Effect of Calcium Compounds on Mechanical Properties of Eco-Friendly Non-Cement Mortar

Abstract

Improvement in the mechanical properties of non-cement mortars using ASTM Class C fly ash (FA) containing high amounts of CaO produced from a fluidized bed-type boiler and blast furnace slag (BS) was studied. This type of FA has been used to carbonate non-cement mortar under supercritical conditions for CO2 fixation, because such mortars cannot be recycled and easily reclaimed owing to the presence of high amounts of CaO that may adversely affect their eco-friendliness because of the high volume expansibility of CaO. Because a substantial increase in the compressive strength was observed for the specimen containing 20% FA, two types of Ca compounds—CaO and Ca (OH)2—were added to the BS specimens to investigate the effects of Ca compounds on the mechanical properties of non-cement mortar specimens. Three types of curing methods—room temperature, room temperature with hermetic sealing, and high temperature with hermetic sealing—were used and the specimens were aged for 3, 7, and 28 days. Because both CaO and Ca (OH)2 enhance the compressive strength at the concentration corresponding to that in the FA, it is speculated that the amorphous materials in the BS is destroyed without adding any acid or reagents because of the increase in pH by the Ca compound present in the specimen. An increase in the specimen pH may affect the strength improvement because of ion elution and curing reaction. It is concluded that enhancement in the mechanical property by the addition of 20% FA to BS can be explained from the results obtained by adding Ca compounds.