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Effect of Cellulose Nanofibrils on Some Common Durability Issues of Cement-Based Systems
- Author / Creator
- Goncalves, Jose Roberto
The most common detrimental processes that lead to reduction in durability in reinforced concrete elements are shrinkage, sulphate attack and chloride attack. Shrinkage is a dimensional instability chiefly caused by a loss of moisture in the hydrating cement paste. Soluble sulphate ions take part in ettringite-induced expansive reactions in concrete, while chloride ions trigger the corrosion process in embedded steel reinforcement.
Cellulose nanofibrils (CNFs) possess unique characteristics, including hydrophilicity, nano-dimensions, and excellent chemical tunability. These features confer upon CNF the ability to control internal moisture movement and also mitigate the ingress of harmful agents, including sulphate and chloride ions, into the cementitious system. A thorough review of the literature confirms that the proposed study is a novel scientific approach.
This study explores the feasibility and demonstrates successfully the ability of CNF to address these three durability concerns. Three types of CNFs were used in the study, each one with a distinct amount of carboxylate grafted upon the fibre surface. The CNF was incorporated in cement-based paste and mortar mixtures at various volume fractions. The mixtures were cast to the same workability and water-to-cement ratio, and the mortars had a fixed cement-aggregate ratio.
Results show that adding CNF moderately increased the shrinkage as measured at 2-days using image analysis. Similar growth in the drying shrinkage was observed in 90-day trials performed according to ASTM C596. However, CNF additions significantly alleviated bleeding and the shrinkage behavior was strongly influenced by CNF's ability to retain water in the early hours of hydration. Further evidence from the gas-sorption analyses reveals that this water accumulated in fresh mixtures due to CNF led to a larger volume of capillary pores and an increase mainly in the pore gel phase. As seen in the long-term trials, when this cumulative water was lost, there was a rise in shrinkage. This study reveals a retardation in hydration upon adding CNF. However, the samples containing CNF uniformly showed less cracking, which is likely due to fibres bridging the micro- and nano-cracks.
The mortars mixtures when dosed with CNF uniformly exhibited noticeable reduction in the ingress of sulphates ions and again, for chlorides ions. Under sulphate exposure, the CNF visibly reduced the penetration of sulphate ions and effected a marked reduction in ettringite as well as the associated ettringite-induced expansion, measured as per ASTM C1012/C1012M. The decrease in the ettringite content (as measured by X-ray analysis) in the CNF-loaded Type GU Portland cement paste mixtures even after 90 days in sulphate solution was comparable to that seen with Type HS Portland cement. At the same time, the compressive strength of CNF-cement mortars was enhanced. Similar improvement was observed in the resistance of mortars loaded with CNF against chlorides ion ingress, as substantiated by the results of the rapid chloride penetrability test (as described in ASTM C1202) and the silver nitrate-based colorimetric method.
The performance verified here can be credited to physical and chemical mechanisms trigged by CNF. Despite a slight increase in the capillary pores ranging from 100 nm – 10 μm as seen from the analysis of SEM images, the porosity in the range of 10 nm to 140 nm was moderately reduced as increasingly amounts of CNF were added to cement pastes. As a result, the pore network in this range experienced a rise in the tortuosity and a reduction in the connectivity, which ultimately hindered the ingress of sulphate ions and chloride ions. Concurrently, the negatively charged ionic groups of CNF chemically scavenged some of the calcium cations soluble in the aqueous phase (as evident from the thermogravimetric analysis), which are essential to the ettringite phase formation. This further explains why CNF loadings slowed down the ettringite content. However, the pH of CNF-cement pastes remained unaltered, notwithstanding the reduction in the calcium content in the pore solution. It is safe to say that CNF will not break the protective passive film of the embedded steel bars. The results show CNF as an all-round promising additive to hamper the ingress of deleterious agents in cement-based systems.
- Subjects / Keywords
- Graduation date
- Fall 2019
- Type of Item
- Doctor of Philosophy
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