2.2.3. Strength Test of Feldspar and Mortar

This test was conducted to investigate the strength characteristics when cement and sand were replaced with porous feldspar. Specimens in which the ratio of ordinary cement to sand was 1:3 (EXP-FM1) and other specimens in which sand was replaced with feldspar smaller than 1 mm and feldspar powder smaller than 40 μm (EXP-FM2) were fabricated in a cubic form (side length: 50 mm). Three specimens were fabricated under each condition. After they were cured in water for 3–28 days, the compressive strength was measured in accordance with the ASTM C109/C109M (KS L 5105) method [26,27]. Table 5 shows the material mix and experimental conditions.


**Table 5.** Test condition of substitute materials with feldspar.

\* PC: Portland cement (%), AG: sand (%), AGF: Feldspar ≤ 1000 μm (%), PF: Feldspar ≤ 40 μm (%), S: Solidifying agent of 0.1% by weight of cement (%).

#### 2.2.4. Thermal Diffusion and Heat Storage Test

In the thermal diffusion and heat storage test, a temperature sensor was embedded at the center of 50 × 50 × 50 mm specimens, which were then subjected to water curing for 28 days [28]. The mixing proportions of the specimens were the same as those in EXP-FM1 and EXP-FM2, as shown in Table 5. The fabricated specimens were installed on top of a 400 × 400 mm hot plate, as shown in Figure 3. Their temperatures were measured every minute using a data logger. The test was conducted under two conditions. First, the temperature of the hot plate was set to 100 °C after installing the two specimens on top of the hot plate. Subsequently, the specimens were separated from the hot plate after a 60 min heating period. They were then cooled at room temperature (22–24 ◦C) to investigate their thermal diffusion effects and heat storage characteristics. In the second method, the thermal diffusion characteristics were investigated by repeating the heating and cooling periods to simulate conditions similar to those of the actual floor heating. Heating and cooling periods were repeated for 300 min at 30 min intervals.

Floor heating circulates water heated to a high temperature through a pipe embedded in the heat storage concrete. In this instance, hot water is repeatedly supplied according to the set temperature. The second method is similar to this process.

**Figure 3.** Thermal diffusion and heat storage test device.

#### *2.3. Pilot Test*

To evaluate the thermal conductivity, heat storage characteristics, and energy efficiency of porous feldspar, a large-scale single-story experimental building composed of temporary structures was installed. The outer wall of the experimental building was insulated to reduce the influence of the external temperature. Inside this experimental building, two temporary houses each of dimensions 3000 (L) × 4000 (W) × 3000 (H) mm were constructed. Figure 4 shows the design drawings of the heat storage experiment. Based on the floor layer construction standard in Figure 1, a heat storage layer composed of typical concrete mortar (PT-1) was constructed in a temporary house and a heat storage layer in which sand was replaced with feldspar (PT-2) was installed in the other temporary house. EXP-FM1 and EXP-FM2 were applied as the concrete mixing ratios of the heat storage layers. In addition, the temporary houses were separated from the ground by 50 cm to minimize the influence of the ground temperature. Construction and measurement for the two conditions were simultaneously performed to minimize the influence of external environmental factors. After installing a 2 kW electric boiler in each temporary house for hot water supply, a watt-hour meter was installed to determine the power consumption according to the experimental conditions. The temperature of the heat storage layer was measured using an infrared thermal-imaging camera (FLIR A615, temperature range: 20–150 ◦C, measurement error: 1 ◦C) (FLIR Systems Srl, Milan, Italy). Temperature sensors were installed at intervals of 80 cm in the heat storage layer to measure the temperature change and power consumption owing to the boiler operation. The measured data were transmitted to the Internet via a wireless router and stored in a cloud service to apply a remote measurement method [29]. The power consumption during the boiler operation was calculated from the images obtained by the CCTVs installed in the watt-hour meters [30].

**Figure 4.** Design drawings of heat storage experiment.

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

#### *3.1. Response Characteristics*

In the development of substitute materials for cement, the cement replacement rate is generally determined by the uniaxial compressive strength. The strength decreases if the mixing proportions of substitute materials are excessive, and the cement content increases if they are too low. In other words, appropriate mixing of cement and substitute materials is important because increasing the cement content is beneficial for strength but not for the environment.

