Carbon dioxide sequestration in cement-based materials : carbonation treatment of recycled concrete powder

Abstract

Urbanisation growth has brought about (i) an increased use of concrete and consequently an increase in Cem I production which is a major contributor to carbon dioxide (CO2) emissions and (ii) an increase of construction and demolition waste (C&DW). The objective of this study is to explore ways of how to provide a partial Cem I replacement whilst mitigating the problems mentioned above, namely by (i) reducing CO2 emissions by using CO2 captured during cement production in a process to produce supplementary cementitious material and (ii) using recycled concrete from construction and demolition waste in the afore-mentioned process to produce supplementary cementitious material (SCM). This dissertation focuses on the production of recycled concrete powder (RCP) and its activation via different treatment methods, to identify the process which yields the best supplementary cementitious material. Treatment processes researched in this dissertation included calcination, dry carbonation, aqueous carbonation, calcination followed by dry carbonation, calcination followed by aqueous carbonation and limewater soaking followed by dry carbonation. Initially, characteristic testing was done on the various treated powders to determine their physical and chemical properties. This was followed by the assessment of these treated powders when introduced as a 20% cement replacement within the mix design of paste and mortar samples, to determine their fresh, mechanical and durability properties. Calcination treatment achieved a material with the highest pozzolanic activity, as XRD results showed it to have the highest percentages of portlandite, tobermorite, gismondine, and belite at 17.4%, 12%, 2.1% and 17.2% respectively, when compared to the other treated samples. Through XRD, compressive strength and durability testing, it was determined that both dry and aqueous carbonation treatments also yielded materials with potential to be suitable supplementary cementitious material. The combined calcination and carbonation treatments yielded the best chemical and mechanical results due to the formation of both calcite and hydrated phases, creating materials of increased reactivity and strength. The dry carbonation in the calcination and dry carbonation process contributed to produce a material with better properties than that produced in the calcination and aqueous carbonation treatment. In fact, calcination followed by dry carbonation achieved the highest compressive strength of 114.90 MPa at 21 days, surpassing that of the cement control.