Investigation of the recyclability of applications involving MgO cement

Ordinary Portland Cement (OPC) is the most popular binder used in construction industry. However, the manufacturing process of OPC has led to some negative environmental impacts, therefore a more environmental friendly alternative to OPC is most welcomed. Reactive MgO cement has been considered a serious contender in this regard as it has a lower clinkering temperature, ability to sequester CO2 in the curing process and can be fully recycled when it is used as the only binder for cement product through calcination. Water-to-cement (w/c) ratio can affect the unconfined compressive strength (UCS), porosity and density of the cement product. For the initial mix containing 53% sands, 37% gravel and 10% MgO cement by weight, it was found that porous blocks prepared from the mix with w/c ratio 1.0 achieved highest 3-day strength and maintained high strength throughout the 28 days curing duration. Porous blocks with w/c ratio 1.1 gave the highest strength for prolonged curing but sign of bleeding was observed in the casting process, which lead to a heterogeneous mix that can affect the quality of the porous blocks. Thus, w/c ratio 1.0 was chosen to be the optimum w/c ratio for the initial mix with aforementioned composition. Calcination can be used to recycle MgO cement. However, it was found that porous blocks made with recycled mix after first round of calcination at 900oC for 4 hours had lower UCS than initial mix. Further strength reduction was observed after second round of calcination. One of the key observations after calcination was the size reduction of aggregates. Sieve analysis revealed that not only coarse aggregate has reduced in size, the total amount of finest particle (MgO cement) that passed through 75 µm sieve has reduced as well. Thus, the strength loss observed in recycled mix could be correlated with: (i) the size reduction of aggregate, (ii) the formation of weak Interfacial Transition Zone (ITZ) around aggregate, (iii) the reduction of free MgO cement content as some of the MgO cement has entrapped in ITZ and (iv) the change of MgO cement reactivity after calcination. No further strength loss and significant size reduction of aggregates were observed in the subsequent calcination. Study to enhance the separation between MgO cement paste and aggregate in order to increase the amount of recycled MgO cement collected was done using preheating at 200oC – 400oC and mechanical separation method, without breaking the aggregate into powder form. MgO cement has the potential to be a more environmentally friendly alternative to OPC. For MgO cement to be more widely utilized, further study has to be done and the advantages of using MgO cement over conventional OPC have to be well advertised to provide confidence to the industry.Bachelor of Engineering (Civil