# MSWI Bottom Ash Application to Resist Sulfate Attack on Concrete
## 1 Introduction  
The quantity of municipal solid waste continues to increase; it exceeded 2.1 billion tons as of 2017 in China, but it is still growing at a rate of 5% to 8% a year [1]. At present, incineration is the most effective way to realize the reduction and reutilization of municipal solid waste and render it harmless [2; 3; 4]. Incleration decreases waste quality by 70% and volume by up to 90% [5]. Thus, the purpose of waste reduction has been initially achieved. In addition, it generates energy from thermal combustion [6]. Waste incineration produces a large amount of bottom ash and fly ash. Bottom ash (BA) refers to the residue discharged from the end of the hearth, which is the main component of ash residue and is close to 80% of the total weight of bottom ash and fly ash. Therefore, further harmless treatment of municipal solid waste incineration (MSWI) bottom ash and its utilization as a resource are still urgent problems to be solved.  
Cement concrete is the best-known type of construction material. Due to its good construction performance, economy, and durability, cement concrete has widely been used in structures such as buildings, bridges, and tunnels [7]. However, it has been found that concrete does not last as long as expected. Many destructive factors, such as sulphate attack, chloride ion penetration, carbonation, and more, can decrease the durability of concrete [8; 9; 10]. The reasons for damage to the durability of concrete due to sulfate attack are divided into two major categories, namely, physical attack andchemical corrosion [11]. Chemical corrosion is mainly caused by chemical reaction between the salt solution and the hydration products of the cement [12]. Physical attack refers to the destruction of concrete by the crystallization of the salt solution [13]. This destruction comes from volume expansion after the salt crystallization. The resistance of concrete to sulfate attack is mainly affected by the content of tricalcium aluminate, the amount of cement, and its compactness. The addition of industrial mineral admixtures can effectively improve the performance of concrete against sulphate attack [14; 15; 16]. After cement is partly replaced by coal ash, silica fume, and slag, and the amount of cement in the concrete is reduced accordingly, the content of calcium aluminate is relatively reduced. Furthermore, compared with cement, such industrial mineral admixtures have lower fineness and particle size; therefore, concrete admixed with these admixtures has greater compactness. Theoretically, concrete mixed with a mineral admixture has better resistance to sulfate attack.  
In this study, a strategy was proposed to improve the sulfate resistance of concrete by mixing it with MSWI bottom ash. This proposal was also based on the chemical constitution of MSWI bottom ash, which is typically rich in silica and calcium oxide with minimum amounts of heavy metals, as it is classified as nonhazardous waste by the China Hazardous Waste List. Consequently, it is available to be reused as a secondary building material [17; 18; 19]. Many studies have strongly stated that MSWI bottom ash has some hydration activity and can be used to manufacture mortar [20; 21]. However, the use of MSWI bottom ash to resist sulfate attack on concrete has been rarely reported, and the transportation and crystallization processes of salt solution in concrete remain unknown. Herein, detailed research was conducted by means of strength analysis, porosity measurements, and capillary rise and crystallization tests. Various water/cement ratios (W/C) and feedings (cement/BA) were found to have great impacts on the mechanical properties and salt solution transportation.