Reaction kinetics, mechanical properties, and microstructure of nano-modified recycled concrete fine powder/slag based geopolymers

Nano-materials are increasingly being used as modified binder for improve the performance of cementitious materials. In this paper, nano-SiO 2 and nano-Al 2 O 3 were incorporated into the geopolymers formed by the mixture of construction solid waste (recycled concrete fine powder) and industrial solid waste (slag). The effects of the type and content of nanomaterials on the early reaction kinetics, physical and mechanical properties and microstructure of geopolymer composites were explored. Results show that the addition of nanomaterials mainly affected the dissolution stage of geopolymerization , played a nucleation effect and improved the reaction rate. The promotion effect of nano-Al 2 O 3 was slightly greater than that of nano-SiO 2 . Based on the results of JMAK model, the formation of geopolymers composites was an one-dimensional heterogeneous nucleation control reaction, which followed a plate-like structure thickening mechanism. The addition of nanomaterials shortened the setting time. The incorporation of nano-Al 2 O 3 was beneficial to improve the fluidity of geopolymer. Furthermore, the addition of the two nanomaterials could improve the compressive strength of geopolymers. The compressive strength tended to first increase and then decrease with increasing nano-SiO 2 or nano-Al 2 O 3 content. The optimal mixing ratio of nano-silica and nano-alumina were 1.2% and 1.6% respectively, and the 28-day compressive strengths were both 59.05 MPa, which were 23.35% higher than that without nano-materials. XRD and SEM-EDS analysis indicate that the incorporation of nanomaterials had no impact on the type of final products, but increased their number, particularly calcium-based gel products. The internal structure of geopolymer with nanomaterials was more dense and uniform than that without nanomaterials. The MIP observed that the addition of the two nano-materials reduced the porosity in the matrix, the number of macropores and the pore fractal dimension . The pore fractal dimension of geopolymer was negatively correlated with the compressive strength.