Role of aggregate and fibre in strength and drying shrinkage of alkali-activated slag mortar

Abstract

This paper presents an investigation on the influences of aggregate-to-binder (A/B) ratio, aggregate size, polypropylene (PP) and steel fibre dosages on the strength and drying shrinkage of alkali-activated slag mortar (AASM). The results indicate that increasing either A/B ratio up to 2.0 or aggregate size up to 1.18 mm first enhances both compressive and flexural strengths of AASM, followed by a reduction as they further increase. This is mainly attributed to the densified aggregate skeleton formed inside AASM. Furthermore, increasing the A/B ratio decreases the moisture loss in AASM and subsequently decreases their drying shrinkage. The use of coarse sand (e.g. 1.18–2.36 mm) enhances the volumetric stability of AASM prepared with the same binder content. For fibre reinforced AASM, the addition of PP fibres reduces the compressive strength of AASM, while the addition of steel fibres has a positive impact. The use of PP fibres not only causes fibre cluster problem, but also creates the weakened interfaces between fibres and binder. Increasing the PP or steel fibre dosage can enhance the flexural strength of AASM, particularly for steel fibres as they have a better bond with the matrix. Incorporation of PP or steel fibres can reduce the drying shrinkage of AASM. As compared to PP fibres, steel fibres are more effective in restraining the drying shrinkage of AASM.