Special topic cements and concretes

Concrete is a particulate composite of stone and sand, held together by an adhesive. The adhesive is usually a cement paste (used also as an adhesive to join bricks or stones), but asphalt or even polymers can be used to give special concretes. In this chapter we examine three cement pastes the primitive pozzolana the widespread Portland cement and the newer, and somewhat discredited, high-alumina cement. And we consider the properties of the principal cement-based composite, concrete. The chemistry will be unfamiliar, but it is not difficult. The properties are exactly those expected of a ceramic containing a high density of flaws. 

The lime is mixed with water and volcanic ash and used to bond stone, brick, or even wood. The water reacts with lime, turning it into Ca(OH)2 but in doing so, a surface reaction occurs with the ash (which contains SiOj) probably giving a small mount of (Ca0)3(Si02)2(H20)3 and forming a strong bond. Only certain volcanic ashes have an active surface which will bond in this way but they are widespread enough to be readily accessible. 

The chemistry, obviously, is one of the curses of the study of cement. It is greatly simplified by the use of a reduced nomenclature. The four ingredients that matter in any cement are, in this nomenclature 

In this terminoiogy, pozzoiana cement is C mixed with a voicanic ash which has active S on its surface. The reactions which occur when it sets (Fig. 20.1) are 

The tobomorite gei bonds the hydrated iime (CH) to the pozzoiana particies. These two equations are aii you need to know about the chemistry of pozzoiana cement. Those for other cements are oniy siightiy more compiicated. 

---

Each chapter is designed to provide the content of a 50-minute lecture. Each block of four or so chapters is backed up by a set of Case Studies, which illustrate and consolidate the material they contain. There are special sections on design, and on such materials as wood, cement and concrete. And there are problems for the student at the end of each chapter for which worked solutions can be obtained separately, from the publisher. In order to ease the teaching of phase diagrams (often a difficult topic for engineering students) we have included a programmed-learning text which has proved helpful for our own students. 

---