Clay Brick Powder as Partial Cement Replacement

In this study, waste clay brick powder (CBP) from a Danish recycling plant was studied with the aim of using the material as a partial cement replacement to reduce the need for cement in cementitious products. The focus is on the fineness of the CBP and its influence on the compressive strength, reactivity of the CBP, and phase development of cement pastes containing CBP over time. The CBP was characterized with respect to mineralogy, Loss on Ignition and thermogravimetric analysis (TGA), particle density, Frattini test, and particle size distribution. The powders were produced by ball milling or disc milling the brick waste, and, thus, CBPs based on the same raw material but with different particle size distributions were produced. Mortar specimens were produced with two CBPs, which were based on the same raw material: CBP-C (coarser powder) and CBP-F (finer powder) added in fractions of 0, 10, 20, and 30 wt% to analyse the effect of the powder fineness on the compressive strength at 28 days. The results showed that the reference (0 wt%) and 10 wt% replacement levels obtained the same compressive strength. Regarding the fineness, the addition of 30 wt% CBP-F resulted in a slightly higher compressive strength compared to that with 30 wt% CBP-C. The reactivity and phase development of the CBP was further studied in cement paste samples containing 0 wt% (reference) and 20 wt% CBP. The pastes were studied using X-ray diffraction (XRD) and TGA at different curing times. The TGA measurements revealed a higher amount of bound water for the pastes containing CBP (normalized to the clinker content). From the XRD measurements, the development of mono-carbonate exceeded the formation of hemi-carbonate, which could explain the higher amount of bound water. Overall, it was found that replacement levels of 10–20 wt% of cement with CBP resulted in good compressive strength of the mortar specimens, which means that it seems possible to replace up to 20 wt% of the cement in cementitious products. However, further research is necessary and longer curing times would be interesting to study.