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In recent years, carbon nanofibers have been investigated as a suitable reinforcement for cementitious composites to yield novel multifunctional materials with improved mechanical, electrical, magnetic, and self-sensing behavior. Despite several studies, the interactions between carbon nanofibers and Portland cement hydration products are not fully understood, with significant implications for the mechanical response and the durability at the macroscopic lengthscale. Thus, the research objective is to investigate the influence of carbon nanofibers on the nanostructure and on the distribution of hydration products within Portland cement paste. Portland cement w/c = 0.44 specimens reinforced with 0.0 wt%, 0.1 wt%, and 0.5 wt% CNF by mass fraction of cement are cast using a novel synthesis procedure. A uniform dispersion of carbon nanofibers (CNF) via a multi-step approach: after pre-dispersing carbon nanofibers using ultrasonic energy, the carbon nanofibers are further dispersed using un-hydrated cement particles in high shear mixing and mechanical stirring steps. High-resolution scanning electron microscopy analysis shows that carbon nanofibers fill nanopores and connect calcium–silicate hydrates (C–S–H) grains. Grid nano-indentation testing shows that Carbon nanofibers influence the probability distribution function of the local packing density by inducing a shift towards higher values, η = 0.76–0.93. Statistical deconvolution analysis shows that carbon nanofibers result in an increase in the fraction of high-density C–S–H by 6.7% from plain cement to cement + 0.1 wt% CNF and by 10.7% from plain cement to cement + 0.5 wt% CNF. Moreover, CNF lead to an increase in the C–S–H gel porosity and a decrease in both the capillary porosity and the total porosity. Based on scratch testing, adding 0.1 wt% CNF yields a 4.5% increase in fracture toughness and adding 0.5 wt% CNF yields a 7.6% increase in fracture toughness. Finally, micromechanical modelling predicts an increase of respectively 5.97% and 21.78% in the average Young’s modulus following CNF modification at 0.1 wt% CNF and 0.5 wt% CNF levels.
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Effect of nano-TiO2 on C–S–H phase distribution within Portland cement paste
We investigate the mechanisms by which titanium dioxide nano-particles (nano-TiO2) interact with cement hydration products. To this end, we synthesize nanomodified cement samples with 1 wt% and 5 wt% of TiO2. We investigate the physical properties using depth-sensing based methods such as statistical nano-indentation and microscopic scratch testing. Fourier transform infrared spectroscopy yields the chemistry, whereas micromechanics modeling provides insights into the nanostructure. The macroscopic plane strain modulus increases by 16% and 83%, respectively, and the macroscopic indentation hardness increases by 37% and 40%, respectively. The fracture toughness rises by 3% and 11%, respectively. Environmental scanning electron microscopy reveals a 30% reduction in crack width for TiO2 cement nanocomposites compared to plain cement. Meanwhile, Fourier transform infrared spectroscopy and statistical deconvolution show an increase in the fraction of high-density calcium silicate hydrates (by 22% and 12% respectively), and in the fraction of calcium hydroxide (by 101% and 251% respectively). Within the framework of the colloidal and granular models of C-S-H, the increase in stiffness and strength after nano-TiO2 modification of cement paste is due to the closely-packed structure and the high atomic coordination number of high-density C-S-H. Similarly, due to the high dimensional stability of high-density C-S-H and calcium hydroxide, our results explain the reported improvements in drying shrinkage and creep properties following cement modification with nano-TiO2.
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- Award ID(s):
- 1829101
- PAR ID:
- 10179177
- Date Published:
- Journal Name:
- Journal of materials science
- Volume:
- 55
- ISSN:
- 1573-482X
- Page Range / eLocation ID:
- 11106–11119
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/https://doi.org/10.1007/s10853-020-04847-5
