According to the determination method of the WCP activity index in JG/T573-2020 Recycled Micronized Powder for Concrete and Mortar, WCP is used to replace 30% of the cement in the preparation of cement, and after 28 days of curing, the compressive strength is measured and compared with the mortar without WCP, that is, the activity index (SAI) of WCP is obtained, and an SAI > 60 is Grade I recycled micronized powder, and 60 < SAI < 70 is Class II recycled micronized powder. Īt present, the activity excitation of WCP has become a research hotspot for scholars at home and abroad, and the activity index of WCP can best reflect the mechanical properties and activation effect of the products. However, WCP itself has potential activity, and the activity index of WCP can reach up to 80 after activity activation, so the WCP activated by the optimal activation method has less influence on the product properties. Compared to silicate cement, WCP itself has higher water absorption and water demand, which can adversely affect the mechanical and working properties of cementitious materials, and this effect becomes larger with increases in WCP admixture. Therefore, certain methods can be used to stimulate the activity of WCP to make it a cementitious material with high hydration properties. In the figure, it can be seen that the main mineral components of WCP are SiO 2, CaCO 3, CaMg(CO 3) 2, etc., all of which have potential activation properties. The XRD patterns of WCP are shown in Figure 2. ![]() The main chemical composition of WCP in some of the representative literature is summarized in Table 1, in which it can be seen that the different sources of WCP lead to complex compositions and large variations in the contents, but its main components are the same as those of cement. WCP is a fine, loose, and irregularly shaped powder that is off-white, as shown in Figure 1 and Figure 2, with a bulk density of 855–917 kg/m 3, an apparent density of 2355–2651 kg/m 3, and a stable specific surface area of 450–500 m 2/kg. The production process of recycled aggregates inevitably produces micronized particles (d < 150 μm), which account for about 19% of the mass of waste concrete, i.e., waste concrete powder (WCP). Since the activation principles of mechanical activation, chemical activation, and thermal activation are different, their activation effects are not the same. Some scholars have found that replacing cement with WCP had negative effects on the mechanical properties and early cracking of the products but could reduce the drying shrinkage of the products. Ma found that active powder (WP) mixed using WCP and waste brick powder could accelerate the hydration reaction of cementitious materials, and the addition of some WP could improve the pore structure of cementitious materials. Chen Xi found that the mixture of WCP, fly ash (FA), and silica fume (SF) could replace 15% of the cement in mortar specimens and could improve the 28-day strength of mortar specimens. In addition, some scholars have improved the hydration of mortar and concrete by WCP compounded by external admixtures. Cheng found that mortar mixed with carbonated WCP had higher flow and compressive strength than mortar without carbonated WCP. ![]() ![]() Due to the large amount of CaO within WCP, some scholars used CO 2 to treat it. ![]() In addition, mechanical grinding can also stimulate the activity of WCP, Yang found that the activity of WCP after 1 h of ball milling was effectively stimulated, and Deng proposed that submicron WCP produced by the wet grinding method can effectively improve the early strength and hydration of concrete. Sui found that WCP generates active substances such as dacite (β-C 2S) and C-S-H gel during heat treatment Li found that WCP can be effectively activated under an alkaline environment to generate a large amount of C-S-H gels. WCP cannot be widely used in construction due to its low activity and huge water requirement, which leads to its need for active stimulation.
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