanning electron mirocscopy had been used to investigate the microstructure of the samples. The mix of shrinkage-reducing admixture and shrinkage-compensating admixture was studied to verify its effective in controlling early-age shrinkage. The phase change materials with a larger latent heat and isothermal phase change process were adopted as the temperature-control component to adjust and control the temperature-rising process during cement hydration process.The results showed that under the experimental conditions and the dosage range ofthe current study, higher alkalinity led to higher cracking sensitivity of cement-based materials. Moreover, it was found that the cracking sensitivity was also influenced by the ratio of water-to-cement when some alkalinity was present. The initial craking time of mortars was largely influenced by alkali types with the following sequence: Na₂SO₄ KOH K₂SO₄ and NaOH. The Na₂SO₄ is the most sensitive component for shrinkage cracking in cement-based materials. This implied that the application of naphtha-based chemical admixtures in cement-based materials might be one of the causes of shrinkage cracking. On the other hand, the results showed that the addition of fly ash or shrinkage-reducing admixtures might lower the cracking sensitivity of mortars with higher alkalinity.The hydration process revolved by resistivity measurement was used to compare the shrinkage cracking of cement-based materials at the early ages. The results demonstrated that the lower the minimum value of resistivity during the very early hydration process, the shorter the initial cracking time. Between them, there existed a good linear correlation (the correlation coefficient >0.90). In addition, the development of the shrinkage ratio vs. weight loss ratio with ages was studied. It's found that the lower weight loss ratio at the same shrinkage ratio or the larger the shrinkage at the same weight loss ratio, the shorter the initial cracking time. So, the ratio of shrinkage and weight loss might be taken as one paramenter, together with the initial cracking time, to evaluate the cracking sensitivity of cement.It was found that the addition of shrinkage-reducing admixtures could greatly reduce surface tension of pore solution and consequently lower the free shrinkage strain, delay the initial cracking time and decrease crack sensitivity of mortar. But, shrinkage-reducing admixtures did not have the function of compensating shrinkage. Moreover, it might accelerate the early hydration of cement pastes under semi adiabatic conditions, which might cause thermal cracking. Therefore, some components in shrinkage-reducing admixtures should be adjusted when shrinkage-redcuing admixtures are applied under higher temperature. Shrinkage-compensating admixture delayed theearly hydration of cement pastes and the initial cracking time although shrinkage-compensating admixture could not lower free shrinkage. The blending of shrinkage-reducing admixture and shrinkage-compensating admixture significantly lowered the free shrinkage, and consequently improved the cracking-resistant of mortar.