Eco-friendly lightweight composite prepared with a geopolymer and wheat straw

Agricultural waste shows promise for use in lightweight building materials. The combination of a geopolymer and wheat straw (WS) is adopted to produce a kind of eco-friendly lightweight composite. Using WS pretreated with sodium hydroxide (NaOH) and sodium metasilicate (Na₂SiO₃) solutions, the influence of WS on the properties of the geopolymer–WS composite (GWSC) is investigated. The pretreated WS avoids the retarding effect of the untreated WS on the hydration of the geopolymer and even presents an accelerating effect. Increasing the content of pretreated WS generally results in a reduction in apparent density and strength. A GWSC with an apparent density of 1000–1500 kg/m³ can be prepared, corresponding to a 28-day compressive strength between 5 and 25 MPa. The water absorption linearly increases with the increase in the pretreated WS content, while an appropriate amount of the pretreated WS can enhance the water resistance of the GWSC. A WS/fly ash ratio lower than 8% can produce a GWSC with qualified thermal insulation performance. Alkaline treatment, particularly sodium hydroxide treatment, significantly improves the bonding strength between the fiber and the geopolymer matrix. In addition, pretreated WS saturated with sodium hydroxide and sodium metasilicate solutions is beneficial to the fluidity of the geopolymer–WS mixture and could act as an internal curing agent for enhancing the hydration of the geopolymer.