Different Effects Of Limestone Powder On Different Hardening System

Limestone powder as a low-cost and readily available mineral admixture is beneficial to the development of green concrete. This thesis studied the properties of paste, mortar, and concrete incorporated with limestone powder, including the mechanical properties, the pore structure of different hardening system and the hydration of cement. The pore characteristics of composite binder containing limestone powder with different fineness and the hydration of cement are given special emphasis.The results of the study show that:①The compressive strength of composite binder containing limestone powder:1) Limestone powder makes greater contribution to the early strength of mortar than paste or concrete. A decrease in water to binder ratio from 0.45 to 0.30, the contribution of limestone powder on the early strength of paste, mortar, and concrete are increased. Fine limestone powder makes greater contribution to the early strength of paste, mortar, and concrete than coarse limestone powder.2) As for the two kinds of water to binder ratio, the middle and later strength of paste decrease with an increase in the content of limestone powder, and the decreased amplitude is larger. For mortar and concrete system, the decreased amplitude is smaller in the case of low water to binder ratio, but it is reverse in the case of high water to binder ratio. Fine limestone powder has less unfavorable effect on the middle and later strength.② The pore structure of composite binder containing limestone powder:1)Limestone powder has favorable filling effect, With the increase of the content of limestone powder, The content of pores larger than 100 nm decreases and the content of microspores that their size less than 50 nm obviously increase. As a whole limestone powder plays a positive role in the pore structure of hardened paste, the unfavorable effect of limestone powder on the pore structure of hardened paste is more evident in the case of high water to binder ratio. Fine limestone powder makes greater contribution to the pore structure of hardened paste.2) There is little influence on the porosity if only a few percent of cement was replaced by the limestone powder in mortar and concrete, And the porosity of composite binder decreases. With the increase of the content of limestone powder, the porosity significantly increases.3)Adding a reasonable amount of limestone powder has little effect on the permeability resistance of concrete, and the limestone powder content is 8%, 15%, 20%,mixed with fine and coarse limestone powder can both make the permeability resistance of concrete reach the same grade of pure cement concrete at later age.③Hydration properties of composite binder containing limestone powder:1) Addition of limestone powder increases the very early-age exothermal rate and cumulative hydration heat, the promoting effect of fine limestone powder is higher than that of coarse limestone powder at the same replacement ratio. Increasing water to binder ratio decreases the promoting effect of limestone powder.2) Limestone powder has a certain promoting effect on the hydration of cement.The promoting effect of limestone powder on the hydration of cement is more evident in the case of a large amount of addition of limestone with low water to binder ratio. An increase in the fineness of limestone powder can also improve promotion effect, but the promotion effect is decreased by age.The experimental results show limestone powder has favorable filling effect and accelerating effect on the early hydration of cement. Limestone powder can make a dense microstructure with low porosity in the hardening system because of its filling effect. The accelerating effect is mainly related to the increased effective water to cement ratio and the increased number of nucleation sites, thus improves the early hydration properties of composite binder containing limestone powder.