Cement is the dry powder and is chiefly made from lime.
The calcium in the final form is bound up in calcium silicates, which are not very soluble in water. Not all concrete radically alters pH. Many municipal holding ponds are made of concrete, both on the supply and the water treatment side. Large changes in pH would not be cool in this case. What determines concrete's chemical behavior after setting is the chemical composition of the aggregate. Crushed shells have also been used in the past, but are not used now in most applications.