Improved strength of alkali activated materials based on construction and demolition waste with addition of rice husk ash

A promising solution to the environmental problem of managing a huge amount of construction and demolition waste (CDW) is its use as precursor in the production of new alkaline-activated cements. Specifically, the main objective of this investigation is to increase the strength of alkaline activated cements synthesized from unseparated CDW of low reactivity due to a high presence of crystalline phases in their composition. The effect of two factors is analyzed: the partial substitution of CDW by rice husk ash (RHA) (20-40 wt%) as well as the modulus of the alkaline activator used (Ms=SiO2/Na2O of 0.5, 1.0 and 2.0). Physical, mechanical and thermal testing, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS) are used to characterize alkali-activated cements. Results show that the incorporation of increasing amounts of RHA and the increase in the modulus of the activator produce changes in the general structure of the gel, obtaining a greater amount of N-A-S-H geopolymeric gel with respect to the main reaction product, the C-A-S-H gel obtained in the control cements containing CDW as the only precursor. The new reaction products and the change in the microstructure make it possible to achieve a compressive strength of 59.6 MPa at 28 curing days in the materials with 40 wt% RHA and Ms= 2.0 compared to 18.5 MPa for the material developed exclusively from CDW. Therefore, this study validates the use of CDW together with RHA as precursors in the production of alkaline activated materials with improved technological properties. These green cements would help to reduce the presence of CDW and RHA in landfills and would generate new environmentally sustainable materials.