MECHANISMS AND MITIGATION OF SHRINKAGE IN ALKALI-ACTIVATED SLAG
Open Access
- Author:
- Ye, Hailong
- Graduate Program:
- Civil Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- October 07, 2016
- Committee Members:
- Aleksandra Radlińska, Dissertation Advisor/Co-Advisor
Aleksandra Radlińska, Committee Chair/Co-Chair
Farshad Rajabipour, Committee Member
William Burgos, Committee Member
Carlo Pantano, Outside Member - Keywords:
- alkali-activated cement
shrinkage mechanisms
carbonation
blast-furnace slag
concrete
early-age cracking - Abstract:
- Ground granulated blast-furnace slag (GGBFS) is an amorphous by-product of the iron industry. It has a latent hydraulic reactivity, which can be catalyzed using proper activators, such as alkali metal hydroxides, carbonates or silicates, to form cementitious materials. Alkaline activated slag (AAS) produces alternative binders that could have important technical, economical and ecological advantages over traditional ordinary portland cement (OPC). However, this new binder suffers from several problems as a building material, including large autogenous and drying shrinkage and micro-cracking. The scientific reasoning behind the considerable shrinkage magnitude and extensive volumetric instability of AAS has not been explained to date. The main objective of this research is to understand the mechanisms of large shrinkage in AAS and to develop and evaluate effective mitigation strategies again these deterioration mechanisms. In the first chapter of this dissertation, the physical models of drying and shrinkage in cementitious materials were reviewed, as fundamental of this research. To investigate the shrinkage mechanisms of AAS, the extensive experimental program was implemented to study the physical-chemical changes in AAS during drying. The main finding of this work is that the shrinkage development of AAS shows pronounced visco-elastic/visco-plastic characteristics, which may be attributed to the reorganization and rearrangement of calcium-alumina-silicate-hydrates (C-A-S-H). The results also indicate that the incorporation of alkali cation (e.g. Na+, K+) into C-A-S-H is responsible for the characteristic viscous deformation of AAS. Several conventional shrinkage mitigation strategies used for OPC were implemented in AAS, including steam curing, sulfate-enrichment, and calcium-enrichment. Our results indicate that the steam curing can stabilize the C-A-S-H in AAS by strengthening the bonding between adjacent particles, hence reducing the magnitude of shrinkage; however, the early-age expansive reaction (e.g. due to the formation of sulfate-bearing phases) is insufficient in offsetting the long-term viscous deformation of C-A-S-H. The effect of carbon dioxide (CO2) in the atmosphere on the volume change of AAS during drying was studied by comparing the shrinkage behaviors of AAS in nitrogen gas and air environments. The results indicate that carbonation of AAS results in expansion, rather than shrinkage as in conventional OPC system, which may be attributed to the crystallization stress of calcium carbonates in the restrained pore structure of AAS. The effect of alkalis (Na+, K+) with various types and sources on the volumetric stability of OPC was examined, since it is proposed in this study that the incorporation of alkali cation in the nanostructure of C-A-S-H is the cause of high shrinkage and characteristic viscous deformation in cementitious materials, regardless it is AAS or OPC. The experimental results support the above statements. The last chapter summarizes the main findings of this research and provides some recommends for the future research.