Relevance of the mineralogy of recycled clay brick powder as SCM for cementitious binder applications

Recycled clay brick powder (CBP) from typical fired clay bricks with different material characteristics is investigated as a supplementary cementitious material (SCM) to gain insight into possible material variations. This type of recycling supports the circular economy by decreasing the clinker content in cementitious products and reducing the brick fraction of construction and demolition waste that gets downcycled as road subbases or landfilled. The influence of mineralogy and material characteristics of five CBPs' on the performance as SCM was evaluated based on early hydration kinetics, pozzolanic reactivity, and compressive strength of mortars with a 40wt% replacement level. Detailed evaluation of the results reveals differences in mineralogy for the five CBPs, and that older bricks (O-CBP) have slightly higher amorphous content (15.8-17.0wt%) than new bricks (N-CBP) (10.4-12.9wt%). These differences are expressed by the compressive strength of mortars, which are higher for O-CBP than for N-CBP, especially for longer curing times (91 d). TGA results show that more water is chemically bound, and more portlandite is consumed when adding CBP compared to inert quartz powder. Two CBPs contain calcium carbonate, revealing synergetic effects by forming mono-carbonate instead of hemi-carbonate. Despite this, the differences in performance between the five types of CBP are limited, suggesting limited variability across different types of bricks and production ages. From a recycling point of view, this is an advantage, as it allows for more recycling of waste brick fractions. Based on these results, CBP can be considered an alternative to low-technical quality calcined clay with the potential to be used as SCM without an additional calcination process despite low amorphous contents. • Five CBPs with different mineralogy showed limited differences as SCM, which can enhance the recycling rate of CBP • Portlandite consumption was evident, confirming the pozzolanic contribution of CBPs in cement pastes despite its low amorphous content • The two CBPs containing calcium carbonate showed synergetic effects by forming mono-carbonate and contributing to compressive strength