No pure cement is not stronger than concrete as cement is only binding materials which bind aggregate and sand with the help of water. If alone cement is used it will shrink and has no compressive strength for which concrete is known for.
Pure cement is actually a component of concrete.
Cement is actually a dry and grayish powder that solidifies when water is added to it. It is the ingredient that hardens concrete when water is mixed.
CONCRETE, on the other hand, is made up of at least three components- cement, sand and aggregates. Cement can be considered as the “glue” that binds all these ingredients.
Concrete can be considered stronger since it contains gravel and aggregates that make it more hard-wearing. Concrete also gets harder with age.
Cement is a binder and concrete forms the bulk.
Filling a void with just cement paste will very highly economical as cement is a costly ingredient in concrete. The bulk is made of aggregates and sand, so the voids between the larger particles are filled by the lower size particles and finally sand. The cement paste coats the aggregates and sand and lubricates them improved compaction.
The strength depends on the water/cement ratio. Lower the ratio, higher is the strength and vice versa. But lower water content makes it difficult to compact the concrete especially if it is a reinforced concrete structure. To overcome the difficulty we use large-sized aggregate in mass concrete structures and lower size aggregates in reinforced concrete structures. For the same volume of aggregates, larger particle size has a lesser surface area so a lesser quantity of the water-cement paste will be adequate.
A concrete mix can be designed for any particular strength dictated by the design of the structure.
So there is nothing like whether concrete or cement being the stronger of the two.