Calcium Leaching in Cementitious Materials

Calcium leaching is a progressive degradation process in which calcium-bearing phases within cementitious materials dissolve into external fluids. The most soluble phase, calcium hydroxide (portlandite), is preferentially extracted, leading to a shift in pore-solution chemistry, a reduction in pH and eventual decomposition of the binding gel, calcium silicate hydrate (C-S-H). As portlandite is removed, pore connectivity increases and mechanical strength declines, rendering concrete more susceptible to ingress of deleterious agents such as chlorides and sulphates. Leaching is often accelerated by exposure to soft or acidic waters, by cyclic wetting and drying, or by acidic industrial effluents. Across infrastructure—from nuclear‐waste repositories to hydraulic dams—the economic and safety implications of calcium removal have driven research into kinetics of dissolution, buffering capacity of residual hydrates and strategies to slow or reverse leaching. Practical applications range from optimised material design incorporating supplementary cementitious materials to advanced reactor configurations for safe extraction of calcium in recycling processes. Understanding the interplay of microstructure, solution chemistry and transport dynamics is crucial to predicting service life and directing remediation efforts.

Research from Nature Portfolio

Recent studies have examined the behaviour of cementitious pastes exposed to hydrochloric acid, elucidating the dynamic equilibrium between dissolution and replenishment of portlandite in the pore solution. Measurements of acid pH change over time have been correlated with loss of compressive strength, revealing that once calcium concentration falls below the equilibrium threshold for C-S-H stability, the silicate gel gradually decomposes. Advanced analytical techniques—X-ray diffraction, differential scanning calorimetry and atomic force microscopy—have mapped the transformations of hydration products during leaching. Notably, delayed hydration of unreacted clinker phases was shown to supply additional calcium hydroxide, thereby temporarily mitigating C-S-H breakdown and extending material durability under aggressive acid attack.

Research from all publishers

Innovations in reactor design have been applied to the indirect mineral carbonation of recycled concrete aggregate, with the adoption of a packed bed reactor boosting calcium extraction yields by up to 55% compared to conventional stirred-batch systems. Continuous solvent refreshment reduces solubility limits and enhances dissolution kinetics, while integrated filtration enables recovery of reagents, paving the way for more sustainable carbon capture processes.

A mobile pilot plant has demonstrated the feasibility of large-scale calcium leaching and subsequent carbonation in the concrete sector. Operating at industrial throughput, the system processes recycled aggregate with tunable CO₂ feed and achieves reliable reagent recycling. Field trials confirm that leached aggregate retains suitability for concrete production, while precipitated calcium carbonate meets quality specifications for various downstream applications.

Numerical simulation studies have revealed that, despite variations in pore connectivity or the presence of micro-fractures, the rate of calcium leaching in fully saturated cement pastes is governed primarily by the total amount of soluble phases (buffering capacity). Virtual microstructures with identical hydrate content but differing transport properties exhibit essentially the same dissolution kinetics, underscoring the dominant role of chemical rather than physical constraints in leaching processes.

Calcium Leaching in Cementitious Materials publication trend

The graph below shows the total number of articles in calcium leaching in cementitious materials across all publications each year (not limited to Nature Index journals).

Technical terms

Calcium leaching: The dissolution and removal of calcium-bearing crystalline and gel phases from cementitious materials into contacting fluids.

Portlandite: Crystalline calcium hydroxide formed during cement hydration; the most readily soluble phase under leaching conditions.

Calcium silicate hydrate (C-S-H): The primary binding gel in hydrated cement responsible for mechanical strength and durability.

Pozzolanic reaction: Secondary hydration whereby siliceous or aluminous materials consume portlandite to form additional C-S-H, refining microstructure and reducing leachability.

Buffering capacity: The ability of residual cement hydrates to maintain pore-solution pH by releasing or consuming calcium ions during exposure to aggressive environments.

References

1. Indirect mineral carbonation of recycled concrete aggregate: Enhancing calcium extraction using a packed bed reactor. Journal of Cleaner Production (2024).
2. Calcium leaching behavior of cementitious materials in hydrochloric acid solution. Scientific Reports (2018).
3. Influence of Micro-Pore Connectivity and Micro-Fractures on Calcium Leaching of Cement Pastes—A Coupled Simulation Approach. Materials (2020).
4. Industrial demonstration of indirect mineral carbonation in the cement and concrete sector. Journal of Environmental Chemical Engineering (2023).