Early Performance Of Cement-Fly Ash Blends At Various Temperatures

To explore the mechanism and hydration behavior of fly ash-cement blends,the electrical resistivity of cement-fly ash blended pastes with 0~60% fly ash replacements and W/B=0.4 was tested during 7d from casting by a non-contact electrical measuring method under 20 ? ? 24 ? and 28 ?.The factors of temperature and fly ash content were considered to modify hydration process and the different hydration properties.The relationships between the electrical resistivity and setting time,the electrical resistivity and compressive strength were plotted,and the activation energy of cement-fly ash blends during hydration was calculated.Additionally,the hydrates microstructure described by the porosity and scanning electron microscope reveals and demonstrates the hydration mechanism of the cement-fly ash at various temperatures.The main research contents and conclusions are as follows:(1)The effect of fly ash on hydration: the electrical resistivity of cement-fly ash blends decreases initially and then increases under 20?,24? and 28?.Before 8h~11h,the more fly ash used,the higher electrical resistivity was reached,while the lower resistivity was reached after 8h~11h at 20?.The addition of fly ash makes the occurring time of the peak points of the resistivity differential curve prolonged and the peak point height reduced.Fly ash delayed the condensation of the pastes,and the initial setting time and final setting time exhibit a proportional relationship of tini/tfin ? 0.825,which is independent on temperature and dosage of fly ash.Fly ash addition reduces the ratio of the compressive strength at 1d,3d,7d to the compressive strength at 28 d,and more fly ash used shows the more obvious reducing effect,however,the fly ash increased the compressive strength at 90 d.The volume density and true density of the hardened cement-flyash results show that before 90 d,the more fly ash,the higher the porosity,however,the porosity of sample FA20 is slightly lower than that of sample FA0 at 90 d because of the secondary reaction of fly ash.(2)The influence of temperature on the hydration: a high temperature makes the occurring time of the feature point of resistivity differential curve ahead,also makes the peak point height increase,the influence of temperature on the resistivity is not obvious before 3d,the larger influence emerges after 3d,the higher temperature,the higherresistivity.High temperature is beneficial to the development of early strength but reduce the later strength.The higher temperature results in the shorter setting time;(3)The quantitative relationship between different hydration performances: it's a good linear relationship between compressive strength and the logarithm of curing age,and the relationship has nothing to do with fly ash dosage and temperature.It's the setting time and the occurring time of the first peak time of resistivity differential curve that have a good linear connection: tini=1.3521tp+2.2155;tfin=1.5971tp+2.7696,which are nothing to do with the hydration temperature and dosage of fly ash,we can predict the setting time from resistivity.The resistivity and porosity have a power function relationship: ?(t)=?·?0(t)/?m,the more fly ash,the bigger m value and ?0(t),the lower ?,and the lower amplitude of ? is greater than the increase of the ?0(t),which means ?·?0(t)reducing;the compressive strength and porosity have a power function relationship: fc=f0·?-n,the more fly ash,the lower f0 and larger n.There is a linear relationship between 1d,3d,7d compressive strength and 1d resistivity,and 28 d,90d compressive strength has a quadratic function relationship with 1d resistivity,we can predict the compressive strength of different ages from the early(1d)resistivity;(4)Apparent activation energy: the apparent activation energy of cement-fly ash hydration calculated by resistivity results were 18.77 KJ/mol,20.67 KJ/mol,22.53 KJ/mol,23.39 KJ/mol when fly ash contents are 0%,20%,40% and 60%,respectively.The addition of fly ash can increase the apparent activation energy of hydration reaction,the more dosage of fly ash,the higher the apparent activation energy.