What is blast furnace cement

Blast furnace cement refers to the cement made of blast furnace slag. Slag, also known as granulated blast furnace slag, is a glassy material formed when the slag melted by blast furnace ironmaking is quenched without crystallization. Its main components are calcium oxide, silicon oxide and alumina. It has high potential activity. Under the action of activator, it can be combined with hydration to form hydraulic cementitious material.

In addition, because the water demand of slag powder is smaller than that of cement, the addition of slag powder can reduce the water binder ratio of concrete, reduce the bleeding water of concrete, increase fluidity, improve workability and pumpability, so it can maintain good workability. Since 30-70% of cement can be replaced in the same amount, the heat release during cement hydration is reduced, and the total heat of hydration will be reduced, so as to reduce the temperature rise of concrete and avoid concrete cracking caused by too fast temperature rise of concrete.

Blast furnace's mechanism

The hydration reaction of cement mixed with slag powder is mainly the hydration reaction of clinker minerals and the chemical reaction of slag powder with calcium hydroxide and calcium sulfate. The calcium hydroxide precipitated after hydration of cement clinker, as an alkaline activator of slag powder, interacts with the active components in slag powder to generate fibrous hydrated calcium silicate, ettringite and other hydrated products. Moreover, the gel structure of hydrated calcium silicate is more dense than that of Portland cement. Most slag powder particles participate in the process of structure formation like "crystal nucleus" in the early hardening stage, Ettringite is formed around the surface of slag powder particles, so as to obtain the best strength. Relevant studies show that as an active mixture of cement-based materials, ground slag powder has filling effect, pozzolanic effect and micro aggregate effect. Appropriate addition can improve the bonding strength between cement slurry and aggregate and form its own tight system. However, when a large amount of ground slag powder is added to the concrete or mortar, the ground slag powder plays a leading role in the contribution to the strength, while a small amount of cement mainly acts as an activator while contributing to the strength to promote the hydration and secondary hydration of the ground slag powder.

Blast furnace's strength effect

Cement is ultimately used in concrete engineering as one of the raw materials of concrete. The three factors affecting the strength of concrete are the strength of cement itself, the bonding strength between cement and aggregate, and the performance of aggregate (gradation composition, self strength, morphological characteristics, etc.). Among them, the biggest influence is the bond strength between cement mortar and coarse and fine aggregate.

When the water quenched blast furnace slag is ground to a specific surface area of more than 500 cubic meters per cubic meter, its glass structure is destroyed, so its potential activity is stimulated. In this case, the improvement of concrete strength by slag powder mainly depends on pozzolanic effect and micro aggregate effect.

Volcanic ash effect

The activation of slag powder changes its bond strength with coarse and fine aggregate. After the ground slag powder is added into the concrete, in the alkaline environment inside the concrete, the slag powder absorbs the calcium hydroxide formed during the hydration of the cement, and can promote the further hydration of the cement to generate more favorable gel, so that the grain size of calcium hydroxide in the interface area becomes smaller, the microstructure of the concrete is improved, the void ratio of the cement paste is significantly reduced, and the bonding force of the aggregate interface is strengthened, so that the strength of the concrete is greatly improved.

Micro aggregate effect

Form a micro self compact structure (i.e. improve the structural compactness through secondary hydration). The average particle size of ground slag is much smaller than that of cement. Its addition fills the gap between cement particles, changes the particle gradation of cement, and makes the concrete form a micro level self-dense solid system, so as to improve the strength. When the fineness of slag powder is very small, not only the potential activity of slag powder can not be brought into full play, but also due to the addition of slag, the amount of cement involved in hydration is relatively reduced, resulting in less hydration products and lower concrete strength.

Blast furnace's effect

(1) After adding slag powder into cement, the hydration heat of cement can be effectively reduced, and with the increase of the content of slag powder, the decline range of hydration heat increases, which can effectively prevent the cracking of mass concrete caused by stress concentration due to high hydration heat.

(2) It can reduce the volume expansion cracking of concrete caused by alkali aggregate (sand and gravel) reaction, so as to reduce the production requirements of low alkali cement.

(3) Because the addition of slag powder can reduce the porosity of concrete, enhance the anti permeability of chloride ion and form an anti-corrosion protective layer for reinforcement, it is more suitable for coastal defense engineering.

(4) The addition of slag powder can reduce the content of tricalcium aluminate and soluble calcium hydroxide in cement, so as to reduce the volume expansion of concrete caused by sulfate erosion.

(5) It can inhibit the dissolution of heavy metals such as high valence chromium in concrete.

(6) It can reduce the color of cement and concrete. The more slag is added, the lighter the color of cement and concrete is. Therefore, it can be used as a substitute for expensive white cement.