Selection of UHPC ultra-high performance concrete raw materials

Due to the very low water-binder ratio and the addition of fiber, the newly mixed UHPC is often more viscous, but as long as the material is suitable and the mix ratio is reasonable, it can also be prepared to work well or self-compacting UHPC). First of all, it is necessary to pay attention to and choose cement with good compatibility with ultra-high efficiency or high efficiency water reducer. In general, low alkali, low C3A cement (sulfate resistant cement) has good compatibility with water reducing agents, and should be tried first. The mineral composition of the cement, such as gypsum content or sulfate/C3A ratio, will also affect the effect of the water reducer. Multiple sources of cement and water reducer should be tested and compared to obtain the best combination is the basis for the preparation of high workability UHPC. Secondly, the amount of water reducing agent should be found through the test its saturated content (that is, above this content, increasing the content will not significantly improve the workability), it is appropriate to use the saturated content to achieve the best possible workability of UHPC. In addition, replacing some cement with mineral materials with fineness similar to cement and better compatibility with water reducer, such as fly ash, mineral powder, glass powder, quartz powder, limestone powder, basalt powder, etc., also helps to improve the workability of UHPC.

Now, the most used superplasticizer is polycarboxylic acid super-efficient superplasticizer, saturated or high content will greatly delay the setting time, you can use coagulant to offset part of the retarding effect of superplasticizer.

The powder of UHPC (refers to particles with a particle size less than 0.125mm) is cement and silica fume, and also includes active mineral materials (fly ash, mineral powder, glass powder, rice husk ash, metakaolin, etc.) and inactive mineral materials (quartz powder, limestone powder, basalt powder, etc.). That is, the particle size distribution of each material is tested, and then the material composition proportion of the theoretical maximum packing density is obtained by computer software analysis and calculation, which is used as the basis for trial allocation. The mix ratio was designed empirically, as shown in Table 4.1. The best volume of the finest particle, silica fume, accounts for about 1/3 of the total volume of the powder. If ultrafine fly ash or ultrafine mineral powder (particle size between cement and silica fume) is used, the amount of silica fume can be appropriately reduced, which helps to improve workability.

In the commonly used UHPC strength range of 150 to 250MPa, high strength, well-graded sand aggregate can be used to reduce slurry content and shrink. The slurry volume (the sum of the volume of powder, water, water reducer and bubbles) is generally in the range of 50 to 90%. High slurry volume content helps to improve workability.

Today, the technical approaches and materials used in the preparation of UHPC are diverse, but the basic principle remains unchanged - particle composition and mix ratio to maximize density. The ultra-high strength of UHPC determines that the water-binder ratio or gouache ratio is generally lower than 0.25. For the preparation of high fluidity or self-compacting UHPC, water reducing agents are required to play a greater role, and the selection of good compatibility water reducing agents, cement and silica ash is the key to success.