Introduction
Ultra-high performance concrete (UHPC) is a fiber-reinforced cementitious material with high strength, high toughness, low porosity and high durability, which is recognized as a new type of structural material with great development prospects and application value in the future. Although in the past 30 years, in-depth research has been carried out on ultra-high performance concrete and its composition, and the formulation process of 150MPa UHPC is relatively mature under vibration and high temperature curing conditions, it is difficult to do so without special treatment (such as thermal curing, pressurization or vibration). Because silica fume has a high activity effect and good bead effect, in most UHPC and RPC, its optimal content reaches 15% 30%, and because silica fume has a large specific surface area, in the case of large dosage, the fluidity is poor, vibration molding conditions must be used, and the activity effect of silica fume at room temperature is limited, so high temperature curing is an important condition for it to obtain excellent mechanical properties. With the gradual and widespread use of UHPC in various fields of construction. Self-compacting and steam-free curing are new performance requirements for UHPC.
Fly ash microbeads (hereinafter referred to as microbeads) is a new type of ultra-fine powder material, which is an ultra-fine and perfect positive spherical powder product with continuous particle size distribution made of high-quality fly ash through unique selection and processing. Fly ash beads have excellent functions such as high activity, low heat of hydration, light weight, corrosion resistance, high compressive strength, good fluidity and good thermal stability, and can be used as a new type of active ultrafine aggregate for high-performance concrete.
In this paper, the effect of fly ash beads on the packing compactness and compressive strength of UHPC was studied by the particle packing compactness test method, so as to prepare 28D compressive strength of 140MPa and 56D compressive strength of 150MPa of self-compacting and non-steaming UHPC.
1 Experimental part
1.1 Test raw materials
1) Cement: Conch p·o 52Grade 5 cement.
2) Fly ash beads: fly ash beads produced by a factory in Inner Mongolia, Figure 1 shows the particle morphology of the beads, from Figure 1, it can be seen that the surface of the beads is smooth, and they are all standard spherical, and the particle size is mostly distributed in 05 4 m, can well fill the lack of continuous particle size distribution between cement and silica fume. Table 1 is the chemical composition of microbeads, fly ash and silica fume, it can be seen that the content of SiO2 and Al2O3 in the beads is higher than that of fly ash, and there are fewer inactive impurities, SiO2 is the main component of the vitreous body, and it is also the main component of the formation of hydrated calcium silicate gel, and the SiO2 and Al2O3 in fly ash contribute greatly to the volcanic ash properties of fly ash, and the higher the content, the greater the volcanic ash activity of fly ash.
3) Silica fume: Microsilica fume produced in Zibo, Shandong, SiO2 94%, specific surface area 20900m2 kg.
4) Quartz sand: 20 40 mesh, 40 70 mesh quartz sand is selected in this paper, and the packing density is tested by traditional methods, when 20 40 mesh 40 0 mesh = 05∶0.At 5, the highest degree of packing compactness, the bulk density is 1915kg m3, and the compactness is 723%。
5) Admixture: Sika 540 powder superplasticizer.
1.2 Test Methods
Kwan et al. have developed a wet test method for powder accumulation through analysis, test comparison and verification, which can accurately measure the packing density of cementitious materials or powders in the real state (mixed with water and under the action of superplasticizer).
The principle of the wet particle packing compactness test method is very simple, that is, the cementitious material (powder) is mixed with different water-glue ratios in different compound proportions, and the ratio of the volume of particles of the mixture colloidal material to the volume of the slurry is calculated according to the density and content of various particles, that is, the particle packing compactness, the larger this value, the smaller the corresponding slurry void ratio. Through the preliminary test, the fluidity of the cementitious material slurry between 190 and 210mm, and the fluidity of the mortar between 270 and 290mm have a higher packing compactness.
According to GB T2419-2005 "Cement Mortar Fluidity Determination Method", the strength of the cementitious material slurry was determined, and 40mm 40mm 160mm test blocks were used. According to JGJT 283-2012 "Technical Specification for the Application of Self-compacting Concrete" and GB T31387-2015 "Active Powder Concrete", the strength of UHPC was determined, and the test block was 100mm cube test block. The pouring is self-compacting, and the maintenance is the standard curing.
