Effect of synthesis parameters on the quality of construction and demolition wastes (CDW) geopolymers

• Tiles and bricks are well geopolymerized; their compressive strength exceeds 49 MPa.
• Concrete shows low geopolymerization potential (max. compressive strength 13 MPa).
• The optimum conditions are: 8–10 M NaOH, temperature 80–90 °C and ageing for 7 days.
• SEM–BSI images show increased homogeneity of the geopolymeric structure.
• FTIR analysis identifies the major fingerprints of a geopolymeric matrix.

In the present study, the geopolymerization potential of construction and demolition wastes (CDW) as well as the effects of the molarity of the alkaline activating solution, the curing temperature, the ageing period and the particle size of the raw materials on the compressive strength of the final products have been studied. For the synthesis of geopolymers, concrete, bricks and tiles collected from various demolished buildings were mixed with the activating solution (NaOH and Na2SiO3). Various synthesis conditions (curing at 60–90 °C, 8–14 M NaOH molarity, particle size) have been considered. Results have shown that tiles and bricks are well geopolymerized, reaching a compressive strength of 49.5 and 57.8 MPa, respectively, while concrete shows limited geopolymerization potential since it reaches a compressive strength of only 13 MPa. The effects on the compressive strength of the specimens were also assessed by considering various molar ratios of the oxides present in the initial paste including SiO2/Al2O3 and H2O/(Na2O + K2O). CDW geopolymers synthesized under the optimum conditions were also subjected to high temperature heating for one hour, freeze–thaw cycles and immersed in distilled water for one and two months to assess changes in their structural integrity. Analytical techniques, X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR) and Scanning Electron Microscopy (SEM) were used for the identification of the morphology and structure of the final products.

Evolution of the compressive strength of geopolymers produced from (a) tiles, (b) bricks and (c) concrete vs. NaOH concentration and heating temperature.Figure optionsDownload as PowerPoint slide