Figure 5 shows the uniaxial compressive strength according to the mixing ratio of cement and porous feldspar powder. The compressive strength of the specimen with only cement was 11.43 MPa, and the compressive strength decreased as the cement content decreased. The compressive strength linearly decreased for EXP-R10 to EXP-R4 in which the cement content was reduced to 40%, and it rapidly changed for EXP-R3 to EXP-R1 in which the cement content was less than 30%. This indicates that the proper mixing proportion of porous feldspar powder is less than 70% for a modest reduction in strength.

**Figure 5.** Compressive strength according to the ratio of cement and porous feldspar.

Figure 6 shows the uniaxial compressive strength according to the silicate mineral type. When feldspar powder was used, the strength was approximately 16–63% higher compared with that when other silicate minerals were used, indicating that porous feldspar can be used as a substitute for cement. As for the characteristics of aluminosilicate minerals, SiO2 and Al2O3 are representative pozzolanic components. It appears that porous feldspar increased the strength through the reaction with Ca(OH2) in the cement hydration process because approximately 80% of its content is accounted for by these two components.

**Figure 6.** Compressive strength according to the type of silicate minerals.

### *3.2. Mechanical Activation*

Figure 7 shows the unit weight and compressive strength according to the particle size of porous feldspar. The unit weight decreased as the particle size of the feldspar decreased. The unit weight for the particle size of 20 μm was 1.06 g/cm3, which was approximately 31% lower than that for 500 μm. As for the compressive strength according to the particle size, the lowest strength was observed for the largest particle size of 200 μm. As the particle size decreased, the strength slowly increased up to approximately 90%, confirming that the physical characteristics were improved by reducing the particle size through mechanical activation.

**Figure 7.** Relationship between the unit weight and compressive strength of feldspar according to the particle size.

#### *3.3. Chemical Activation*

Figure 8 shows the compressive strength according to the mixing condition of porous feldspar. The compressive strength of EXP-A2 in which 100% cement was mixed with solidifying agent corresponding to 0.1% of the cement weight ranged from 15 to 19 MPa, showing that the compressive strength was improved by approximately 33% compared with that of EXP-A1 in which only 100% cement was used. The compressive strength of EXP-A3 in which 70% of the cement weight was replaced with porous feldspar and solidifying agent corresponding to 0.1% of the cement weight added ranged from 15 to 18 MPa, which was approximately 30% higher than that of EXP-A1 in which only cement was used. Figure 9 shows the surface structures of the specimens analyzed using SEM. For EXP-A3 in which cement was replaced with porous feldspar, reaction products of the chemical reactions of the inorganic solidifying agent and porous feldspar were observed. Table 6 shows the results of analyzing (SEM-EDS, TESCAN VEGA3 SBH) the chemical compositions of the specimens used in the tests. Na and Cl, which are the major components of the solidifying agent, were detected in EXP-A2 and EXP-A3, in which the solidifying agent and porous feldspar were added. In EXP-A3, the Si and Al contents were 29.6% and 7.2%, respectively, values two to three times higher than those of the other samples. They appear to have increased the strength through the reaction with Ca(OH)2 generated from the cement hydration process.

**Figure 8.** Compressive strength of cement and feldspar mixture.

(**a**) EXP-A1 (**b**) EXP-A2

(**c**) EXP-A3

**Figure 9.** SEM image of specimens. (**a**) EXP-A1, (**b**) EXP-A2, and (**c**) EXP-A3.


**Table 6.** SEM-EDS elemental composition of specimens (EXP-A1, EXP-A2, EXP-A3).

\* N.D: Non-detection.

#### *3.4. Evaluation of Substitute Materials*

The compressive strength test was conducted to evaluate the applicability of porous feldspar to the heat storage layer in the heating floor layer. For a relative comparison, a specimen was fabricated in the same manner by mixing cement mortar and sand in a ratio of 1:3. The mixing proportion of porous feldspar less than 70% was proposed in Section 3.1, and the inorganic solidifying agent corresponding to 0.1% of the cement weight was added based on the experiment results in Section 3.3 to prevent the rapid reduction in strength and to increase the addition of porous feldspar.

