Experimental Study On Mechanical Properties And Hydration Process Of High Volume Fly Ash-cement-based Materials

High volume fly ash concrete has superior economic,social and environmental benefits,so besides hydraulic structures,it has been widely used for engineering fields as bridge,road and housing construction.The strength development and hydration process of cement-based materials will show some specific characteristics when the large amount of fly ash is added.Therefore,the development of compressive strength,flexural strength,chemically bounded water content and fly ash reaction degree of fly ash-cement-based materials under different water to binder ratios(0.4,0.5 and 0.6),fly ash content(0,30%,40%,50%,60% and 70%)and curing temperatures(20?and 35?)were investigated in this study,and the relationship between strength development and hydration process of fly ash-cement-based materials was explored.The main conclusions were drawn as follows:(1)Under standard curing,both compressive strength and flexural strength of fly ash-cement mortar developed very rapidly at early ages while slowly at later ages.However,as the fly ash content increased,mortar strength grew relatively faster at later ages.With the increase of water to binder ratio and fly ash content,the strength of fly ash-cement mortar was reduced,and the compressive strength presented a good linear relationship with fly ash content.The fly ash content showed a more significant effect on mortar strength as the water to binder ratio reduced.(2)Under high-temperature curing(35 ?,RH>95%),the development of compressive strength and flexural strength of fly ash-cement mortar was also rapid at early ages while tending to be gentle at later ages,the compressive strength was improved compared with that under standard curing condition.The early-stage strength development of control cement mortar was fast under high-temperature curing,but the later age strength was basically not grow.The early-age strength of fly ash-cement mortar presented a linear decline with the increasing of fly ash content,and at later age,the strength firstly increased and then declined with the increase of fly ash content.(3)Under standard curing,both chemically bounded water content and fly ash reaction degree of fly ash-cement paste were increased along with the curing age,and the development rate at early ages was higher than that at later age.As the water to binder ratio increased,chemically bounded water content and fly ash reaction degree were improved,but the water to binder ratio had a minor influence at early age,and it became more remarkable at later ages.The higher content of fly ash,the lower the chemically bounded water content and fly ash reaction degree in the system.(4)Under high-temperature curing,the increase rate of chemically bounded water content of fly ash-cement paste at early age was also higher than that at later ages,and compared with that under standard curing,the early-age chemically bounded water content in the system was evidently improved,but the later-stage development of chemically bounded water content became gentler.With the improve of water to binder ratio,the chemically bounded water content of the system was increased.The chemically bounded water content in the system basically linear declined with the increasing fly ash content.(5)Quantitative prediction equations of chemically bounded water content of fly ash-cement paste under standard curing and high-temperature curing were established and verified.The results indicated that the established prediction equation of chemically bounded water content of fly ash-cement-based material shows a high accuracy.Under standard curing and the same water to binder ratio,chemically bounded water content approximately presented a linear relationship with the compressive strength,and the lower the water to binder ratio,the larger the slope of fitted line.According to the test results,the prediction equation of compressive strength of fly ash-cement mortar based on chemically bounded water content under standard curing was established.