Low-Calcium Silicate Cement Corrosion Mitigation via pH Buffering
Solution Overview
Problem
Conventional concrete technologies face issues with corrosion of iron or steel components in low calcium silicate based carbonatable non-hydraulic cement and concrete products, leading to structural failures due to pH changes and chloride penetration, which existing technologies have not adequately addressed.
Innovation Solution
The development of novel curing methods and formulations that maintain a high pH in the pore solution of carbonated low calcium silicate cement-based materials, incorporating additives like calcium nitrate tetrahydrate and silica-rich layers to prevent corrosion of steel reinforcement, while achieving compressive strengths of up to 10,000 psi or more.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If carbonation curing is used to reduce CO2 footprint, then environmental sustainability is improved, but pore solution pH drops leading to steel corrosion
Solution Approach 1:
Silica fume is introduced as an intermediary substance that reacts with carbonic acid to form additional calcium silicate hydrate gel, which buffers the pH drop during carbonation. This mediator allows the system to maintain both carbonation benefits and corrosion protection
Solution Approach 2:
The chemical composition parameters of the cementitious system are changed by incorporating silica fume and adjusting the water-to-cement ratio. These parameter changes create a pore solution with higher silica content that better resists pH reduction during carbonation, preventing steel corrosion while maintaining carbonation curing benefits
2Use of energy by moving object
If low calcium silicate cement is used to reduce energy consumption, then energy efficiency is improved, but compressive strength development is reduced
Solution Approach 1:
A composite cementitious system is created by combining low calcium silicate cement with silica fume. This composite material leverages the low carbon footprint of low calcium cement while using silica fume to enhance strength development through additional C-S-H gel formation and pozzolanic reactions
Solution Approach 2:
Silica fume acts as a self-enhancing component that utilizes the carbonation process itself to generate additional binding products. The silica fume reacts with carbonic acid to form more C-S-H gel, allowing the material to self-strengthen during the carbonation curing process without additional energy input
3Loss of substance
If carbonation reaction occurs to sequester CO2, then CO2 sequestration is improved, but pore solution pH drops causing corrosion initiation
Solution Approach 1:
The harmful carbonic acid formed during CO2 sequestration is converted into a beneficial substance. Instead of allowing carbonic acid to lower pH and cause corrosion, the silica fume reacts with it to form additional calcium silicate hydrate gel, transforming the harmful byproduct into a strength-enhancing binding material
Solution Approach 2:
Silica fume serves as a chemical intermediary that intercepts carbonic acid before it can significantly lower the pore solution pH. This intermediary reaction produces additional C-S-H gel while buffering the pH, allowing CO2 sequestration to proceed without triggering corrosion
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution effectively delays or prevents corrosion of steel components, maintaining a favorable pH environment that enhances the service life and durability of carbonated concrete products, ensuring adequate compressive strength and water resistance without compromising freeze-thaw durability.
Implementation Method 1
The development of novel curing methods and formulations that maintain a high pH in the pore solution of carbonated low calcium silicate cement-based materials, incorporating additives like calcium nitrate tetrahydrate and silica-rich layers to prevent corrosion of steel reinforcement
Implementation Method 2
Carbonatable cement refers to cement that is principally cured by reaction with carbon dioxide, CO2, in any of its forms, such as, gaseous CO2 in the presence of water, CO2 in the form of carbonic acid, H2CO3
Implementation Method 3
Hydration of OPC in the presence of water produces a solution within the porous material that consists mainly of alkali hydroxides, such as, Ca(OH)2, NaOH and KOH. Depending on the composition of the cement and the aggregates, the pH of the pore solution is typically between 12.5 and 13.5
Implementation Method 4
Penetration of salts from the environment may also lead to a remarkable change in the composition of the pore solution
Implementation Method 5
Corrosion of plain carbon steel in conventional concrete is initiated when a passive film on the surface of iron or steel (e.g., plain carbon steel) is removed either by lowering of the pore solution pH or by combination of low pore solution pH and presence of chlorides
Data Source
AI summary
Methods for producing compositions that prevent, mitigate or delay the onset of corrosion of iron or steel (e.g., plain carbon steel) components used as reinforcement or otherwise at least partially embedded in carbonated concrete composite materials and objects based on carbonatable calcium silicate cement are disclosed.