2. Test ratio design and test results
In this experiment, the effect of mineral admixture content on the compactness and strength of net slurry accumulation under the condition of low water-glue ratio was studied, and the mix ratio and test results are shown in Table 2 and Table 3. In order to ensure that the slurry and mortar have a good self-compacting effect and a high degree of packing compaction, the slurry should be guaranteed to have a fluidity between 190 and 210mm under the slurry test condition, and the mortar should be guaranteed to have a fluidity between 270 and 290mm under the cone truncation test condition. A0 and B0 are pure cement and cement silica fume = 1 01 blank control group.
In Table 2, the strength of the A4 ratio is higher, and the basic compound ratio is used, and Table 3 is the cementitious system in which silica fume is mixed with cement and microbead composite cementitious material, and the same quality is substituted for the compound blending system, and the mix ratio and test results are shown in Table 3.
3 Analysis of test results
3.1. Effect of microbead content on slurry performance
The effect of microbeads on the packing density and strength of the pulp is shown in Figure 2, and the effect of the amount of microbeads on the water demand and strength of the pulp is shown in Figure 3.
As can be seen from Fig. 2 and Fig. 3, the compressive strength of the slurry is positively correlated with the wet particle packing compactness of the slurry, and with the increasing amount of microbeads, the wet particle packing density of the slurry increases first and then decreases. Because the particle shape of the beads is better, and the particle size is smaller than the cement, the void between the cement particles is effectively filled, and the water-cement ratio is continuously reduced under the condition of ensuring good fluidity, but the strength is not constantly increasing, mainly because if the dosage is too large, the amount of microbeads exceeds the optimal cement gap filling amount, and the excess microbeads can not play a filling effect and the beads can not have a chemical reaction with water, so the optimal dosage of microbeads is 20%. As the part of fly ash with better particle shape and good activity, the activity of microbeads is also slow, so its activity in the later stage is more obvious.
3.2 Effect of silica fume content on the performance of slurry
The effect of silica fume content on the water demand of the slurry is shown in Figure 4, and the effect on the compactness and strength of the slurry is shown in Figure 5.
Due to the small particle size and large specific surface area of silica fume, it can be seen from Figure 4 and Figure 5 that under the same fluidity, the net slurry of silica fume doped with 10% (the first histogram) has a large water demand, poor fluidity, low density of wet particle accumulation, and low compressive strength, which needs to be mixed with microbeads. From Figure 5, it can be concluded that the 28d compressive strength of silica fume is slightly improved by the self-compacting pulp, and the 56d strength is about the same as that of single-doped beads, and the optimal silica fume content is 6% 12% considering the influence on fluidity. Therefore, under normal temperature conditions, the active effect of silica fume is limited, and scholars hZanni used SINMR (nuclear magnetic resonance spectroscopy) to quantitatively analyze the degree of reaction of silica fume at room temperature, and the conclusion was consistent. The results showed that only 10% of the silica fume reacted after 56 days of water culture at a curing temperature of 20. Under the condition that the curing temperature is 90 for 48 hours, 49% of the silica fume reacts at the age of 56 days.
By test, in cement microbeads silica fume = 1 02∶0.1. Under the cementitious system, when the sand glue ratio is 08 1, the mortar slurry has good fluidity, mixed with 2% steel fiber, can be prepared at room temperature curing 28d compressive strength of 140MPa, 56D compressive strength of 155MPa self-compacting UHPC.
4 Conclusion
1) The good morphological effect of fly ash beads can greatly reduce the water demand of the slurry and improve the fluidity of the slurry. In addition, the activity of microbeads is higher than that of general fly ash, and the dual effect of morphology and volcanic ash will improve the compressive strength of the slurry, and the strength of the slurry will be significantly improved in the later stage, and the optimal content is 10% to 25%.
2) Under normal temperature conditions, the activity of silica fume is limited, and the compressive strength of the microbead cement composite system can be slightly improved at room temperature for 28 days, and the optimal dosage is 6% to 12% due to its adverse effect on fluidity.
3) When the cementitious system is cement, microbeads, silica fume=1 02∶0.1. The cementitious slurry has good fluidity and compressive strength, which can reach 150MPa. Under the above cementitious system, when the sand glue ratio is 08 1, add 2% steel fiber, can obtain 28D compressive strength of 140MPa self-compacting UHPC at room temperature.
4) There is a strong positive correlation between the compressive strength of the cementitious material slurry and the packing density of wet particles.