Study On The Effect Of Concrete Admixtures On Drying Shrinkage Of Hardened Cement Pastes And Its Mechanisms

The cracking caused by shrinkage is one of the most important factors which have significant influences on the durability of concrete. The workability and mechanical properties of high-performance concrete (HPC) were greatly improved by the aids of mineral and chemical admixtures, which become the two essential components of concrete. Meanwhile, these two components could be additional factors which affect the cracking caused by deformation and make the influences on cracking more complex. So it is necessary to get a new insight of the effects of mineral and chemical admixtures on drying shrinkage of cement and their mechanisms in scientific theory followed by solutions to the cracking of concrete caused by shrinkage in engineering practice.A rapid disc-method was applied in this research to study the effects of silica fume, fly ash, blast furnace slag, naphthalene superplasticizer and polycarboxylate superplasticizer on the drying shrinkage of cement pastes cured in standard condition. Concerned of mass concrete and steam-cured products, the early heat treatment was designed, that is the cement pastes cured at 60℃for the first day before cured at 20℃. And pore structures of cement pastes were measured by MIP and N2 adsorption experiments. These results could give further insights on the interpretations of the drying shrinkage mechanism of hydrated cement pastes.On the basis of the research results, the following conclusions could be reached:①The effects of mineral admixtures on the drying shrinkage of hardened cement pastes depend on the types, the dosage, the water to binder ratio(w/b), and the degree of hydration. When the amount of fly ash and ground blast furnace slag is more than enough, the drying shrinkage of hardened cement pastes will be reduced. When the dosage of silica fume is about 5.0% to 7.5%, the drying shrinkage of the hardened cement paste with lower water to binder ratio is slightly reduced.②Chemical admixtures could increase the drying shrinkage of hardened cement pastes, which depends on the types and the dosage of the superplastisizer, the water to cement ratio(w/c), and the degree of hydration. Commonly, the drying shrinkage of hardened cement paste with superplasticizers is more than the one without superplasticizers, and the drying shrinkage is increased by the dosage of superplasticizers. In order to reduce the drying shrinkage in engineering practice, the superplasticizers should be minimized when other properties are guaranteed. ③With the same mixing proportion, the drying shrinkage of hardened cement pastes with different ages are also different.The hardened cement paste without admixtures gets its greatest drying shrinkage at the early age.When water to binder ratio is lower enough, the hardened cement paste with silica fume reaches its greatest drying shrinkage at the early age. But when the water to binder ratio is higher, the drying shrinkage is increased by the ages. And for hardened cement pastes with fly ash and ground blast furnace slag, in addition to a greater drying shrinkage at the early age, there is also a most disadvantaged age with the greatest drying shrinkage, which happens in about 1 month.Hardened cement pastes with superplasticizer still get the greatest drying shrinkage at the early age. For hardened cement pastes using polycarboxylate superplasticizer, there is still a later (about 2 months) growth process for drying shrinkage.④When the mixing proportion is given, the drying shrinkage of hardened cement pastes are affected by the early heat treatment.Early heat treatment reduces the drying shrinkage of hardened cement pastes without admixtures.For hardened cement paste with silica fume, the early heat treatment could still reduce its drying shrinkage. And the same effect happens on the hardened cement paste with fly ash and ground blast furnace slag, but it is interfered by age and dosage for the later one.The drying shrinkage of hardened cement pastes with superplasticizer is still reduced through the early heat treatment. And for hardened cement paste using polycarboxylate superplasticizer, the early heat treatment increases the amout of later (about 2 months) drying shrinkage.⑤The pore structures of hardened cement paste are measured through the methods of MIP and N2 adsorption, and according to the results, mineral admixtures has increased the total porosity of hardened cement paste, and both nonporous pores of less than 32 nm and the large pores of more than 10μm have increased. Porosity of hardened cement paste with chemical admixtures has decreased. Meanwhile, the nonporous pores of 20~50nm have significantly decreased, and the large pores of more than 10μm increase slightly. The above results could explain the drying shrinkage development of hardened cement pastes tested.The above results could guide the application of concrete admixtures in engineering practice.