Selection of UHPC ultra-high performance concrete raw materials

Ultra-High PerformanceConcrete (UHPC), also known as Reactive Powder Concrete (RPC), is one of the most innovative cement-based engineering materials of the past three decades. Achieve a big leap in the performance of engineering materials. "Ultra-high performance concrete" consists of two aspects of "ultra-high" - with ultra-high durability and ultra-high mechanical properties (compressive, tensile and high toughness).

Origin of material composition

The original UHPC (Densit) used new raw materials at that time (late 1970s) - silica ash and naphthalene superplasticizers, and the rest were traditional materials; In the 1990s, UHPC, represented by RPC, introduced grinding quartz powder, and the superplasticizer was replaced by polyacrylic acid and polycarboxylic acid super-efficient superplasticizer.

Entering the new century, the research of UHPC mineral raw materials involves nano-silica, nano-calcium carbonate, nano-carbon tube, ultrafine fly ash, ultrafine mineral powder, ultrafine cement, rice husk ash, metakaolin, glass powder, etc. To date, the finest practical mineral material is still silica fume, due to its good particle shape (spherical), high pozzolanic activity, and mature commercial supply. The use of other ultrafine mineral materials helps to reduce the amount of silica fume. The use of ordinary fine fly ash, mineral powder instead of part of the cement, glass powder instead of quartz powder, have achieved good results. The actual application of UHPC, the compressive strength is generally in the range of 150~250MPa, and the coarse and fine aggregates are generally selected natural rocks with high strength, such as quartz, granite, basalt and so on. Higher strength or need very high wear resistance, artificial aggregates can be used, such as sintered bauxite, silicon carbide, metal aggregates, etc. Great progress has also been made in improving the effect and efficiency of fiber for strengthening and toughening UHPC.

The localization and greening of the mineral components of UHPC is now the focus of a lot of research and development efforts to reduce the production and formulation costs of UHPC and make UHPC more low-carbon. The experimental study of Wang Chong et al., Chongqing University, shows that UHPC can be prepared with conventional technology and materials. In their experiments, they used a coarse limestone aggregate with a maximum particle size of 20mm, a fine medium sand aggregate with a fineness modulus of 2.3, a 900kg/m3 composite cementing material (50% cement, 10% silica fume, 20% mineral powder and 20% limestone powder), no fiber, a water-binder ratio of 0.16, and an ordinary laboratory mixer. The maximum slump of the concrete is 268mm, and the maximum compressive strength is 175.8MPa and 182.9MPa in 90 days and 365 days, respectively. Southeast University studied the preparation of green C200 UHPC (C200 GRPC), the gelling material group into 40% cement, 25% ultrafine mineral powder, 25% ultrafine fly ash and 10% silica fume, using 4% volume of steel fiber (lf=13mm, df=0.175mm), The compressive strength, bending strength and breaking energy of UHPC exceed 200MPa, 60MPa and 30,000J/m2, respectively.

The research groups of H. Yigen iter and H. Yaz Husk respectively tested using ordinary fine fly ash (specific surface area of 290m2/kg) and mineral powder (specific surface area of 396m2/kg) to replace 20% to 60% of cement (specific surface area of 380m2/kg). Under three curing conditions, namely 20oC standard curing, 100oC steam curing for 7 days and high temperature and high pressure curing (210oC, 2MPa pressure) for 8 hours, the influence of fly ash and mineral powder instead of cement on the strength of UHPC was studied. The results are shown in FIG. 1 and FIG. 2. Under standard curing conditions, the 7d strength of UHPC decreased with the increase of fly ash and mineral powder replacement rate of cement, but the 28d and 90d strength did not change significantly, which was consistent with the early Bache test results of fly ash replacement of cement. After steam or high temperature and high pressure curing, the strength of UHPC without fly ash or mineral powder is greatly improved. With the increase of fly ash replacing cement, the strength decreases. The strength of mineral powder replacing 20% cement is the highest, and then the strength decreases with the increase of the replacement rate. The replacement rate of cement by fly ash and mineral powder reaches 60%, and the UHPC strength can still reach a very high level. Therefore, using a large amount of fly ash and mineral powder, the cement content of UHPC can be reduced to the level of ordinary high-strength concrete, that is, 350~ 550kg/m3.

Basic raw material

The basic raw materials of UHPC are: cement, silica ash, high efficiency or super efficient water reducing agent, aggregate (dmax≤ 1mm fine quartz sand or ordinary granular sand and coarse aggregate), fiber (steel fiber, PVA fiber). The typical water-binder ratio (w/b) was 0.15~0.20.

K. Wille et al. analyzed and counted the main indicators and intensity variation range of 50 UHPC mix ratios, as shown in Table 4.1. The composition of these UHPC, part of no coarse aggregate, part of coarse aggregate (dmax= 7 ~ 16mm), most of the 20oC curing, a small part of the fiber and 90oC thermal curing. In general, the UHPC strength of coarse aggregate is relatively high, and the amount of cement and silica fume is relatively low. The strength of UHPC using fiber and thermal curing is relatively high.

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.