Concrete inFocus - Spring 2013 - (Page 16)

materials Fly Ash FAQ F ly ash continues to generate many questions in the ready mixed concrete industry. Here are the most frequently asked questions about fly ash and its use in concrete. Concrete Infocus thanks Boral Material Technologies, Inc. (BMTI), a leading marketer of fly ash, for supplying the answers. Where does fly ash come from? Fly ash is produced through the combustion of coal used to generate electricity. After coal is pulverized, it enters a boiler where flame temperatures reach up to 1,500°C. Upon cooling, the inorganic matter transforms from a vapor state to a liquid and solid state. During this process individual, spherical particles are formed. This is fly ash. It is then collected by either using electrostatic precipitators, bag houses or a combination of both. Fly ash from these systems is collected in hoppers and then transferred to storage silos. Fly ash is tested for physical properties such as fineness, loss on ignition and moisture before it is allowed to be shipped to its end user. What is the difference between Class C and Class F fly ash? The primary difference between Class C and Class F fly ash is the chemical composition of the ash itself. While Class F fly ash is highly pozzolanic, meaning that it reacts with excess lime generated in the hydration of portland cement, Class C fly ash is pozzolanic and also can be self cementing. ASTM C618 requires that Class F fly ash contain at least 70 percent pozzolanic compounds (silica oxide, alumina oxide and iron oxide), while Class C fly ashes have between 50 and 70 percent of these compounds. Typically, Class C fly ash also contains significant amounts of calcium oxide—over 20 percent. Most Class F fly ash contains little calcium oxide; however, some Class F fly ash sources may contain intermediate levels (8 to 16 percent) of calcium oxide. While both classes of fly ash greatly reduce concrete permeability as compared to the cement only mixes, Class F tends to give proportionately greater permeability reduction. Due to the higher levels of pozzolanic compounds, Class F fly ash mitigates against sulfate attack, alkali silica reaction, corrosion of reinforcement and chemical attack. While Class C fly ash generally improves concrete durability as related to these forms of attack, higher replacement percentages may be necessary to effectively mitigate them. Fly ash particles. Image courtesy of Pittsburgh Mineral & Environmental Technology, Inc. ( Photo by Randolph W. Shannon, PMET microscopist. 16 ı SPRING 2013

Table of Contents for the Digital Edition of Concrete inFocus - Spring 2013

Corporate Suite
Enviro Scene
A Legacy in Construction
CalPortland Slip Form Success
Fly Ash FAQ
Acceptance Test Reports for Ready Mixed Concrete
First Batch Plant Certifications in Mongolia
Index of Advertisers
Taking It to the Streets
Impact of Specifications on Concrete Quality

Concrete inFocus - Spring 2013