Activation of natural pozzolans, fly ashes and blast furnace slag

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One disadvantage of the use of lime-pozzolan cements (LPCs) is their slow strength development; however, continuous increase of strength at later ages due to pozzolanic reactivity indicates that it is potentially possible to increase early strength development. Three methods were investigated to accelerate the strength development of an LPC made with a natural pozzolan from Bolivia: (1) addition of chemical activators to the LPC pastes (chemical method), (2) elevated temperature curing of LPC pastes (thermal method) and (3) prolonged grinding of the natural pozzolan (mechanical method). Extensive optimization tests indicated that Na2S04 and CaC12 were the most effective chemical activators, and 4% Na2S04 or 4% flaked calcium chloride (CaC12.2H20) based on the mass of the LPC was an optimum dosage. The addition of Na2S04 improved the early strength significantly. The addition of CaC12 .2H20 was not helpful to the early strength gain, but increased the later-age strength by two or more times, especially at low curing temperature. These findings were confirmed by testing two LPCs made with two different Guatemalan natural pozzolans. Elevated temperature curing and prolonged grinding of natural pozzolan accelerated both the pozzolanic reaction and early strength development. An inverse relationship between temperature and the ultimate strength of lime-pozzolan pastes exists. Chemical activators showed different temperature sensitivity: Na2S04 showed the most efficient activation at 500C while CaC12 was most efficient above 500C. Increasing the fineness had a significant effect on control and Na2S04 pastes, but no significant effect on CaC12 pastes. A comparison based on strength development and economy indicated that the chemical method of activation was the best one among the three activation methods examined. The addition of Na2SO4 or CaC12.2H2O (a) to two lime-Guatemalan natural pozzolan, (b) to lime-LFA (LFA - a low calcium fly ash from Sundance, Alberta) and (c) to lime-blast furnace slag pastes confirmed the above findings. Na2SO4 increased both the early and later-age strengths. CaC12.2H2O increased slightly the early and later-age strength of lime-HFA (HFA - a high calcium fly ash from Laramie, U. S. A.) pastes. The main hydration products of lime natural-pozzolan, lime-LF A and lime-slag pastes in the absence of activators were calcium silicate hydrate (C-S-H) and C4AH13 • The addition of Na2SO4 increased the alkalinity of the solution, which accelerated the early pozzolanic reaction, and resulted in the formation of an AFt phase (ettringite), both of these effects explain the increase in early strength of the pastes. The addition of CaC12 decreased the alkalinity of the solution, retarded the early dissolution of a pozzolan and caused less C-S-H to form, but favoured the formation of C4AH13 or the solid solution of C4AH13 - C3A.CaC12• l0H2O. As the quantity of C-S-H increased, C4AH13 or the solid solution of C4AH13 - C3A.CaC12 • lOH2O acted as reinforcement; thus, CaC12 pastes give a high strength. The main hydration products of lime-HF A were C-S-H and ½A??. The addition of Na2SO4 not only resulted in the formation of AFt, but also increased the amount of ½A??, while the addition of CaC12 eliminated C3A??, and resulted in the formation of C4AH13 - C3A.CaC12 • lOH2O.
Bibliography: p. 240-253.
Shi, C. (1992). Activation of natural pozzolans, fly ashes and blast furnace slag (Unpublished doctoral thesis). University of Calgary, Calgary, AB. doi:10.11575/PRISM/10784