**3. Alkali-Activated Material Characterization**

As previously stated, the characterization of AAMs is generally based on mechanical tests performed in compliance with reference standards for common construction materials, due to the lack of specific tests methods.

In the case under study, the evaluation of the quality of the AA pastes was based on the compressive strength of 40 × 40 mm cubic samples. The reference standard is the EN 1015-1, which is commonly used for hardened mortars. According to the aforementioned standard, the compressive strength is calculated applying an increasing load (from 50 N/s to 500 N/s) on cubic samples, so that the failure occurs in a range from 30 to 90 s. The maximum load is registered and used for the calculation of the compressive strength.

Different mixtures were prepared, according to the following variables:


The right mix design was chosen in terms of workability, mechanical properties and low environmental impact curing conditions.

After several trials, the mix design considered the 70% of waste basalt powder and 30% of metakaolin as precursors mix. The SS/SH ratio for the activator was fixed equal to 4, while the precursors/activators ratio was 0.75. Once mixed, the AA paste was casted in 40 × 40 mm cubic molds and one day in the oven at 70 ◦C was chosen as optimized curing conditions. In order to have a complete mechanical characterization of the AA mixture, the compressive strength tests were carried out after 3, 7, 14, 21 and 28 days of curing.

The mechanical results are presented in Table 2.


**Table 2.** Compressive strength results for the alkali-activated (AA) cubic samples.

The AA mixture highlighted a remarkable compressive strength if compared to common Portland cement concrete after just 3 days of curing. However, a light deflection of the mechanical properties is registered for samples cured from day 7 to day 28. This reduction does not affect the performance of the material and, based on results, the complete curing of the experimental mixture can be considered as concluded after 7 days. Furthermore, a slight variation in mechanical results is a common phenomenon for AAMs, due to the high influence of mixing and casting operations on the final performances of the material.

#### **4. Paving Blocks Characterization**

Once defined the final mix design and the curing conditions, the AA paste was used for the production of the experimental paving blocks. Two versions of the modular element were produced. The PBP was entirely made in AA mixture, while PBA was mixed with aggregates, according to a specific grading distribution, and the experimental AA paste as binder.

Figure 1 shows the particles size distribution, which fits with the gradation band commonly used for concrete paving blocks. Common limestone aggregates suitable for construction materials were used. They were mixed together with the AA paste according to a specific ratio in order to have a suitable workability of the final material.

**Figure 1.** Grading distribution for Paving Block Aggregates (PBA).

The aggregates were mixed with the AA paste in order to improve the mechanical properties of the final mixture and to evaluate whether or not the presence of aggregates could affect the performance of the experimental paving blocks.

Once mixed or not with aggregates, the resulting material was casted in plastic molds, specific for the production of concrete interlocking modular elements. As defined during the mix design phase, the material was cured into the oven at 70 ◦C for 24 h. The obtained paving blocks were than de-molded

with the injection of compressed air. The samples were than stored for seven days before being tested in order to achieve the maximum of their mechanical properties, as verified during the preliminary AAMs characterization.

The plastic mold and the final samples are shown in Figure 2.

**Figure 2.** (**a**) Plastic mold; (**b**) Final paving blocks after de-molding.

In the following paragraphs, based on EN 1338 standard, the laboratory characterization of the experimental paving blocks is presented.
