Multi-scale Performance and Durability of Carbon Nanofiber/Cement Composites Florence Sanchez, Ph.D. Lu Zhang, Ph.D. Chantal Ince

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1 Multi-scale Performance and Durability of Carbon Nanofiber/Cement Composites Florence Sanchez, Ph.D. Lu Zhang, Ph.D. Chantal Ince Civil and Environmental Engineering

2 Motivation Increase interest in multi-functional, structural composites Demand for smart structures with superior mechanical and functional properties Importance of weight savings Nanofiber/cement composites Expected to improve mechanical properties Additional smart properties: electromagnetic field shielding, self-sensing capabilities, self control of cracks Key aspects: proper dispersion and degree of interfacial interaction between the carbon nanofibers (CNFs) and the cement phases Need for understanding the mechanisms of action of CNFs in cement pastes and the impact of their long-term use Carbon nanofibers

3 Complex Interaction with the Environment Mechanical Loads H + CO 2 H 2 O Fibers Pores Porous Media Developed by F. Sanchez Weathering forces Temperature (elevated, cyclic) Radiation (solar, nuclear, thermal) Freeze/thaw cycling Wetting/drying cycling Water (solid, liquid, vapor) Chloride attack (sea water, deicer salts) Acid attack (HCl, H 2 SO 4, acetate,...) Sulfate attack (soil, sea water,...) Microbiological attack Air constituents (O 2, O 3, CO 2 ) Acidic air pollutants (NO X, SO X,...) Internal chemical changes Decalcification Slow hydrolytic decomposition of hydration products Desiccation / change in moisture Exchange reactions with cement paste Formation of soluble/ insoluble Ca salts Carbonation Substitution reactions in CSH Reaction product accumulation, crystallization, and polymerization Alkali-silica reactions Activation / deactivation of fiber surface Internal stresses Bulk cement paste Loss of alkalinity Increase in volume of crystallized phase Increase in stresses Change in microstructure and morphology Shrinkage Cement-fibers interface and interfacial zone Change in fiber surface properties Change in bond strength Change in local microstructure and morphology Local material loss Weakening of fiber Increase in deterioration processes Material damage Loss of compressive strength Loss of tensile strength Loss of mass Change in Young s modulus Change in fracture toughness Change in porosity Increase in permeability Deformation Expansion Cracking Spalling Debonding Delamination

4 Nano-Reinforcement/Cement Interface Bulk Cement Paste O O O OH Si Si Si O O O Interfacial Zone Nano- Reinforcement OH O Ca O C Ca O OH OH Ca C OH O Ca O C Ca OH O Ca O C

5 Research Objectives Determine the effect of surface treatment and admixture addition on the incorporation of CNFs in cement composites Determine the effect of decalcification on the interface between CNFs and cement phases Investigate how microstructural and morphological alteration of the cement paste due to decalcification affects the role of the CNFs and in turn the macroscale properties of the material

6 Materials Two types of cement pastes Portland cement (PC) Portland cement with 1 wt.% silica fume (SF) Carbon nanofiber (CNF) size ~ nm dia., 1-3 µm long Surface treatment: HNO 3 Mix design CNF loadings: and.5 wt% Water/cementitious material:.33 Specimens Cylinders 2in dia. x 4in height Curing 28 days minimum, room temperature, 1% RH

7 Microstructure Studies CNF/Portland Cement Composites Int. J. Materials and Structural Integrity, In press CNF pocket As received CNFs CNF pocket HNO 3 surface treated CNFs Randomly entangled Appearance of combed yarn Pocket wall Entangled network of CNFs intermixed with hydration products Anchored CNFs

8 Paste with silica fume Microstructure Studies CNF/Cement Composites Composites Science and Technology, 69 (29) Groove left by CNF Individual CNFs anchored in paste Hole left by CNF Individual CNFs well anchored inside of hydration products Silica fume facilitated CNF dispersion

9 Microstructure Studies CNF/Portland Cement Composites Surface treatment with HNO 3 CNFs found acting as bridges between hydrates Surface coating on CNFs

