## 1 Introduction  
Due to the large volume of construction and demolition wastes and the deficiency of natural resources, the recycling of building material wastes is urgent and of great importance. The use of building material wastes in concrete can realize up-recycling for building materials. Concrete wastes can rarely be reused as recycled aggregate to prepare concrete because recycled aggregates \"contaminated\" with cement paste have lower mechanical properties owing to the presence of cement paste. The workability, mechanical strength, and durability of recycled concrete are adversely affected by recycled aggregate [1; 2; 3]. Many experiments were carried out to enhance the quality of recycled aggregate, e.g., incorporation of thermal and mechanical treatment [4] and carbonation treatment to strengthen the recycled aggregates [5; 6]. In the process to treat the recycled aggregate, some fine powders including cement paste, sand, and coarse aggregate powder are produced, and some studies focused on the quality improvement and application. Increasing the particle size of recycled cement powder with 450 \({}^{\circ}\)C treatment and partially replacing the cement powder with ground-granulated blast-furnace slag could effectively improve the quality of recycled cement [7]. [PERSON] and [PERSON] [8; 9] reported that new calcium silicate hydrate (C-S-H) was formed and the optimum temperature was at 700-800 \({}^{\circ}\)C. [PERSON] and [PERSON] reported that clinker could be reproduced from cement paste by 1450 \({}^{\circ}\)C thermal treatment [10]. The waste concrete powder (WCP) contained not only cement paste powder but also the fine and coarse aggregate powders; therefore, it had a relatively complicated composition and special properties. It was expected that WCP could be simply used in concrete as an admixture. Admixture is a material other than water, aggregates, hydraulic cement, and fiber reinforcement that is used as an ingredient of concrete or mortar and is added to the batch immediately before or during its mixing [11]. In addition, optimal packing can be achieved with some admixtures as finer fillers, concrete absorptivity, and porosity can be reduced, and concrete has a more watertight structure [12]. Admixtures are classified as active mineral admixtures and inert ones [13], where active mineral admixtures are referred to those that contain latent active SiO\({}_{2}\) or Al\({}_{2}\)O\({}_{3}\), e.g., fly ash, slag, and silica fume. Inert mineral admixtures include no latent active compositions, e.g., CaCO\({}_{3}\) powder. The use of powders as admixtures in mortar or concrete is judged by standards. On the basis of the stipulation of ASTM C618, cement mortar replaced with 20% admixture as cement can generate greater than or equal to 75% of the strength of the reference mortar (without the replacement of cement) at both ages of 7 and 28 days to be regarded as pozzolanic material [14]. Chinese standards have similar requirements for different admixtures; for example, in Chinese standard JG/T 486, the ratio of mechanical strengths at 7 days and 28 days between mortar with 30% powders replaced with cement and mortar without powders is regarded as an index to judge if a mineral powder can be used as a concrete admixture [13]; another method to determine the pozzolanic activity of an admixture is the Frattini test according to BS EN 196-5:2011 [15; 16]. The use of WCP as admixtures in mortar has rarely been investigated. [PERSON] et al. reported that the flow ability and the mechanical strength of mortar used WCP as admixtures decreased, and WCP was certified as a nonreactive powder [17]. [PERSON] et al. confirmed that cement paste contained 24% reactive residual anhydrous clinker, and the compressive strengths of mortar with 24% cement paste replacing cement decreased; however, the compressive strength of mortar with 20% cement paste replacement could attain 83% of that of the reference specimen [18]. [PERSON] et al.