**1. Introduction**

The construction sector can be considered the largest resource user and waste producer in the world. About 3 billion tons of construction and demolition waste are generated annually in the world [1], while in the European Union (EU), 37.1% of total waste (approximately 800 million tons) was generated in the construction industry in 2020 [2], and this trend is constantly increasing. For this reason, the reuse and recycling of construction and demolition waste (CDW) is a key point not only for increasing resource efficiency, but also for reducing the use of large amounts of primary materials, energy consumption, and waste generation.

Despite the fact that many ways of utilizing CDW are known [3–8], there are many problems that prevent their large-scale use as a substitute for primary raw materials [9]. One of the largest components of CDW waste is concrete waste (other than excavated soil), approximately 70–80% of whose volume consists of fine and coarse aggregates, causing continuous depletion of natural resources [7]. Fine and coarse aggregates can be extracted from concrete waste using different processes (crushing, magnetic separation and sieving process), which can then be used as recycled concrete aggregates in different structures [7,10–14]. At the same time, the fine powder fraction (<150 μm) mainly from the cement stone part of concrete can also be used in recycled concrete or geopolymer [15–17]. However, in order to increase the reactivity of the CDW particles, milling may also be necessary, which can be performed in various mills (planetary mill [18], ball mill [17–19], vibratory mill [18], and stirred media mill [20]). The advantages and disadvantages of CDW powder fineness were summarized in a study by Tang et al. [21].

The aim of the research work is to increase the pozzolanic reactivity of the CDW powder fraction using a high energy-density mill (stirred media mill) and to study the relationships between its reactivity, structural characteristics, and the properties of the produced cement stone.

#### **2. Materials and Methods**

The base materials for the experiments were CDW with particle size <63 mm from Mento Ltd. (Bodrogkeresztúr, Hungary) and Portland cement (CEM II/A-S 42.5 R). As a first step, the CDW sample was crushed with a jaw crusher with a gap size of 20 mm. The fine fraction (<200 μm) was removed with sieving, and the 20-8 mm fraction was used in a Deval apparatus for autogenous milling for 60 min. This way, due to the abrasion of the CDW, a higher amount of cement stone could be obtained. The fine products of the previous steps were used as feed for the mechanical activation. The mineralogical composition of CDW was determined with a Bruker D8 Advance X-ray powder diffractometer (XRD) (Cu-Kα radiation, 40 kV, 40 mA) in parallel beam geometry (Göbel-mirror), as can be seen in Table 1. The high amount of quartz and carbonates come from the sand and hydrated cement. Mechanical activation was carried out in a stirred media mill with 530 cm<sup>3</sup> volume, using ZS type, Ø 1–1.2 mm ceramic beads and 5 m/s circumferential velocity. Both the material and grinding media filling ratio was 0.7. The applied milling times were 1, 3, 5 and 10 min. The 0 min milling refers to the pre-processed sample that was not mechanically activated in the stirred media mill.


**Table 1.** Mineralogical composition of CDW (wt. %).

The particle size distributions and SSA of the mechanically activated samples were analyzed with a HORIBA LA950-V2 laser particle size analyzer. The pozzolanic reactivity was assessed based on the lime sorption test carried out according to the MSZ 4706-2:1998 standard. For the structural analysis, a Jasco FTIR-4200 analyzer equipped with a diamond ATR was used, with 4 cm−<sup>1</sup> spectral resolution. The spectra were recorded between 400–4000 cm−<sup>1</sup> and baseline-corrected.

To further test the reactivity of the mechanically activated CDW cement, samples were prepared to examine the possibility of Portland cement replacement using the mechanically activated CDW cement stones. In the specimens, the Portland cement was replaced with 0%, 10%, 20% and 30% mechanically activated cement stone, and a 0.33 water-cement ratio was applied to all mixtures. The uniaxial compressive strength of the 20 × 20 mm cubical specimens was measured with an SZF-1 type hydraulic press.

### **3. Results and Discussion**

*3.1. Mechanical Activation of CDW*

3.1.1. Particle Size, SSA

The effect of mechanical activation on the particle size and geometric SSA of CDW is shown in Table 2. Milling resulted in a decrease in the particle size of CDW, while the specific surface area increased. After 10 min of milling, the specific surface area of CDW increased by 33%, but at the same time, no significant decrease in particle size was observed. This can presumably be attributed to the high quartz content (Figure 1), which hindered the efficiency of the milling.

**Table 2.** The characteristic particle size and SSA values of the ground cement stones.


**Figure 1.** The FTIR spectra of the cement and ground CDW samples.
