# Properties of a Lightweight Fly Ash-Slag Alkali-Activated Concrete with Three Strength Grades
## 1 Introduction  
Concrete is the most widely used building material on this planet, which is mainly produced by ordinary Portland cement (OPC), natural aggregates and other components. Cement production needs to consume natural resources and releases large amounts of carbon dioxide (CO\({}_{2}\)) into the environment. [PERSON] et al. [1] found the manufacturing process of OPC contributes 5-8% of global greenhouse gas emissions, and the annual worldwide cement production would increase to 4.38 billion tons by 2050 at an annual growth rate of 5%. Alkali-activated concrete (AAC), also known as \"geopolymer concrete\", can be manufactured through an activation process between alkaline liquids and materials rich in silica and alumina, such as blast furnace slag powder (BFSP), which is a by-product from the steel making, and fly ash (FA), which is a by-product of coal combustion from coal-fired power plants. It has become a promising green construction material for sustainable development and attracted much attention from the academic field in recent years [2, 3]. It wasestimated that the use of AAC instead of OPC concrete can help to reduce CO\({}_{2}\) emissions by 60% to 80% [4; 5; 6]. The comparison of mechanical properties and durability between AAC and OPC concrete has been widely reported [7; 8; 9; 10; 11], but most of these studies were focused on heat-cured fly ash-based AAC. Recent studies indicated that the combination of high calcium raw materials and low or no calcium raw materials can improve the early strength of the AAC and avoid heat-curing. [PERSON] et al. [8] and [PERSON] et al. [9] found that a fly ash-slag-based AAC manufactured at room temperature could achieve the same setting time and compressive strength as OPC concrete, and had a higher splitting tensile strength and lower Young's modulus and Poisson's ratio. Some studies [12; 13] found that the compressive strength of fly ash-slag-based AAC increased with the increase in slag content. A microstructure analysis of AAC indicated that the calcium content available for geopolymerization was closely related to the slag content in binder materials [14; 15]. The polymerization of a low-calcium FA with an alkaline activator mainly produced sodium alumino-silicate hydrate gel (the NASH network), while the blended low-calcium FA and BPSP with a high calcium content was polymerized not only to generate the NASH network, but also to form calcium alumino-silicate hydrate gel (the CASH network). Therefore, it can be considered that fly ash-slag-based binder can be used to prepare an AAC with superior mechanical properties at room temperature. [PERSON] et al. [16] and [PERSON] et al. [17] investigated the relationships between the engineering properties of AAC and compressive strength, and found that the ultrasonic pulse velocity (UPV) increased with an increase in compressive strength. Some researchers compared the fracture properties between AAC and its cement counterpart, and found that the fracture energy of the AAC increased, whereas its characteristic length decreased with an increase in compressive strength [7; 18; 19; 20]. The application of lightweight aggregates in structural concrete is an important development in concrete construction. The structural lightweight concrete has the advantages of a light self-weight, good heat insulation performance and high fire resistance; thus, it is very appealing, especially in the production of the smaller and lighter precast concrete members used in high-rise buildings. Sintered fly ash aggregate is an artificial lightweight aggregate that is produced by expanding the pulverized fuel ash at approximately 1200-1300 \({}^{\circ}\)C in a rotary kiln. The particle shape of the sintered fly ash aggregate is mainly spherical, with a dense outer shell and highly porous inner structure. A lightweight ordinary Portland cement concrete (LOPC) containing sintered fly ash aggregate has been recently studied in some research [21; 22].