10 Interface Studies Molecular Dynamics Modeling J. of Colloid and Interface Science, 323 (28) Polarity of functional group drives affinity to cement phases H-bond network developed across the interface, bridging the structures Optimal number of O-containing groups required for efficient graphitic structure/cement interaction

11 Macroscopic Property Studies CNF/Cement Composites Splitting tensile strength (MPa) max median min PC pastes Ref..5wt% * As received CNF* Surface treated with nitric.5wt% CNF SF pastes Ref..5wt% CNF* PC paste - Ref.5 wt% CNF Post compression

12 Durability Studies Water absorption studies Decalcification studies 7M NH 4 NO 3 solution Accelerates calcium leaching by formation of Ca(NO 3 ) 2 Liquid-to-Surface ratio = 5 ml/cm 2 Immersion: 7, 3 and 95 days - No renewal Material characterization Mineralogical changes Solid phase mineralogy and element mapping of leached cement pastes Microstructural changes Mechanical performance effect

13 Water Absorption Capacity CNF/Cement Composites.3.3 g water/cm PC paste Time (hr) g water/cm SF paste Time (hr) PC - Ref PC - CNF SF - Ref SF - CNF PC paste CNF pockets hydrophobic (Gore-Tex effect) SF paste CNF pockets hydrophilic (C-S-H coating)

14 Mass loss (%) Mass Loss and Penetration Depth CNF/Cement Composites Time (days) PC - Ref PC - CNF SF - Ref SF - CNF ph Greater mass loss for PC pastes Greater visual degraded depth for SF pastes Greater visual degraded depth with CNFs Influence of volume fraction of CNF pockets (~13% and 3%) Ref CNF PC pastes SF pastes Exposure to NH 4 NO 3 95 days

15 Element Mapping (LA-ICP-MS) CNF/Portland Cement Composite Ca/Ca "apparent" core Visual degraded depth SEM Calcium CH front Mg/Mg "apparent" core Magnesium Al/Al "apparent" core Si/Si "apparent" core Aluminum Silicon Distance from edge (µm) Na/Na "apparent" core C/C "apparent" core Sodium 3.6 Carbon Distance from edge (µm) AFt Visual degraded depth did not provide complete delineation of degraded state of the paste Sound zone altered

16 Mineralization of CNF Pockets CNF/Portland Cement Composite Relative Content Si C Ca Distance from edge (µm) CNF pocket Mineralization of pocket CNF pockets acted as sink for Ca, Si, Al Impregnated CNF meshwork

17 Mechanical Effect of Decalcification CNF/Cement Composites Compressive strength (MPa) wt% CNF.5 wt% CNF max median min PC pastes Compressive strength (MPa) wt% CNF.5 wt% CNF max median min SF pastes d 3 d 95 d d 3 d 95 d d 3 d 95 d d 3 d 95 d Days of exposure to NH 4 NO 3 Days of exposure to NH 4 NO 3 No residual effect of CNFs on ultimate compressive strength

18 Mechanical Effect of Decalcification CNF/Portland Cement Composites Compressive load (MPa) PC pastes - w/cm=.33 Reference.5 wt% CNF.5 wt% CNF - AN95d Reference - AN95d Displ. (mm) Decalcification resulted in a change in failure mode from brittle cracking to slow ductile load dissipation

19 Conclusions SF and surface treatment with HNO 3 facilitated CNF dispersion and improved interfacial interaction Unchanged compression and tensile strengths with CNFs but residual load-bearing capacity post failure Hydrophobic/hydrophilic effect of CNF pockets Important role of CNF pockets in the decalcification process Kinetics of degradation affected by volume fraction of CNF pockets CNF pockets acted as sink for Ca, Si, Al Decalcification changed the failure mode from brittle cracking to slow ductile load dissipation, which was more pronounced for the PC paste with CNFs MD a useful technique for understanding interfacial interaction between cement phases and reinforcing structure

20 Acknowledgements National Science Foundation (CMMI-51534) Rossane Delapp, Senior Research Scientist David Delapp, Ph.D., Senior Research Engineer