Experimental Study On Mechanical Properties Of Fly Ash-slag-based Recycled Aggregate Geopolymer Concrete

With the vigorous development of Chinese infrastructure,the shortage of nonrenewable resources such as river sand,river water,and rocks aggregate,as well as environmental pollution caused by calcined cement and lime,and the problem of a large number of old building demolition wastes are facing unprecedented challenge.In order to alleviate the above problems and explore a new way out,this research focuses on the "recycled aggregate sea sand geopolymer concrete",a new type of environmentallyfriendly and energy-saving concrete.Fly ash,blast furnace slag or metakaolin is used as a gelling material,and strong alkaline solutions such as sodium hydroxide,potassium hydroxide,sodium silicate,etc.are used as activators.Which is very different from the common portland cement concrete currently used in terms of raw material sources and hydration mechanism,but according to existing literature,geopolymer concrete shows far better mechanical properties and durability than ordinary portland cement,which has the advantages of high early strength,strong durability,environmental protection of raw material sources,and simple preparation process.It will undoubtedly be the new direction of building materials science in the future.The main highlight of this article is that most of the raw materials used are renewable resources(such as sea sand)and industrial by-products(such as fly ash and slag),which have the advantages of nontoxic side effects,environmental damage and sustainable development.In this study,geopolymers were used to replace ordinary portland cement,recycled aggregates were used to replace natural aggregates,and sea sand was used to replace river sand.A "recycled aggregate sea sand geopolymer concrete" was deployed.Orthogonal analysis and full factor analysis were used to study the influence of various factors on the mechanical properties of "recycled aggregate sea sand geopolymer concrete" through the relationship between macroscopic phenomena and microstructure.The main conclusions are as follows:(1)The change of any factor of NaOH solution concentration,slag content and water-cement ratio will cause obvious compressive strength changes,but the influence rules and mechanisms are not the same.Geopolymer concrete has the highest compressive strength at medium concentration of NaOH solution,while the compressive strength at low or high concentrations is lower than that at medium concentration.Increasing slag content and decreasing water-cement ratio can enhance the compressive strength of geopolymer concrete;(2)Some substance in seawater and sea sand will hinder the hydration reaction of geopolymers in the early stage and reduce the early strength of the sea sand geopolymer,but the hindering effect becomes insignificant in the later stage and the strength will rise.The sea sand geopolymer and river sand geopolymer show a certain difference in mechanical properties;(3)For the compressive strength of geopolymer concrete,the influence of slag content,water-cement ratio and NaOH solution concentration on the specimen decreases in sequence,that is,the slag content has the largest influence,the NaOH solution concentration has the smallest influence,and the NaOH solution concentration is not the higher the better,if the concentration increased when it reaches a certain concentration,it will adversely affect the compressive strength;(4)The cohesive force of geopolymer is much stronger than that of ordinary Portland cement.In geopolymer concrete,the interface transition zone(ITZ)is no longer the weakest link,and the strength of aggregate can be fully utilized.It is also one of the fundamental reasons why the compressive strength of geopolymer concrete is much higher than that of ordinary Portland concrete.