Study On Hydration And Mechanical Properties Of Early-age Cement-silica Fume Cementitious Materials

The early crack phenomenon of high performance concrete restricts the high development of concrete and engineering construction,and the concrete shrinkage is the main factor of the early crack.In this paper,the 7 days shrinkage of the cement-based materials with different water-binder ratio?W/B=0.4,0.5?and different dosage of silica fume?SF=0%,5%,10%,15%?which after the final setting were studied,then the heat of hydration,compressive strength,electric resistivity of the cement-based materials were tested.And,the function mechanism of silica fume on cement-based materials was discussed by some microstructure analysis methods,which consist of XRD,SEM et.al,the correlation of various hydration properties have been established.Also,the time of the secondary reaction was studied by using the selective dissolution method.The research contents and results are showed as follows.?1?The SF have dramatically small particle size and remarkably high specific surface area,and the SF samples present the excellent filling effect and high secondary reactive activity.The results showed that:?a?the dosage of SF from 5%to 15%,the initial solidification time of samples with different W/B advanced 0.37 to 1.23 h and their final solidification time advanced 0.32 to 0.82 h,whereas the tini/tfin never changed with the dosage of SF.The dosage of SF between 0%and 15%,the t_ini/t_fin of sample with 0.4 W/B is 0.89 to 0.94,the t_ini/t_fin of sample with 0.5 W/B is 0.83 to 0.91 and the average t_ini/t_fin is0.90.?b?At the age of 7 days,with the increase of SF which from 5%to 15%,the compressive strength of the sample with 0.4 W/B raised from 54.2 MPa to 56.9 MPa,the compressive strength of the sample with 0.5 W/B grown from 33.9 MPa to 42.7 MPa.So,at the same W/B,the compressive strength increased by the adding of SF and the promotion of compressive of SF on samples reduced by the increment of W/B.?c?The dosage of SF from 5%to 15%,the total heat release of samples with the W/B of 0.4 rose from 288.58 J/g to 364.30 J/g in the age of 72 h,the total heat release of samples with the W/B of 0.5 increase from 338.17 J/g to 389.42 J/g in the age of 72 h.Therefore,total heat release of hydration increased with the rise of SF dosage and W/B.With the increment of SF dosage,the rate curve of the heat hydration of samples with the W/B of 0.4 presented the second peaks which consists of 13.6 h,12.6 h,11.6 h and 10.6 h,respectively,whereas that second peak of the samples with the W/B of 0.5 is 13.1 h,12.1 h 11.1 h and 10.1 h.In the same W/B,once the SF dosage increased 5%,the present of the second peak time advanced 1 h,this means that the dosage of SF accelerated the hydration reaction of cement.?d?The quick growth period of the sample shrinkage which in the same W/B decreased with the increase of dosage of SF,and the sample with the W/B of 0.4 reduced from 10.4 h to 7.6 h,and the small the W/B is,the shorter the growth period is.The quick growth of autogenous shrinkage?AS?with the W/B of 0.4 which in the SF dosage of 10%is 8.4 h,while that of 0.5 is 8.7h.The ratio of the autogenous shrinkage to the drying shrinkage?RA-D?grown rapidly in the early time and it raised with the increase of SF dosage and the decrease of W/B.The dosage SF from 5%to 15%which is 0.4 W/B,the RA-D increased 6.98%to 30.16%in one day,raised from 15.1%to 28.19%in the 3 days,whereas grown from 16.78%to 26.16 in 7 days.At the age from 1 to 7,the sample with the W/B of 0.4 is 21.66%to 21.15%,whereas that the W/B of 0.5 is 6.06%to 5.78%,this means that the AS only takes a small part in the DS.?e?In the age of 7 days,the electric resistivity of samples with the W/B of 0.4 increased with SF,which from 17.10?.m to25.98?.m,that the W/B of 0.5 increase from 10.96?.m to 21.89?.m.When the dosage of SF is 10%,the electric resistivity of samples with the W/B of 0.4 is 21.74?.m,whereas that the W/B of 0.5 is 18.25?.m.This means that the electric resistivity increased with the increase of SF dosage and the decrease of W/B.With the increase of SF,the second peak time of resistivity differential curve from 794 min advance to 718 min at the W/B of 0.4and the second peak time from 890 min advance to 739 min at the W/B of 0.5.Therefore the second feature point of resistivity differential curve ahead with the increase of SF.?2?There is a correlation between the hydration properties,because these hydration properties change are based on the free water decrease and the porosity decrease in the process of the cement hydration.These hydration properties have same trend under different dosage of SF and different W/B.?a?There was a good linear relationship between the heat released and the dosage of SF during the same age:y=ax+b?R²=0.96⁰.99?.Within the ages of 1 to 3d,for the slurry of W/B of 0.4,the a increased from 3.80 to 5.37,the b increased from 183.85 to 288.77.For the slurry of W/B of 0.5,the a increased from 2.60 to4.06,the b increased from 206.93 to 326.11.?b?There is a good linear relationship between the AS and the compressive strength:y=ax+b?R²=0.94¹.0?.For the slurry of W/B of 0.4,the a increased from 0.21 to 0.22,the b decreased from 14.43 to-165.44.For the slurry of W/B of 0.5,the a increased from 0.66 to 0.73,the b decreased from-0.23 to-200.51.?c?There was a good linear relationship between the resisticity and the compressive strength during the same age:y=ax+b?R²=0.98¹.0?.For the slurry of W/B of0.4,the a increased from 0.32 to 0.64,the b decreased from 4.91 to-0.61.For the slurry of W/B of 0.5,the a increased from 0.29 to 0.48,the b decreased from 1.39 to 0.97.?d?The resisticity and the AS have a power exponential relationship:y=aLn?x?+b?R²=0.95¹.0?.For the slurry of W/B of 0.4,the a increased from 3.47 to 71.16,the b decreased from-0.97to-468.70.For the slurry of W/B of 0.5,the a increased from 5.09 to 97.30,the b decreased from-9.52 to-543.93.According to the correlation between the hydration properties,unknown performance can be predicted by known performance,and the appropriate SF dosage can be predicted.That can provide reference for engineering.?3?The selective dissolution method and resistivity combined to determine the pozzolanic effect time of SF.It find that the time of pozzolanic effect time of SF was close to the time of the frist peak of the resistivity differential curve.The time of pozzolanic effect time of SF can be predicted the resistivity method.