Introduction

Portland cement(PC)concrete is the most widely used construction material in the world.However,the increasing emphasis on the sustainability has highlighted the adverse effects of PC production on the environment and motivated researchers to explore new cementitious materials like geopolymer cement(GC)as a partial or complete alternative to PC for two main reasons:(1)through chemical activation,industry waste such as fly ash(FA)and ground granulated blast furnace slag(GGBFS)can be utilized[1];(2)the carbon footprint of GC is significantly lower than that of PC,making it a more environmentally-friendly choice[2].However,it should be noted that similar to PC concrete(PCC),the GC concrete(GCC)still has a brittle nature.Therefore,an in-depth understanding of its fracture properties,such as the fracture energy and tension softening behavior[3]is a prerequisite for the safe application of GCC in practical structural applications.

The properties of GCC largely depend on the used raw materials.Initially,the GC was strictly defined as an alkali-activated aluminosilicate(e.g.metakaolin as a precursor)without(or with little)other components[4].As a byproduct of coal power plants,lowcalcium content FA can be also activated to form geopolymer,leading to a major hydration product of N-A-S-H gel with a three-dimensional spatial structure[5].However,high alkali concentration and heat curing are always needed to achieve a reasonable compressive strength due to its high activation energy[6].The heat curing methods are acceptable in precast geopolymer industry,yet very challenging to be implemented in in-situ constructional operations[7].Therefore,it is imperative to develop other chemical processes for room-temperature hardening of GCC.Therefore,in alkali-activated systems,calcium-containing solid aluminosilicate sources,such as ground granulated blast-furnace slag(GGBS)or high-calcium fly ash(FA)were usually introduced under high alkaline environment to form C-(A)-S-H gels[1].Such chemical product shows some similar features to that of C-S-H gel that is dominant in the hydraulic cement.The big advantage of slag/fly ash blended geopolymer concrete lies its ambient temperature curing capacity.The compressive strength of slag/FA blended geopolymer concrete usually increases with the amount of slag[8,9].

Up to now limited research has been conducted on the fracture properties of GC concrete in consideration of different material compositions.A recent Ph D project was completed at the Hong Kong Polytechnic University on the static fracture and dynamic mechanical performance of GC concrete[8,10-12].This paper presents a summary of the research findings arisen from this project on the static fracture properties of GC concrete.