# Modeling of the Chemical Re-Alkalization of Concrete by Application of Alkaline Mortars

## TL;DR

This study explores how applying alkaline mortars can re-alkalize carbonated concrete to prevent steel corrosion, focusing on the role of chemical properties over structural ones.

## Contribution

The study introduces a modified Fick’s law model with a time-dependent factor to evaluate CRA, highlighting the importance of initial alkalinity and potassium gradients.

## Key findings

- High initial alkalinity (pH > 14) improves re-alkalization during the suction phase.
- Potassium concentration gradients at the interface drive the diffusion phase of CRA.
- Structural parameters like capillary porosity do not strongly correlate with CRA deceleration.

## Abstract

Since the number of existing steel-reinforced concrete buildings affected by carbonation-induced corrosion is steadily increasing, there is a high demand for durable repair methods. Chemical re-alkalization (CRA) represents one such approach, relying on the transport of alkaline pore solution from a repair mortar into carbonated concrete. With the introduction of clinker-reduced binder systems such as hybrid alkali-activated binders (HAABs), their suitability for CRA and governing material parameters require further clarification. In this study, material-related chemical and structural influences on CRA were investigated using an adapted form of Fick’s second law of diffusion, incorporating a time-dependent attenuation factor, β(t). The CRA progression was evaluated over 28 days, distinguishing between an initial suction phase and a subsequent diffusion phase. The results show that a high initial alkalinity of the mortar pore solution (pH > 14) significantly enhances re-alkalization during the suction phase, reflected by suction factors a > 1. In contrast, progression during the diffusion phase is primarily governed by the potassium concentration gradient at the mortar–concrete interface, while structural parameters such as capillary porosity show no systematic correlation with the deceleration factor b (−0.46 ≤ b ≤ −0.26). The findings indicate that, within the investigated range, mortar pore solution chemistry has a stronger influence on CRA than structural properties, providing guidance for the targeted design of alkaline repair mortars.