Assessing the porosity and shrinkage of alkali activated slag-fly ash composites designed applying a packing model

• A particle packing model is used to design alkali activated slag-fly ash composites.
• The relation between porosity and ingredients of alkali activated binder is studied.
• Activator modulus of 1.4 presents the optimum strength in all cases.
• Lowering activator modulus and slag content is efficient to reduce drying shrinkage.

This paper addresses the fresh behaviors, gel structure, strength, porosity and drying shrinkage of alkali activated slag-fly ash composites designed by applying the modified Andreasen & Andersen model. The results show a large variation of slump flows and setting times when using different slag/fly ash ratios and activator moduli. The microstructure analyses by FTIR and TG show the gel structure remains stable after 1 d of curing, and mixes with higher slag contents and lower activator moduli show slightly higher bound water content. The main reaction product is a chain structured C-A-S-H type gel regardless of slag/fly ash ratio and activator modulus, but a slightly higher main absorption band is shown in samples with high fly ash contents. A 28-d compressive strength of about 90 N/mm2 is achieved and a higher content of slag leads to a higher strength and lower porosity in general. An optimum activator modulus of 1.4 in terms of strength is shown, while an increase of activator modulus between 1.0 and 1.8 benefits the pore structure refinement. Both slag content and activator modulus strongly affect the drying shrinkage, and using a high amount of fly ash and low activator modulus can effectively reduce the drying shrinkage.