Why is fly ash sometimes used in grout? Does it work the same as portland cement? How can fly ash be used to replace portland cement in some grouts?
Fly ash is material that is obtained from coal combustion. Fly ash itself does not work as a cement. However, if finely divided, it reacts with calcium hydroxide in the presence of moisture to form compounds very similar to the strength-producing compounds found when portland cement hydrates. Calcium hydroxide is one of the main components of portland cement. When fly ash is added to portland cement it therefore acts as a cementitious material. ASTM C 476 and C 270 permit the use of cement meeting ASTM C 595 "Standard Specification for Blended Hydraulic Cements." This specification allows 15% to 40% fly ash by weight of portland cement as a cement replacement. The maximum allowable amount of fly ash depends on the type of portland cement and the strength-gain characteristics of the blended cement. In addition to its cementitious properties when mixed with portland cement, fly ash enhances the grout's flow.