1. Working principle
Silica fume has the characteristics of fine particle size (the average particle size is 0.1um, only a few hundred percent of cement particles) and large surface area (15000 m2/kg). After adding silica powder to the concrete, the effect is as follows:
(1) physical function. Because of the fine particles, it fills the pores between cement particles, making the slurry more compact, improving the microstructure of cement and aggregate, thereby improving the macroscopic physical – mechanical properties of concrete, such as strength, impermeability, durability, elastic modulus and so on.
(2) chemical reaction. The main ingredients of silica fume is silica (SiO2), in alkaline excitation conditions, can react with cement hydration precipitation of Ca(OH2), produce silicate colloid with hydraulic properties – hydrated calcium silicate gel (CSH gel), the gel block in the capillary tube, the capillary pores become smaller, and is not continuous, which greatly improves the consistency of concrete. The hydrated calcium silicate gel strength is much greater than that of Ca(OH)2 gel. Therefore, microsilica fume has volcano ash effect and particle effects, can improve the workability of fresh concrete and cohesion, to comprehensively improve the quality of concrete.
2. The performance of silica fume in improving the performance of concrete
(1) improve the early strength and ultimate strength of concrete
When silica fume replacement ratio is less than 30%, the steam temperature is 80℃, the compressive strength one day of mortar is 2 times without adding micro silicon fume (100MPa); without using autoclave curing, is almost 3 times (150MPa); using standard curing, the compressive strength of mortar increased significantly. When the silica fume is combined with high efficiency alkali water agent, the water binder ratio (W/C+Si) of concrete will be reduced to 0.13-0.18, and the cement particles will be filled with microsilica fume. The compressive strength of concrete is 3-5 times of that without microsilica.
(2) increase the density of concrete
In concrete adding silica fume can increase reaction silicon content, at the electron microscope, the concrete mixing silica cement gap has the crystal growth. In addition, the micro silicon powder particles are very small and fill the concrete micropores evenly. In foreign countries, the micropore volume of concrete with microsilica is obviously reduced by the cement injection method.
(3) improve the performance of concrete segregation and exudation properties
After pouring concrete, the phenomenon that water is separated from concrete often occurs. That is to form water film on the surface, also known as floating slurry, which makes the upper layer concrete distribute unevenly and affects the quality of the building. Foreign research shows that, micro silica incorporation, that the substitution rate of Si/ (Si+C) is more, the concrete material is more difficult to segregation and bleeding. When the replacement rate is up to 15%, even if the concrete slump was 15-20cm, almost no segregation and bleeding; when the substitution rate is 20-30%, pouring the concrete directly into the tap water can not segregation. Due to the properties improvement of micro silica fume on the concrete segregation and bleeding, the mixed micro silica fume concrete can be used in harbour, tunnel and underwater engineering.
(4) improving the anti-permeability, chemical corrosion resistance and specific resistance of concrete
As the addition of micro silicon fume improves the compactness of concrete and greatly reduces the cement void, it improves the permeability resistance of the microsilica fume powder. Foreign studies have shown that when the replacement rate of microsilica fume in concrete is 10-20%, it significantly improves the impermeability and chemical corrosion resistance of concrete, and also improves the corrosion resistance of steel bar. At this time, the increase of density and the increase of SiO2 content, effectively prevent the invasion and corrosion of acid ions. In addition, the specific resistance of microsilica is high ( up to 1.2 x 1011 Ω·cm, 90-150℃), so the concrete specific resistance can increase 1.9-16 times.
3. The construction method of silicon powder concrete
(1) the mixing of silicon powder concrete. Mix with the mixer, according to the formula, the material will be accurately weighed. The feeding sequence should be water → gravel → sand → cement → silicon fume → mixing by stirring machine for 1.5min, then add latex to the admixture, then stir 1.5min and out. Concrete mixing well symbol: various materials uniform mixing, good flowability, non segregation, slump control within 3cm, you may not use if it is more than 4cm.
(2) transportation of silica fume concrete. The mixed silica powder concrete must not exceed 10min for horizontal transportation and vertical transportation.
(3) the silica fume concrete warehousing pouring. The pouring process should be finished first time and must not stop halfway. Because mechanical failure and other causes produce cold joint, casting should be stopped, casting concrete has a certain strength (generally wait for 24h), after the construction seam treatment (The processing method is the same as the base plane), then casting the left concrete. Pouring should be gathered from left or right direction, through silo, first use pervibrator, then use plate vibrator mention pulp, slurry vibrating time is slightly longer than ordinary concrete, vibration compaction should not leak vibration.
(4) maintenance. The new pouring silica fume concrete will stop for 1h – 3h after vibrating and finishing (according to the specific temperature of the spot), immediately cover the plastic film, prevent rain and surface evaporation to generate cracks. 24h strictly prohibited for people to step on or pile up debris. After 36h – 48h, remove the plastic film and cover straw bags, according to hydraulic concrete maintenance procedures timely watering maintenance.
4. The problems that using micro silica fume in concrete:
(1) mix proportion design
A, compatibility: micro silica fume powder should be compatible with water reducing agent, fly ash, mineral powder, fiber and so on.
B, slumps: due to the interstitial effect, the viscosity of concrete is higher, so the concrete with silica fume slump design requirements is higher 2-3cm than the ordinary concrete;
C, mixing amount: making high strength and shotcrete, the amount of microsilica fume is 6-12% of cement. For underwater engineering concrete, its mixing amount is slightly higher. When adding silica fume as pump aid, the mixing amount is reduced to about 2%. The final mix ratio is determined by the mix ratio test.
(2) feeding and mixing
A, the usual concrete industry uses densified silica fume, so artificial feeding or automatic feeding can be selected.
B, due to the unique ultrafine characteristics of micro silicon fume powder, micro silica fume requires immediate put into after aggregate feeding, and the powder silica fume should not be added to the ready mixed concrete.
C, The mixing time of adding microsilica in concrete is extend 30-40s than ordinary concrete, in order to obtain good uniformity.
D, mixing micro silica fume concrete transportation has no special requirement, and is same as ordinary concrete.
(3) pouring and curing
A, The vibration of micro silica fume concrete must be dense and must not be leaked and lack, and over vibration. The general vibration time is 10-30s.
B, concrete continuous supply. All time is no more than 100min;
C,It is very important to prevent the premature evaporation of water in the curing process of silica fume concrete. Therefore, the pouring is finished, the film is immediately covered, the sacks are laid and the water conservation.