(271j) Aerated Concrete Blocks Production Using Secondary Steelmaking Slag

Aerated
concrete blocks production using secondary steelmaking slag

Introduction:

Steel market plays a pivotal role in the
diversification of the oil-based economies to grow its non-oil based
commodities and create an industrial sector that competes internationally. One
of the main commercial processes for steelmaking is the Electric Arc Furnace
(EAF) steelmaking. This process is commonly preceded by an ironmaking process
where iron ore pellets are reduced to metal iron and then sent as direct
reduced iron to the EAF. In the EAF, also called primary steelmaking step, the
reduced iron is melted and combined with flux (mainly dolomite) for impurities
removal forming a separate phase of liquid EAF slag. Finally, the liquid iron
goes through an alloying process in the Ladle Refining Furnace (LRF), also
called secondary steelmaking step, where the composition of the steel undergoes
final adjustment and burnt lime is added to remove impurities forming a
separate phase of liquid LRF slag. Both liquid slags (EAF and LRF) are air-cooled
after being poured out of the furnaces forming solid slags.

In the steelmaking process described there are
several solid wastes that is formed at each step of the iron/steelmaking
process. DRI dust (containing > 50% Fe) is generated from the reduction
step, EAF slag is generated from the primary steelmaking step, and LRF slag (~50%
CaO) is generated from the secondary steelmaking step. While there are research
directions focused on the use of these solid wastes, many steelmaking
industries accumulate these solid-wastes in landfills resulting in hefty
landfill costs. This project aims at
reducing the environmental and economical impact of steelmaking plant by utilizing
the LRF slag in the production of Aerated Autoclaved Concrete blocks (AAC).

AAC are highly insulating construction materials,
with low thermal conductivity, very light weight, and exceptional fire
resistant making them excellent materials for buildings insulation. Moreover, unlike
conventional concrete, AAC are produced from a chemical process that has zero CO2 emissions. AAC is
conventionally made from lime powder, Aluminum (Al) powder, silica sand,
cement, gypsum, and water. The chemical process of producing AAC generates hydrogen
gas by the reaction of the Al powder with calcium hydroxide expanding the
slurry to almost double its size giving the AAC its lightweight property. In
addition, the high-pressure autoclave curing process increases its mechanical
strength significantly by reacting CaO with SiO₂ and H₂O
forming crystalline hydrates called tobermorite. Due to the high composition of
CaO and SiO₂ in the LRF slag, we propose in this project to produce
AAC using LRF slag to replace the CaO and SiO_2.

Outline of
method:

In an attempt to study the viability of
partially replacing lime and silica with LRF slag, AAC blocks samples were
produced by varying the mass composition of lime with the LRF slag while
keeping the overall mass composition for both of these materials constant.   The produced samples then were tested for
their mechanical strength and crystal structure. Special attention was given to
the amount of Al powder added and to the water-to-dry components
(liquid-to-solid) ratio because of their direct effect on the mechanical strength
of the AAC. Detailed experimental steps are not included for brevity.

Results brief
overview:

The prepared AAC specimensÕ compressive
strength properties and microstructural features are analyzed and evaluated in
comparison to conventional AAC. The compressive strength was tested using MTS
alliance RF / 150 Material testing system. Our results show that the AAC
produced from pure slag (sample #4) meets the range
of the compressive strength of a standard AAC block; hence the slag is a
promising candidate for the production of AAC. Table (2) shows the compressive
strength of AAC blocks with varied LRF slag compositions.

Table  SEQ Table \* ARABIC 1 Compressive strength
of AAC blocks with varying LRF slag composition

The crystal structure of the produced AAC
samples was captured using Scanning Electron Microscope imaging technique and
was compared to the crystal structure, tobermorite, of a standard AAC. Sample 4
(100% LRF slag) showed a needle-like tobermorite structure as compared to a
lath-like structure for the conventional AAC. Nevertheless, the results
indicate that inclusion of LRF slag enhances crystal growth (tobermorite) due
to increase of Ca/Si ratio which increases compressive strength.

Figure  SEQ Figure \* ARABIC 1  Crystal structure of conventional AAC
(left) and AAC made from LRF slag (right)

Acknowledgement

This work was conducted in collaboration with Emirates Steel
Company, HiTech concrete company, and Al Jazeera Factory for Construction
Material.

References:

Y. Chen, J.
Chang, Y. Lai and M. Chou, "A comprehensive study on the production of
autoclaved aerated concrete: Effects of
silica-lime-cement composition and autoclaving conditions", Construction
and Building Materials, vol. 153, pp. 622-629, 2017.

F.
H. Ibraheem, R. Muhsin Fathi Shaweas and S. Mustafa Shakir Mahmood,
"Production of thermostone in Koya city", First International
Symposium on Urban Development: Koya as a Case Study, pp. 175-180.

AAC | AAC Block Technical Specification- Eco Green.
[Online]. Available:
http://www.ecogreenproducts.in/technical_specification.php. [Accessed:
25-Feb-2018].