Research Of Effects And Mechanism Of Manufactured Sand Characteristics On Portland Cement Concrete

With the rapid development of infrastructure construction and increasing attention being paid to environmental protection, there is a lack of the natural sand used to meet the requirements for engineering construction, so the use of manufactured sand (MS) to prepare concrete has become a trend. However, compared with natural sand, manufactured sand has remarkable differences on its shape, gradation, surface texture and the content of particles whose size is less than 0.075mm. Therefore, it has a practical and theoretical significance to research the influence of these characteristics of manufactured sand on performances of Portland cement concrete and its mechanism.Firstly, the influence of six stone dusts with representative lithologic characters on the action effect of chemical admixtures and the concrete performances were researched, which include quartzite, gneiss, granite, basalt, limestone and marble. The results showed that the change of lithologic characters of stone dust had an obvious effect on the adaptation of chemical admixtures, especially the polycarboxylate superplasticizer, while had no significant effect on the action of mineral admixtures. The change of lithologic characters of stone dust had a slight influence on concrete workability, strength and volume stability, and had little effect on the durability of MS concrete.Secondly, the effect of limestone dust content in the manufactured sand on concrete properties was studied. The results showed that the proper amount of stone dust could improve the gradation of MS, increase the paste volume and fill the void among particles, which could improve the workability and strength of MS concrete. However, the optimum content of stone dust in MS was different for MS concretes of different strength grade. The proper amount of stone dust could improve the durability of concrete, including the resistance to chloride ion permeation, the frost resistance, the sulfate resistance and abrasion resistance. But it was harmful to control the drying and plastic shrinkage that the stone dust content was more than 7%～10%. So, it is provided that the limited value of stone dust in MS in the national standard "Sand for Building" can be raised to 5 %, 7 %, and 10% or even higher.Thirdly, the effect of characteristic indexs of MS on the concrete performance was studied, such as particle shape, graduation, crushing value, et al. It was found that the routine test method of the sand roughness could not reflect the particle shape of MS accurately influenced by its graduation and stone dust content. It was also found that the angular MS could reduce the workability of MS concrete due to the increase of yield stress, and improve its strength and volumetric stability. However, there is no obvious correlation between the strength of MS concrete and its crushing value. The mortar rheological test results showed that the proper increase of particle content of 0.15～0.3mm and 0.3～0.6mm was helpful to improve the workability of MS concrete.Fourthly, influence of the change of MS MB value on the concrete properties was studied by adding clay powder into MS. The results showed that MB value had no correlation with the lithologic character and stone dust content of MS, while was in direct proportion to clay powder content. It was also found that MB value was influenced by liquid limit index of clay powder. With the increase of MS MB value, the plastic viscosity of concrete and its adsorption of the chemical admixtures, especially polycarboxylate superplasticizer, were increased. Due to the increase of MS MB value, the workability and volumetric stability of MS concrete was deteriorated significantly. But the mechanical properties and durability of MS concrete were also influenced by its strength grade. The proper MB value could improve the strength and durability of MS concrete for low-strength concrete, while for high-strength concrete, the increase of MB value could lead to the deterioration of strength and durability of MS concrete.Finally, the promotion of stone rust on cement hydration and its filling effect on concrete void were studied by using XRD, SEM, MIP, micro-hardness, micro-calorimeter, et al. The results showed that stone dust had crystal effect, and its CaCO₃ particulate participates the hydration of C₃A, promoting the early hydration of cement. With the higher CaCO₃ content in stone dust, the hydration promotion could be more notable. Proper amount of stone dust in MS densified the interfacial transition zone (ITZ) and improved pore structure of MS concrete, which enhanced the properties of MS concrete. However, if the quantity of stone dust was too high, the free stone dust could exist in ITZ, which was harmful to the bond between aggregate and hardened cement paste, and decreased the performance of MS concrete.