Studies on Cement Design and Steel Corrosion towards Sustainable Reinforced Concrete
- Author(s): Guo, Peng
- Advisor(s): Sant, Gaurav N
- Bauchy, Mathieu
- et al.
The two major construction materials in civil engineering, cement, and steel, are studied using Density Functional Theories (DFT) simulation and empirical methods, respectively. In this thesis, DFT method is used for the study of cement material reactivity study to get access to the electronic-scale investigation; for steel corrosion research, 3-dimensional topography at high resolution is captured for nanoscale pitting evolution observation. The simulation work aims at understanding how to enhance Ordinary Portland Cement (OPC) reactivity and reduce energy consumption as well as reduce CO2 emission from cement industry. Ca2SiO4 (C2S) is the second major phase in OPC clinker, however, it reacts with water at a slow rate and does not contribute early-age strength to concrete structures. To uncover the nature of this low reactivity, 3 major polymorphs of C2S naming γ-, β- and α'L-C2S crystal structures are calculated to determine the least stable phase, revealing that the stability of three C2S ranks in the sequence of γ- > β- > α'L-C2S. As β-phase is the dominant C2S, points defects with foreign ions (K+, Na+, Mg2+, Sr2+, B3+, Al3+, Fe3+, Ge4+) have been introduced into β- C2S crystal structure in order to improve the instability and potentially reactivity. From the doped β- C2S structure, at the same molar concentration, B element shows the highest instability, following by Al element; Na and Mg elements exhibit little trend to activate β- C2S. Ionic size and doping preference have been further studied to uncover the rule of doping in β- C2S as well as future doped structure design. The doping preference in β- C2S crystal structure highly correlates to the ionic size difference between the foreign ion and the substituted ion. Ions with smaller ionic size difference from foreign ions are easy to be replaced. The investigated aspects include lattice parameter, mechanical performance, DOS, PDOS, liner charge distribution, coordination number with oxygen atoms, formation enthalpy as well as vacancy formation enthalpy. Results show that Mg atoms introduced the most instability to doped structures at the same oxide concentration (in mass).
Steel corrosion, specifically, rebar in the concrete structures, has aroused increasingly research interest in order to improve the understanding its nature as well as to protect steel from rapid failure. For steel corrosion study, AISI 1045 medium carbon steel has been selected to conduct pitting corrosion initiation and propagation observation using Vertical Scanning Interferometry (VSI). The corrosion environments are at neutral pH with/without 100 mM NaCl for comparison. Over time, the 3D steel surface morphology is captured with VSI for pitting corrosion analysis at the nanoscale. The comparing items include corrosion product chemical composition, pit initiation mechanism, pit density, average pit depth, and surface height change frequency distribution. Evolution of pitting corrosion over large surface areas has been examined. Special efforts have been paid to quantify the pitting nucleation, growth, and propagation rate. Chloride ions enhance the corrosion rate significantly by shortening the initiation time as well as increase the pit density, pit depth as well as steel surface height retreat. For both with and without chloride ion cases, the surface defects, e.g., MnS inclusions initiate pitting corrosion. Initial pits remove passivation film around themselves, where more pits will generate and propagate. In this study, the pitting corrosion follows the adsorption mechanism. This study provides new insights into steel corrosion at neutral pH environment and extends the understanding of the nature pitting corrosion process.
In sum, from the results of DFT simulation of doped C2S structures, it can be concluded that the introduced instability highly depends on the ionic size mismatch between dopants and the ions being replaced. In steel corrosion study, it is found Cl- ions accelerate pitting corrosion by such factors: (i) destabilization of passivation films, (ii) prevention of repassivation, and, (iii) preservation of aggressive microenvironments within the pits. Both of the studies can be helpful to enhance the performance of reinforced concrete structures.