The Romans. I'll try to get the details and repost.

Concrete is a material used in building construction, consisting of a hard, chemically inert particulate substance, known as an aggregate (usually made from different types of sand and gravel), that is bonded together by cement and water. The Assyrians and Babylonians used clay as the bonding substance or cement. The Egyptians used lime and gypsum cement. In 1756, British engineer, John Smeaton made the first modern concrete (hydraulic cement) by adding pebbles as a coarse aggregate and mixing powered brick into the cement. In 1824, English inventor, Joseph Aspdin invented Portland Cement, which has remained the dominant cement used in concrete production. Joseph Aspdin created the first true artificial cement by burning ground limestone and clay together. The burning process changed the chemical properties of the materials and Joseph Aspdin created a stronger cement than what using plain crushed limestone would produce. The other major part of concrete besides the cement is the aggregate. Aggregates include sand, crushed stone, gravel, slag, ashes, burned shale, and burned clay. Fine aggregate (fine refers to the size of aggregate) is used in making concrete slabs and smooth surfaces. Coarse aggregate is used for massive structures or sections of cement. Concrete that includes imbedded metal (usually steel) is called reinforced concrete or ferroconcrete. Reinforced concrete was invented (1849) by Joseph Monier, who received a patent in 1867. Joseph Monier was a Parisian gardener who made garden pots and tubs of concrete reinforced with an iron mesh. Reinforced concrete combines the tensile or bendable strength of metal and the compressional strength of concrete to withstand heavy loads. Joseph Monier exhibited his invention at the Paris Exposition of 1867. Besides his pots and tubs, Joseph Monier promoted reinforced concrete for use in railway ties, pipes, floors, arches, and bridges. http://inventors.about.com/library/inventors/blconcrete.htm

Read a book that proposed the theory [ I stress theory] that the limestone body of the Egyptian pyramids [ the million or so blocks] were NOT cut from a quarry, transported down the Nile and re-erected, BUT were actually a 'concrete' mix poured into a mould on site, and made with crushed limestone mixed with a solution [ secret/ lost] that was imported from central Saudi Arabia. This would explain how such a vast project could be completed within twenty or so years, using only laborers, and NOT using roller logs and ropes [ with the exception of the much fewer GRANITE blocks. The existing drawings of the construction method, many men hauling the blocks, only applied to the GRANITE construction, not the limestone. The 'proof' presented was that limestone in its natural state has all the larger shell bits lying in the same direction, the same as when they were deposited on the ocean floor, then compressed. The book claims that the pyramid blocks show these same shell bits in all DIFFERENT angles, just as though the material had been MIXED, then SET. Egyptologists know that pyramid building ceased abruptly for no known reason. The book claims that the supply of the SETTING AGENT, whatever it was, from Saudi Arabia, ceased suddenly, probably having run out, putting an end to these mammoth projects. The MIXTURE/ CONCRETE theory would also explain how the blocks fitted so close that not even a knife blade could be inserted. When one block was set, even partially dry, and the mould for the next block installed, then the face of the set block, showing, would form the first face of the new moulding, creating a perfect fit, and a perfect 'lock'. After a few dozen of these, no earthquake could possibly dislodge them. A very interesting theory.