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The Romans first invented what today we call hydraulic cement-based concrete. They built numerous concrete structures, including the Pantheon in Rome, one of the finest examples of Roman architecture that survives to this day, which has a 42-meter-diameter dome made of poured concrete . The name concrete comes from the Latin "concretus", which means to grow together. This is a good name for this material, as the chemical hydration process, which mainly occurs over the time scale of hours and days, causes the material to grow together from a viscoelastic, moldable liquid into a hard, rigid solid. In our world today, concrete has become ubiquitous, and in fact it is hard to imagine modern life without it. About five billion tonnes of concrete are used around the world each year, enough for close to one tonne for each person per year, at a volume of about 400 liters per person. The cement used mostly in today's concrete is called portland cement. The process to produce portland cement was invented by Joseph Aspdin in the early 1800's in England. The name portland may have been originally a marketing ploy, as portland building stone was very popular in England at that time , and Aspdin may have wanted people to favorably compare concrete made with his cement to the popular building stone.
It is important to remember that cement is the powder that reacts with water to form cement paste, a hard, solid material that forms the matrix for the concrete composite. The addition of sand (fine aggregates) that are up to a few millimeters in diameter makes mortar, and the addition of rocks (coarse aggregates) of up to a few centimeters in diameter makes concrete. It has always been known that concrete is a porous material, whose properties depend on its pore space. There are many different kinds of pores in concrete, ranging from the air voids that are entrapped in the mixing process, which can be quite large, up to a few millimeters in diameter, to the capillary pores, which are essentially the space occupied by the leftover water from mixing, down to the nanometer-scale pores that exist in some of the hydration products produced by the cement-water chemical reaction.
Until recent years, the overwhelming focus has been on concrete's compressive strength, which has been mainly related to the overall porosity of the cement paste matrix and the amount and structure of the aggregates. Mechanical strength depends on defects and not on any overall average property, and so is very difficult to relate to microstructure. This has caused relatively little attention to be paid to the details of the pore space. Unfortunately, it has perhaps led to the the idea that concrete is simply a commodity material, with nothing needed to be understood about the microstructure. However, more recently, it has been recognized that much of the concrete in the infrastructure in the U.S. and Europe and elsewhere has been deteriorating faster than expected, with much of this deterioration due to the corrosion of reinforcing steel coming from the ingress of chloride and other ions from road salts, marine environments, and ground soils. Hence close attention is now being paid to the transport properties of concrete (diffusivity, permeability, sorptivity, etc.) which, although still difficult to relate to pore structure and microstructure, are easier to study in a fundamental way than is compressive strength . This has led to new attention being paid to the microstructure of concrete, with the realization that concrete is a complex composite, whose improvement and control require the usual materials science approach of processing, microstructure, and properties.
This chapter briefly reviews some of the main ideas that have been proposed and partially validated to attempt to explain the microstructure of concrete and its effect on transport properties. The main ideas used are percolation and composite theory, combined with quantitative computer simulations. This chapter reflects the authors' view of concrete microstructure, and draws heavily on computer simulations of the microstructure. Not every part of this view has been validated experimentally, though much has, so that we expect some parts to change over time as new experiments (and new simulations!) are performed.
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