Effect of carbonation on bound chlorides in cementitious and alkali activated binders

Abstract

Concrete is the most popular building material in modern times. It is widely used in construction due to its excellent mechanical properties, durability and ease of handling and placement. Cement is one of the essential components of concrete that binds the aggregate and embedded elements together. However, 3.6bn tonnes of cement is produced worldwide annually, and this contributes to 4% of the global CO2 emission. Alkali activation of aluminosilicate industry by-products, such as ground granulated blast furnace slag, fly ash or calcined clays, can provide a viable binder system that can contribute towards the cement market. Activators such as sodium hydroxide and sodium silicate or their potassium equivalent are widely used. The durability of these new binder systems is being investigated and the available data indicates a mixed performance.

Chloride induced corrosion of reinforced concrete is one of the most common deterioration mechanisms that affect concrete structures exposed to marine or urban environments across the world. The chloride ions that enter concrete can be found either as free, which are able to migrate towards steel and cause corrosion, or as bound, which are fixed within the cement matrix. Therefore, chloride binding capacity of the concrete is a very important property to resist the chloride corrosion. Carbonation induced corrosion of steel bars is another common deterioration mechanisms in concrete, especially in structures exposed to urban/industrial environments. Carbonation is a two-part process, involving transport and reaction. CO2 from the atmosphere ingress into concrete (transport) and react with the mineral phases in cement (reaction). These reactions can lead to micro cracks and decrease in pH of the pore solution in concrete, which both can accelerate further deterioration.

This thesis investigates the influence of carbonation on the chloride transport and binding in cementitious and alkali activated systems. For cementitious systems, influence of the different factors viz. water binder ratio, pulverized fuel ash content, and silica fume content, on the bound chlorides were studied both prior to and after carbonation. For alkali activated systems, the chloride binding in single precursor and multiple-precursor alkali activated materials prior to and after carbonation were studied.

Based on the results, in cementitious system, before carbonation, all the uncarbonated pastes rendered similar apparent pH values. Amongst the three factors studied in this research, the w/b showed the most significant influence on the bound chloride content. After carbonation it was found that there existed a linear relationship between bound chlorides and apparent pH for all the mixes. Therefore, the effect of carbonation on the chloride binding capacity of the mixes could be predicted using a measurement of their apparent pH.

In terms of the alkali activated systems, alkali activated slag bound more chlorides compared to alkali activated fly ash and alkali activated metakaolin due to the formation of Friedel’s salt and the structure of hydrotalcite to alkali activated fly ash and alkali activated metakaolin, chloride binding was mostly due to physical absorption. The influence of carbonation on the bound chlorides in different alkali activated systems is mainly on the chemical bound chlorides. The mix of slag, fly ash, and metakaolin cannot improve the bound chloride content in different multiple precursor alkali activated systems.

These findings contribute to a deeper understanding on the influence of carbonation on chloride binding in different binders. This will help to establish the specifications for the concrete structures exposed to the combined effects of chlorides ingress and carbonation.

Date of Award	Dec 2021
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	Chinese Scholarship Council (CSC)
Supervisor	Sree Nanukuttan (Supervisor), Daniel McPolin (Supervisor) & Muhammed Basheer (Supervisor)