Incorporating synthetic zeolites into 3D printed concrete for carbon capture

The materials used for 3D Concrete Printing are typically high in cement and consequently have significantly more embodied carbon than more than “typical” mortars and concretes. It is widely recognised that this needs to be minimised against a backdrop of national targets for achieving Netzero, for which the construction sector plays an important role. Towards this, there is growing interest in developing lower carbon printable mixes using cement replacements, and more recently calcined clay alternatives. An alternative and potentially complementary approach to reduce the impact of the material is to consider the capture of carbon by the mortar or concrete over its life, through carbonation. In this approach, the idea is to capture as much carbon as possible, and so the research challenge is to develop printable mortar mixes that are capable of increasing carbon uptake. The addition of zeolites has potential since they can be found naturally on Earth and can be synthesised at low temperatures where they act as molecular sieves to both physically and chemically absorb CO2 from the atmosphere via mineral carbonation.

This work examines the impact of varying qualities of zeolite on the carbonations effect, and the material rheology, critical for 3DCP applications. A method for selecting and replacing the existing dry components was developed to ensure a good particle size distribution, needed for pumping and extrusion of the material. In the study, zeolite LTA was incorporated into a well published 3DCP mortar mix in different proportions: - 0 %, 10 %, 20 %, 30 %, replacing flyash. It was shown that the rheological values (plastic viscosity and yield stress) increased dramatically above 10 % substitution while at 30 % substitution, the material becomes unextrudable.

In terms of carbon uptake, this study found that the addition of zeolites significantly reduces the carbonation properties and in turn the CO2 uptake. The standard material can capture CO2 up to 10 % with a carbonation degree (Dc) value equalling 51 % while the addition of zeolites can only uptake as much as 5 % and a Dc value of 27 %. It appears that the addition of zeolites reduces the material’s porosity, which enhances both the microstructural and durability properties. This is observed in the compressive strength values which significantly increase at increasing zeolite LTA concentration for both air and CO2 cured samples.

In this study, the addition of zeolites had the opposite effect of capturing CO2 from the atmosphere: zeolites are known to be natural CO2 adsorbents. When added to fresh printable mortar material, the zeolites create a more close, dense microstructure due to their pozzolanic behaviour where the zeolites will react to form more calcium silicate hydrate products by reacting with the portlandite material. Zeolites do have potential applications in printable mortar material such as viscosity material agents (VMAs) and accelerators as well as improving mechanical and durability properties. The results from the study suggest that the use of alternative materials should be considered for carbon capture.