A cross mixture design to optimise the formulation of a ground waste glass blended cement

The aim of this study is to optimise the mechanical properties of mortars in which waste glass is employed as a partial substitution of cement. For this, a cross mixture design containing 28 runs was set up to study the influence of the mixture proportions of Portland cement, waste glass and silica fume in the blended cement and also two process variables namely: waste glass fineness and waste glass type. Compressive strength results showed that, whatever the age of the mortar this response is significantly higher when the lower fineness of the ground waste glass is used. In particular the incorporation of ground waste glass of 40 μm fineness decreases the compressive strength. In the contrary, this response is maintained constant when adding up to 20% of the 20 μm ground glass to the pure cement. Moreover, although the separate addition of both ground glass and silica fume to pure cement decreases the compressive strength, isoresponse curves show a synergistic effect between these two admixtures when they are simultaneously added. Indeed for mixture containing 70% pure cement, the compressive strength at 90 days are, respectively, 53.6 and 57.9 MPa for 30% waste glass and 30% silica fume. This response reaches 60.1 MPa when the blended cement contains 15% waste glass and 15% silica fume. Finally, analysis of isoresponse contour plots of the combined mixture design allowed us to choose the optimum formulae giving the high values of compressive strength at 28 and 90 days which is 80% cement, 20% waste glass.

► Ground waste glass is used as a partial substitution of cement in mortar.
► Pozzolanic activity of ground waste glass is evaluated by mechanical tests.
► A cross mixture design is set up.
► A synergistic effect exists between ground glass and silica fume when they are simultaneously added to cement.
► Using the isoresponse contour plots, the optimum formulae is 80% cement, 20% waste glass.