Effect Of Viscosity Modifying Admixture On Properties Of Self Compacting Concrete

Self compacting concrete(SCC) is a kind of high performance concrete which can automatically compact without vibration. However, the SCC tend to segregate with increasing the fluidity. Viscosity-modifying agents(VMA), such as inorganic sol, cellulose ether, and welan gum, etc, are often introduced to prevent the segregation. In this paper, a kind of polymer with low, but different molecular weight(abbreviated as P4 and P8) was chosen as VMA, and the effect of the dosage on the performance of the fresh and harden cementitious materials were studied. The results showed that:(1) The addition of both P4 and P8 showed minor effect on the fluidity of fresh cement paste, but great impact on the flow time of paste after 20min’s standing and then re-stirring. With the increase of dosage, the fluidity decreases, and addition of both help improve the segregation resistance of SCC.(2) While incorporating P4, the cement hydration can be promoted, and the accelerating effect increased, and then decreased with the increase of dosage. The promoting effect was most obvious when the dosage was 0.2%. While P8 was added, the cement hydration can be promoted when the dosage was lower than 0.2%, and the promoting effect declines with the increase of dosage. However, when the dosage is above 0.3%, the addition of P8 began to show a retarding effect on the cement hydration, and the dosage is higher, the retarding effect is more obvious.(3) The strength of the hardened SCC was significantly improved by incorporating both P4 and P8, and with increasing the dosage, the strength was enhanced then decrease. The enhancement was most obvious when the dosage was 0.2%. Through introducing both P4 and P8,the chloride ion penetration resistance was obviously promoted according to the colorimetric testing method.(4) Though the capillary porosity increased in the hardened cement paste while introducing P4 and P8 according to the MIP analysis, the SEM result showed a better combination between the fly ash particles and the cement hydration products, the pores are fully filled with hydration products, and the microstructure of hardened cement paste is more dense. That might be the main reason why the compressive strength and chloride ion penetration resistance of the hardened SCC were improved while incorporating P4 and P8 as VMA.