Performance optimization and hydration characteristics of BOF slag-based autoclaved aerated concrete (AAC)

Highlights

• Steel slag completely replaced lime as the main calcareous raw material in AAC production.

• The addition of steel slag showed no significant loss of strength in AAC products.

• The bulk density of AAC can be reduced by adjusting the W/S ratio and NaOH content.

• The addition of steel slag can hinder the formation of xonotlite.

Abstract

Basic oxygen furnace (BOF) slag, a by-product of the steelmaking process, has never been effectively utilised owing to its various drawbacks. In this study, BOF slag was used as the calcareous raw material of autoclaved aerated concrete (AAC) to successfully and completely replace lime with BOF slag. Moreover, the slurry was regulated with NaOH and a thickening agent, successfully reducing bulk density from the B07 level to the B05 level without reducing mechanical performance. The reaction products of AAC were analysed using X-ray diffraction and scanning electron microscopy. The results demonstrate that the addition of BOF slag can hinder the formation of xonotlite which offsets the other negative effects of BOF slag to a certain extent. In addition, kirschsteinite and part of C₂F were involved in generating tobermorite during the autoclaving process.

Introduction

As a significant category of lightweight concrete, autoclaved aerated concrete (AAC) is a widely used building material and a substitute for sintered clay bricks. The practical need for AAC is increasing owing to its light weight, sound insulation, machinability, excellent fire resistance, and excellent insulation [[1], [2], [3], [4]]. In China, AAC is classified into sand-based AAC (SAAC) and ash-based AAC (AAAC) based on the properties of the siliceous material used. SAAC mainly utilises quartz sand and other siliceous materials with low activity, whereas the main siliceous materials of AAAC are various pozzolanic materials with relatively high activity. In addition to these siliceous materials, the production of traditional AAC requires the following components: (1) calcareous materials (mainly cement and lime), (2) gas-forming material (aluminium powder), and (3) admixture (gypsum) [5,6]. Moreover, additives such as superplasticisers, hydrophobic agents, and fibres have been applied for practical purposes to improve their behaviour [[7], [8], [9], [10]]. Recently, for in terms of energy conservation and emission reduction, studies on AAC preparation using various industrial wastes have been widely conducted, including fly ash, air-cooled slag, red mud, iron tailings, phosphogypsum, and copper tailing [[11], [12], [13], [14], [15], [16], [17]].

Steel slag, a well-known by-product of the steelmaking process, accounts for 10–15% of crude steel production [[18], [19], [20]]. Steel slag is mainly a complex oxide comprising CaO, Fe₂O₃, SiO₂, Al₂O₃, MgO, MnO, and P₂O₅ [21,22] and includes mainly open hearth, basic oxygen furnace (BOF), electric arc furnace (EAF), and ladle slags. Most BOF slags have similar chemical compositions and mineral phases to those of Portland cement clinkers. However, the contents of CaO, SiO₂ and Al₂O₃ in BOF slag are relatively low. In terms of mineral composition, BOF slag contains calcium silicates (mainly C₂S), which endow BOF slag with cementitious properties [20]. Research on the use of steel slag in cement and concrete has been widely conducted [[23], [24], [25], [26], [27], [28], [29]]. However, steel slag has not yet been efficiently utilised owing to several drawbacks, with low cementitious activity being the primary factor limiting its application. The content of minerals with hydration activity (C₃S and C₂S) is much lower than that of the cement clinker. Moreover, the crystallisation of C₃S and C₂S minerals is excessively developed owing to the long cooling process of the steel slag in the furnace, resulting in poor hydration activity. The poor soundness of steel slag is another critical drawback, with free CaO and MgO being the main factors influencing the soundness of steel slag [18,30,31], which exhibits low hydration activity during hydration, leading to the appearance of cracks and breakage in steel slag-based products after a period of time. Several studies have reported that steel slag activity can be improved by increasing its fineness [18,21,32,33] as high-fineness steel slag shows high activity to a certain extent; however, the poor grindability of steel slag has been ignored. As a result, steel slag cannot be extensively used because the excessive pursuit of fineness increases costs.

Thus, it is feasible to apply BOF slag to autoclaved aerated concrete. Most of the calcium oxide and silica in BOF slag can be used as raw materials to form tobermorite. Furthermore, during high-pressure steam curing, the activity of BOF slag is further increased, which can effectively circumvent the original defects of BOF slag, even without a high degree of fineness. The current challenge lies primarily in three aspects: First, the alkalinity required for slurry foaming decreases because quicklime is replaced by BOF slag, which affects the foaming effect; Second, the reduction in water demand because of the reduction in the amount of quicklime affects the capacity of the final product; Finally, a large amount of BOF slag introduces minerals that do not participate in the reaction (most notably, wustite), thereby reducing the effective strength component.

This study aimed to investigate the feasibility of BOF slag as the main feedstock for replacing quicklime and sand in AAC production. Specifically, the influence of BOF slag content on the slurry foaming performance and AAC mechanical properties is discussed. Additionally, the autoclaved products were characterised to evaluate the mechanism. In conclusion, this work is expected to provide a new methodology for utilising BOF slag and offer a novel way to regulate the performance of AAC products.