A Study On Early-age Shrinkage Of Concrete And Cement Paste

This paper focuses on early shrinkage of concrete and cement paste which is the hot topic nowadays and makes a in-depth research from experimental study, mechanism analysis, and theory three aspects.3 days early shrinkage of recycled concrete from initial setting time with different replacement rates of recycled aggregate was investigated by self-made equipment. The differences between unimproved recycled concrete and the improved which utilizes new mixing method that R-aggregates is coated with slag and nanoCaCO3 are observed, and relevant mechanism is analyzed. The results illustrate that early shrinkage of recycled concrete outweighs natural concrete, but there is not a definite link between the replacement rate of recycled aggregate and the early shrinkage. The treatment can effectively reduce the shrinkage of recycled concrete in 10 hours, but cannot have a sustaining effort. Early shrinkage of recycled concrete is determined by pores and cracks in the old cement mortar of recycled aggregate which accelerate the loss of water, and prewet of R-aggregates which enlarges pore size. The treatment can fill up the pores and cracks and lower soluble ion concentration to reduce the early shrinkage.To further analyze the mechanism of shrinkage of concrete, and exclude the effect of aggregates, the early shrinkage of cement paste was studied. Effect of the naphthalene series superplasticizer and polycarboxylates, influence of temperature deformation and water evaporation of cement paste on the early shrinkage of cement paste were analyzed. The results illustrate that addition of water reducers substantially increased early shrinkage of cement paste. We infer that the water-reducing agent thins the diameter of pores which leading to an increase of the pressure, on the other hand, the addition of water reducers into cement paste increases the ratio of Water evaporation and free water. Two reasons above contribute to the increase of shrinkage.To analyse the main effect of early shrinkage of cement paste, based on capillary pressure theory, effect of surface tension of water in capillary, total porosity, pore size distribution, and the contact angle on shrinkage of cement paste were introduced, with the changing of different dosages and types of water reducers and different phases of shrinkage. These studies provide a base to start a further research on early shrinkage of concrete and cement paste...