Limestone powder has been widely used in concrete structures because of its easy availability and positive effect on reducing the heat of hydration. The addition of limestone powder has a significant influence on the hydration and microstructure of cement paste and therefore affects the strength and durability of concrete. This paper examines the hydration and microstructure of cement paste blended with limestone powder using multiple techniques. The results reveal that when the content of limestone powder is up to 50 wt%, the compressive strength, cumulative heat and portlandite show a significant reduction. However, the addition of limestone powder has no effect on the species of hydration products and pore solution composition in cement paste specimens. The porosity and relative threshold diameter of cement paste samples increase while the pore volume (<50 nm) decreases with the increase in limestone powder content. The total electrical resistance of continuously and discontinuously connected pores in cement paste samples has an exponential relationship with the threshold diameter and a linear relationship with the pore volume (<50 nm) and porosity.