Physical, mechanical and microstructural properties of ultra-lightweight high-strength geopolymeric composites

A light weight, high strength and low carbon content are the main characteristics of modern concrete that meet requirements for functionality and environmental considerations of engineering structures. In this study, ultra-lightweight high-strength geopolymeric composites (ULHSGCs) were developed using fly ash cenospheres and hollow glass microspheres as aggregates and ground granulated blast furnace slag and silica fume as binders. This investigation focused on the physical, mechanical and microstructural properties of the ULHSGCs. A series of tests were conducted to examine the influence of the key factors on the flowability, setting times, mechanical properties and microstructure. The various tested parameters included the (1) silica fume content (10 %, 12.5 % and 15 %), (2) Na2O content (5 %, 6 % and 7 %); (3) alkali modulus (1.25, 1.5 and 1.75), (4) fibre type (polypropylene fibre and polyethylene fibre), and (5) fibre length (12 mm and 18 mm). The results showed that the density of the proposed ULHSGCs was less than 1500 kg/m3, while the compressive strength was above 70 MPa and the tensile strain capacity was up to 5.3 %. A higher Na2O content and a lower alkali modulus could provide better mechanical properties and fluidity but with a shorter setting time. The addition of 0.5 vol% fibre could effectively improve the mechanical properties without affecting the fresh properties. Due to good bonding to the matrix, the polyethylene fibres allowed the ULHSGCs to achieve pseudo strain hardening under tensile or flexural loading.