Performance and Mechanism of Cementitious Composites Modified with Cellulose Nanocrystals

纖維素納米晶改性水泥基複合材料的性能與機理

Student thesis: Doctoral Thesis

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Award date18 Aug 2021

Abstract

Nanotechnology of cementitious materials in concrete has achieved significant progress. On the one hand, as a gene for concrete, exploring the structure of Calcium Silicate Hydrate (C-S-H) gel from nanoscale is the focus of most research. It can be helpful to understand the hydration and hardening of the cementitious material, the generation and development of mechanical properties, as well as the interface structure in concrete. On the other hand, following the concept of “restructuring large structures through molecular reorganization”, the application of nanomaterials to achieve the modification and design of C-S-H gel structures is the most fundamental and promising method for solving a series of problems such as cracking, low strength, poor durability, etc. in concrete.

As a biopolymer nanomaterial, cellulose nanocrystals (CNC) can be sustainably obtained from trees, cotton, flax and other plants. CNC has the characteristics of low aspect ratio, high crystallinity, excellent mechanical properties, low thermal expansion coefficient and high surface activity, which makes it a modified material of cementitious composites with great potential, and meets the development trend of green concrete. However, the application research of CNC in cement-based materials (CBMs) is only at the early stages, and the advantages and approaches brought by the characteristics of CNC are still unclear. Systematic research on CNC modified CBMs is currently needed.

In this thesis, CNC was used as a modified material in different scales of concrete, from macroscopic to nanoscopic. The effect of CNC on the properties of cementitious composites and the modification mechanism was explored.

In the early stage of hydration, the addition of CNC has a significant increase in the flexural strength of the cement while the impact on the compressive strength is not obvious. The addition of 0.5 wt.% CNC can achieve a 41.86% increase in flexural strength for the pure cement samples cured for 3 days. With the increase of the curing age, the improvement effect of CNC on the compressive strength of cement increases. When the content of CNC in cement is 0.5 wt.%, the flexural and compressive strengths after 28 days of curing increased by 27.63% and 18.43%, respectively.

The addition of CNC can promote the hydration of cement, thereby generating additional Ca(OH)2, but the content of Ca(OH)2 does not increase proportionally with the increase of CNC content. When the additive amount of CNC is 0.5 wt.%, the increase in the Ca(OH)2 content reached a maximum of 10%. In addition, the deepening of the hydration degree also led to a 15% decrease in the porosity of the cement sample, and the pore size of the cement sample was found to be significantly reduced by 45%. It is worth noting that during the 3-day hydration age, the addition of CNC prolongs the hydration induction period, reduces the hydration heat release rate of cement.

The addition of CNC reduces the rate of hydration exotherm and the total reaction exotherm of tricalcium silicate (C3S). When CNC reaches the maximum content of 0.5 wt.%, the peak of heat release is delayed by 1.1 h, and the maximum heat release rate is reduced by 22.3% compared with the hydration exothermic characteristics of pure C3S. The presence of CNC will provide additional nucleation sites for the formation of C-S-H, resulting in a decrease in its degree of polymerization. In addition, the nucleation effect of CNC was confirmed through separate hydration experiments. The Ca2+ and silicate generated by the C3S hydration reaction will be complexed on the surface of the CNC to form C-S-H and continue to grow around the CNC particles to form a denser network gel structure.

The existence of CNC greatly reduces the crystallinity of monosulfoaluminate (AFm), and with the increase of CNC content, the crystal size of AFm decreases exponentially. When the addition amount of CNC is 0.5 wt.%, the crystal size of AFm is reduced to 1/5 of the comparative sample. In addition, CNC changed the crystal structure of AFm, causing its morphology to change from a typical hexagonal plate shape to a disc-shaped weak crystal with edge gelation. During the hydration process of C3A-CaSO4, CNC will form Al-O-C chemical bond with AFm through the -OH carried on its surface, and inhibit the continuous crystallization and growth of AFm.

The large amount of -OH carried on the surface of CNC will complex Ca2+ in solution, thereby promoting the crystallization and continued growth of Ca(OH)2 on its surface. The presence of CNC does not cause the change of the C-S-H layer spacing, while it will improve its crystal structure so that it has higher crystallinity and the dreierketten structure is more orderly. The addition of CNC leads to a decrease in the average degree of polymerization in the chemically synthesized C-S-H and a significant increase in the dimer content. When the CNC content is 0.5wt.%, the average chain length of C-S-H is reduced by 21%.

By exploring the effect of CNC on the properties of cementitious composites at different scales and analyzing the modification mechanism, it is confirmed that the change in the nanostructure of cementitious composites is the root cause of the changes in macroscopic properties and the modification of cementitious composites by CNC is an effective way to achieve a leap in macroscopic properties. The wide range of sources and sustainability of CNC make it have the prospect of being widely used in cement-based materials. However, the stability of CNC in an alkaline environment and the durability of CNC-modified cement-based materials require long-term experimental verification to ensure its smooth application in actual construction projects.