Silica Fume in Concrete
Silica fume can be used as a cement replacement to improve the properties of concrete. Since silica fume is collected from smelting silicon or manufacturing ferrosilicon alloys, its formation undergoes changes among three states of matter. Due to the rapid state changes that gases from the furnaces condense into liquids then back into solids, silica fume is a noncrystalline (an amorphous) structure with high pozzolanic activity.
Silica fume production process:
Fine particle size
The average particle size of silica fume is about 0.1-0.15 μm, which is 100 times finer than that of cement, so silica fume can fill in the voids in cement. Because of this, silica fume can reduce bleeding, preventing the accumulation of water underneath the aggregate, and can improve the strength at the interfacial transition zone between the cement paste and the aggregate particles.
Ca(OH)2 + SiO2 + H2O → C-S-H
Silica fume reduces the size of capillary pores by reacting with calcium hydroxide (Ca(OH)2), a compound that is adverse to the strength, to form calcium silicate hydrate (C-S-H). The filling effect of silica fume not only improves the compactness of the concrete, but also accelerates the cement hydration during the early stages as the calcium hydroxide is being used up, offsetting the reduction of the early strength caused by the use of fly ash.
Silica fume can enhance the density, compressive strength and durability of concrete, reduce its permeability and improve frost resistance. Since concrete containing silica fume has lower water-cement ratio, the heat of hydration is also reduced, which can further prevent cracking.
Reducing the heat of hydration
The heat of hydration of cement mixed with 0, 10% and 30% of silica fume was tested and shown that though the peak temperature of silica fume added cements appears slightly earlier than that of cements without silica fume, the total heat release (THR) of those with silica fume is lower. The 7-day THR is reduced by 29% and 19%, with 10% and 30% of silica fume respectively. For high-strength concrete with cement content 540 kg/m3, 10% of silica fume replacement can decrease the adiabatic temperature rise by 9%.
Silica fume improves the cohesion of the mix and prevents segregation. During the concrete pumping process, segregation causes blockage easily and even pump explosion. And segregation occurs in the pouring process may also cause issues such as honeycombing, cavitation.
Controlling alkali aggregate reactions (AAR)
Generally, alkali aggregate reactions can be divided into two types: alkali-carbonate reactions (ACR) and alkali-silica reactions (ASR).
Three conditions that cause the alkali aggregate reactions are:
- High alkali content;
- Reactive components in the aggregates;
- Sufficient moisture and temperature.
As mentioned above, silica fume is used as a cement replacement and reduces water content in the voids of concrete. In dry conditions, alkali aggregate reactions are difficult to occur. It was found that at early ages, by adding 5%, 10%, 15%, 20% of silica fume, the expansion caused by alkali aggregate reaction is suppressed. But when the dosage exceeds 15%, the influence of silica fume is not very significant.
Silica fume concrete mix design
Compatibility: Silica fume concrete has a high water demand due to the high surface area of silica fume. Therefore, only when silica fume is used together with compatible superplasticizers, can it be fully utilized.
Slump: Since silica fume can fill interstitial voids, the concrete has high viscosity (which controls the workability), the slump should be increased by 2-3cm.
Dosage: For high-strength concrete and shotcrete, the dosage of silica fume is 6-12% by mass of cement; for underwater concrete, the dosage should be slightly higher; and for the purpose of improving pumpability, the dosage is reduced to about 2%.
Silica fume concrete construction
Mixing of silica fume concrete
For the use of mixers, accurately weigh the materials according to the ratio, and feed in the order of stone, gravel, sand, cement, silica fume and mix them for 30 seconds. After that, add water and mix for 1.5 minutes, and then add admixtures, mix for 1 minute and discharge. The symbol of good concrete mixing is that the mixing of various materials is homogeneous, with good fluidity, no segregation, and the slump control is within the required range. Never add silica fume in powder form to the wet concrete already mixed.
Transportation of silica fume concrete
The horizontal and vertical transportation time of the silica fume concrete is the same as that of conventional concrete.
Silica fume concrete pouring
The pouring process should be finished at one time and should not be stopped halfway, except that cold joints occur due to machinery failure or other causes, in this case, not until the concrete has a certain strength (generally after 24 hours) and the joints are treated should the remaining part be poured. It is suggested to pour in one direction. And after spreading, use internal vibrators then surface vibrators for consolidation. The vibrating time of silica fume concrete is slightly longer than that of conventional concrete.
Fresh silica fume concrete should be covered with plastic film 1 hour to 3 hours after the vibrating process to prevent rain and surface evaporation from causing cracking. It is strictly forbidden to step on or pile up miscellaneous items within 24 hours. After 36 to 48 hours, remove the film, cover the concrete with straw, and spray water in time according to the maintenance procedures.