Long-Term Performance of Blended Cement Paste Containing Fly Ash against Sodium Sulfate Attack
Publication: Journal of Materials in Civil Engineering
Volume 30, Issue 12
Abstract
Fly ash, a supplementary cementitious material, is normally used in conjunction with portland cement to improve the durability properties of concrete in the field, such as sea-crossing bridges, high-speed railways, and mass concrete in dams. Currently, these concrete structures are frequently exposed to sulfate environments. The object of this paper is to investigate the relationship among content of calcium hydroxide (CH), formation of gypsum, and compressive strength, as well as the relationship among the formation of ettringite, pore structure, and expansion properties in blend cement paste at long-term sulfate attack, up to 1,110 days. Mercury intrusion porosimetry (MIP) and thermo gravimetric (TG) analysis were used to evaluate the pore structure and content of CH, respectively. The results show that there is a significant negative relationship between CH content and the coefficient of compressive strength during sulfate attack as more gypsum formation is contributed for the loss of strength. Compared to the reference sample, the addition of 20% and 40% fly ash decreased the CH content by 16.7% and 25.1% through the pozzolanic reaction, which reduced the formation of gypsum and prevented the decalcification of calcium silicate hydrate (C-S-H). Crystallization pressure, due to the formation of ettringite in a limited space, is discussed to explain the cracking of cement-based materials. Several harmful pores () were formed in the reference sample due to crystal pressure with the formation of ettringite, which caused the expansion and exacerbated the diffusion of sulfate ions. The pozzolanic reaction of fly ash also improved the pore structures and reduced the degree of supersaturation of ettringite by preventing the ingress of external sulfate ions.
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Acknowledgments
The authors would like to express sincere thanks to the National Natural Science Foundation of China (Grant Nos. 51608004 and 51778003), Anhui Natural Science Foundation (1708085QE102), Natural Science Foundation of the Anhui Higher Education Institutions (KJ2016A818), The Project of Anhui Provincial for Sending Visiting Scholars to Research Abroad (gxgwfx2018048), and Anhui Jianzhu University Doctoral Foundation (2015QD03) and Opening Foundation of State Key Laboratory of High Performance Civil Engineering Materials (2018CEM002).
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©2018 American Society of Civil Engineers.
History
Received: Jan 7, 2018
Accepted: May 29, 2018
Published online: Sep 12, 2018
Published in print: Dec 1, 2018
Discussion open until: Feb 12, 2019
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