Concrete has been used as a building material by man for a very long time, and columns constructed over 3500 years ago can still be found standing in Egypt, and the coliseum in Rome is another ancient example. The cements used in these early constructions were made from a mixture of ground lime, calcium and shale, mixed with rock and sand.
The cements which we use today are normally mixtures of tricalcium aluminate (3CaO • Al2O3), tricalcium silicate (3CaO • SiO2), and dicalcium silicate (2CaO • SiO2) combined with iron and magnesium compounds. Gypsum is frequently added to help decrease the speed of hardening which initially occurs by the hydration of the tricalcium silicate upon the addition of water. The subsequent hydration of the other substances completes the curing process, which produces heat, and which can take years.
Varying the balance of these substances produces cements with different qualities useful in different circumstances and conditions, and the addition of additives can improve concrete strength, wear-resistance and cracking. Fast drying cements are useful for small projects, but the heat produced can cause cracking, making them unsuitable for large structures. Low-heat cements containing a high proportion of dicalcium silicate are normally used for large-scale pours.
For concrete pumping a correctly balanced mix is imperative, and this is generally 1 part cement, 2 parts sand and 3 parts aggregates. However there are many different factors to consider in blending the perfect mix.
When pumping cement it is important that there is sufficient mortar to bind the rock, and enough sand to contain and fill the rocks. Good concrete mixer becomes more and more important. Generally mixtures with larger size aggregates make for a stronger final product, but the mix should contain a range of aggregate sizes to ensure that no air spaces remain, as these would debilitate the finished structure. Mixtures with larger aggregates separate more easily, so less water is added and more cement powder.
There must be enough cement to slurry the line, as when pumping the cement by concrete mixer is the lubricant which permits the mixture to slide through the pipe. The smaller the rocks the more important the amount of cement is, and generally the more cement in the mixture the easier it is to pump.
The correct amount of water is essential, as while more water will make the mix more fluid and easier to move, too much water will prevent the correct chemical reaction essential for achieving maximum strength.