英语翻译Modern cement,called Portland cement,was invented in 182

英语翻译
Modern cement,called Portland cement,was invented in 1824.It is a mixture of limestone and clay,which is heated and then ground into a power.It is mixed at or near the construction site with sand,aggregate small stones,crushed rock,or gravel,and water to make concrete.Different proportions of the ingredients produce concrete with different strength and weight.Concrete is very versatile; it can be poured,pumped,or even sprayed into all kinds of shapes.And whereas steel has great tensile strength,concrete has great strength under compression.Thus,the two substances complement each other.
They also complement each other in another way:they have almost the same rate of contraction and expansion.They therefore can work together in situations where both compression and tension are factors.Steel rods are embedded in concrete to make reinforced concrete in concrete beams or structures where tensions will develop.Concrete and steel also form such a strong bond─ the force that unites them─ that the steel cannot slip within the concrete.Still another advantage is that steel does not rust in concrete.Acid corrodes steel,whereas concrete has an alkaline chemical reaction,the opposite of acid.