Investigating the effects of mass ratios and molarities on the compressive strengths and formation of efflorescence in geopolymer concretes

• Main Author: Wei, Longtai
• Other Authors: Ting Seng Kiong
• Format: Final Year Project
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Engineering::Civil engineering::Construction technology
• Online Access: https://hdl.handle.net/10356/154376
• Institution: Nanyang Technological University

This study focuses on the effects of mass ratios of sodium silicate solutions to sodium hydroxide (NaOH) solutions and molarities of NaOH solutions on the average compressive strengths and degrees of efflorescence formation for fly ash based geopolymer concretes. This study also focuses on the effects of efflorescence on the average compressive strengths of the fly ash based geopolymer concretes. A fly ash to alkaline activator mass ratio of 3 was selected for all specimens. Mass ratios of sodium silicate solutions to NaOH solutions (thereafter referred to as mass ratios) of 1.5, 2 and 2.5 were used. NaOH solutions of molarities 12M, 15M and 18M were selected as well. 9 specimens each were casted from the following mix designs: 1.5 and 12M, 2 and 12M, 2.5 and 12M, 1.5 and 15M, 2 and 15M, 2.5 and 15M, 1.5 and 18M, 2 and 18M, 2.5 and 18M. The results for the average 28th day compressive strengths indicated that a higher molarity and a lower mass ratio resulted in the greatest average 28th day compressive strength. The results for the recalculated average 90th day compressive strengths (not accounting for readings which did not fall within ± 1 standard deviation of the initial average 90th day compressive strengths) indicated the same relationship as the average 28th day compressive strengths, with the sole exception of specimens casted with 12M NaOH solutions, where it was observed that a mass ratio of 2 instead of 2.5 led to the lowest average 90th day compressive strength. For a given mass ratio of 1.5 to 2.5, it appeared that more efflorescence was formed as the molarity of the NaOH solution increased from 12M to 18M. For a given molarity of 12M to 18M, it appeared that more efflorescence was formed as the mass ratio increased from 1.5 to 2.5. The effects of efflorescence on the average compressive strengths remain inconclusive, as a number of specimens displayed higher compressive strengths after the formation of efflorescence as compared to without efflorescence. It is recommended to increase the number of specimens crushed with efflorescence and without efflorescence, to provide a more fruitful insight on the effects of efflorescence on average compressive strengths. It is also recommended to use other mass ratios and molarities in the mix designs.