UHPC is the abbreviation of ultra-high performance concrete with high strength, high toughness and excellent durability, which is mostly used in bridge, water conservancy, marine and other engineering projects. Silica fume, as a highly reactive mineral admixture, is an important material for preparing UHPC and improving its usability and strength at room temperature. HSA has compiled the effects of different admixtures and types of silica fume on the performance of UHPC matrix under room temperature curing conditions for your reference.
The effect of silica fume doping on the performance of UHPC matrix
1.The effect of silica fume doping on the fluidity
Silica fume doping from 15% to 30%, the overall performance of the UHPC matrix flow gradually decreasing trend. Especially from 15% to 25%, the flow rate decreases significantly. Because the silica fume particles are nearly two orders of magnitude smaller than cement particles, the fineness and specific surface area are much larger, so the water requirement of the matrix increases with the increase of silica fume dosing. After the doping amount is greater than 25%, the effect on the flow rate is not obvious, and even some groups appear slightly increased. Because silica fume particles in addition to filling in the larger pores, with the further increase in the amount of doping, silica fume particles also exist in the relatively large cement particles between the role of lubrication, thereby improving the matrix fluidity.
2.The effect of silica fume admixture on strength
The effect of silica fume on the strength of UHPC is mainly manifested in two aspects:
- The microfilling effect due to the fine particles themselves, silica fume particles fill in the pores and between the larger cement particles, reducing the number of large pores in the matrix, optimizing the cementitious material gradation and strengthening the system compactness;
- The volcanic ash effect of silica fume changes the hydration process of UHPC cementitious components, reduces Ca(OH)2, which is weak in strength, and significantly increases the content of the generated C-S-H gel substance.
The compressive strength of the matrix was reached when the silica fume admixture was 15%, after which the compressive strength showed an overall decreasing trend with the increase of the silica fume admixture. Because the volcanic ash reaction of silica fume is very weak under room temperature conditions, a large part of silica fume does not react and cannot effectively exert the effect of improving strength. Comparing with the net slurry of 52.5 grade silicate cement, the compressive strengths at 0.16 to 0.22 water-cement ratio are all between 112 and 114 MPa, which shows that the compressive strengths of UHPC matrix are all increased compared with the net slurry after the incorporation of silica fume.
The amount of silica fume dosing for UHPC matrix prepared by different grades of cement
As the overall performance of UHPC matrix has a greater relationship with the cement species, and the difference between 52.5 grade silicate cement and 45.5 grade medium heat cement, for the two different cements, the production of silica fume in the preparation of UHPC matrix also has different requirements.
1. Dosing amount of silica fume for 52.5 grade silicate cement preparation
The flow and strength laws in the test of 52.5 grade silicate cement mixed with semi densified silica fume are consistent with the results of primary silica fume (undensified silica fume). When zirconium silica fume and white silica fume were added, the flow rate did not change much at 15% and 20%, and already had good fluidity, especially zirconium silica fume, which had a flow rate of 175 mm at 0.16 water-cement ratio. after that, the flow rate increased significantly with the increase of silica fume; all four kinds of silica fume reached the compressive strength of UHPC matrix at 15% admixture. Therefore, the suitable doping amount of silica fume in UHPC prepared by 52.5 silicate cement is 15%.
2. Dosing amount of silica fume for 45.5 grade medium-heat cement preparation
In the test of mixing semi densified silica fume into 42.5 grade medium-heat cement, the flow rate and strength pattern are consistent with the results of mixing primary silica fume. When zirconium silica fume and white silica fume were blended, the flow rate increased gradually with the increase of blending amount, and the strength of the specimens did not differ much when the blending amount was 15% and 20%, and was slightly higher at 20%. Therefore, the suitable dosing of silica fume in the matrix prepared by 42.5 grade medium-heat cement is 20% on a comprehensive consideration.
Effect of silica fume type on the performance of UHPC matrix
In the case of certain amount of doping, the type of silica fume has different effects on the performance of UHPC matrix, and the effect of the type of silica fume is also different for different cement varieties used in UHPC.
1.The effect of silica fume type on 52.5 grade silicate cement
In the vertical comparison of the same water-cement ratio, for 52.5 grade silicate cement, in terms of increasing the fluidity of the matrix, zirconium silica fume > white silica fume > primary silica fume > semi densified silica fume. Although the composition of primary and semi densified silica fume is the same, semi densified silica fume has a much higher packing density and a poorer dispersion in the hardened slurry, resulting in less flowability. In contrast, zirconia silica fume and white silica fume have significantly higher flowability compared to the two gray silica fumes, especially zirconia silica fume.
Although the high fluidity of zirconia silica fume helps to reduce the water-cement ratio with guaranteed workability, the strength of the UHPC matrix doped with zirconia silica fume does not reach the expected high strength, and the compressive strength of the UHPC matrix doped with zirconia silica fume at 0.16 water-cement ratio is only 121.6 MPa, indicating that the contribution of zirconia silica fume itself to improve the strength of UHPC is small. The improvement of flowability by white silica fume is better than the two gray silica fumes, and the improvement of UHPC strength is greater among the four silica fumes due to the higher SiO2 content of white silica fume, which is involved in the reaction activity.
The upper limit of compressive strength of UHPC matrix with 52.5 grade silicate cement appeared in the ratio of 15% white silica fume doped with water-cement ratio of 0.18, and the strength was 141.7 MPa, which increased by 26.5% relative to the net cement paste; the upper limit of strength of semi densified silica fume doped with semi densified silica fume appeared in the group with flow degree of 180 mm, which is consistent with the previous conclusion that the matrix at a flow degree of about 180 mm has better workability and is conducive to improving the strength.
2. Effect of silica fume type on 42.5 grade medium-heat cement
For the matrix prepared by 42.5 grade medium-heat cement, the improvement of fluidity is also zirconia silica fume > white silica fume > primary silica fume > semi densified silica fume; in terms of strength, except for the matrix mixed with zirconia silica fume, the upper limit of strength of all the groups appears at the water-cement ratio of 0.18, when the fluidity of the matrix is also closer to the ideal.
The strength of zirconium silica fume is higher at 0.16 water-cement ratio, because the matrix flow is too high after that. The upper strength limits of the matrix mixed with the four silica fume types: white silica fume (142.6 MPa) > primary silica fume (138.4 MPa) > semi densified silica fume (134.2 MPa) > zirconium silica fume (126.9 MPa), which increased the strength relative to the net slurry by 18.8%, 15.3%, 11.8%, and 5.8%, respectively, further demonstrating that the contribution of white silica fume to the strength of UHPC is greater, and the primary silica fume is second.