Figure 10 shows the results of the compressive strength test. As for the strength characteristics according to the curing time, the strength showed a tendency to increase over time for both materials. Especially, for EXP-FM2 with porous feldspar, the compressive strength at 3 days was 10.53 MPa even though the cement content was reduced by 5%. This result indicates that the strength was improved by approximately 43% compared with that of EXP-FM1. The compressive strengths of EXP-FM1 and EXP-FM2 at seven days were 7.3 and 14.94 MPa, respectively, showing that the strength of EXP-FM2 was two times higher. Both compressive strengths satisfied the quality criterion of South Korea (strength at seven days: 7 MPa) [31]. The compressive strengths of EXP-FM1 and EXP-FM2 at 28 days were 14.14 and 18.97 MPa, respectively, confirming that the strength of EXP-FM2 with porous feldspar was approximately 35% higher than that of EXP-FM1 even though its strength increment slightly decreased.

**Figure 10.** Compressive strength for different mixing ratios and curing times.

#### *3.5. Characteristics of Thermal Di*ff*usion and Heat Storage*

Figure 11 shows the temperature changes in the EXP-FM1 and EXP-FM2 specimens when they were heated on the heating plate for 60 min and then cooled at room temperature (22–24 ◦C) for 150 min. The maximum temperature of the EXP-FM2 specimen, which replaced the sand with porous feldspar, was 66.4 ◦C. This was 7 ◦C higher than the maximum temperature (59.4 ◦C) of

EXP-FM1, a comparison target, confirming the high thermal diffusivity of the specimen containing the porous feldspar. In contrast, when cooled at room temperature after heating for 60 min, EXP-FM2 exhibited a sharp decrease in temperature at the beginning of the cooling period and maintained an approximately 1.3 ◦C higher temperature than EXP-FM1. This could be because the heat loss in EXP-FM2, which exhibited relatively higher temperatures, was more, owing to the equilibrium between the temperature inside the specimen and the outside temperature during the cooling period at room temperature.

**Figure 11.** Result of heat storage (heating for 60 min).

Figure 12 shows the results of the test in which the heating and cooling periods were repeated at 30 min intervals. The maximum temperatures during the heating period were approximately 59 ◦C for EXP-FM1 and 65 ◦C for EXP-FM2, resulting in a difference of approximately 6 ◦C. Contrarily, the minimum temperatures during the cooling period were approximately 45 ◦C for EXP- FM1 and 48 ◦C for EXP-FM2, resulting in a difference of approximately 3 ◦C. As the test was repeated, the maximum and minimum temperatures exhibited similar values.

In the cooling process, EXP-FM2 had higher temperatures than EXP-FM1 unlike the results shown in Figure 11. This is because a method similar to the actual heating process was selected in the cooling process without separating the specimens from the heating plate. A point to be noted from these results is whether the created mortar, in which the sand was replaced with porous feldspar and the cement content was decreased, is suitable as a heat storage layer in flooring material. In Section 3.4, it was confirmed that the specimen containing the porous feldspar satisfied the strength criterion for the heat storage layer. In addition, the results of the thermal diffusion and the heat storage test confirmed that EXP-FM2, which replaced the sand with porous feldspar, had faster thermal diffusion and better heat storage characteristics than EXP-FM1, a comparison target. In general, low density of concrete causes high thermal diffusivity but leads to a low specific heat capacity. In this test, however, EXP-FM2, which had a lower density, was superior to EXP-FM1 in terms of both heat transfer and heat storage. It appears that the heat storage effect was high owing to the porosity characteristics of feldspar, a substitute material for sand.

**Figure 12.** Characteristics of heat diffusion and storage (repeated heating).

#### *3.6. Pilot Test*

#### 3.6.1. Characteristics of Thermal Conductivity and Heat Storage

Figure 13 shows the temperature distribution of the heat storage layer measured using the thermal-imaging camera during the heating time after hot water was supplied to the floor. The temperature of the supplied hot water was 55 ◦C. At 10 min after the boiler operation, the heat storage layer mixed with porous feldspar (PT-2) exhibited a temperature approximately 2 ◦C higher than that of the heat storage layer composed of ordinary concrete (PT-1). The temperature difference increased to approximately 3 ◦C at 2 h after the boiler operation, indicating that PT-2 had better heat transfer than that of PT-1. Figure 14 shows the temperature variation during the cooling period after the boiler operation was stopped. PT-1 exhibited a fast cooling speed and no thermal image could be obtained after 90 min. The average temperature was 21 ◦C after 120 min and 18 ◦C after 180 min, which was identical to the temperature before the boiler operation. In contrast, PT-2 with porous feldspar exhibited temperatures approximately 2 ◦C higher than those of PT-1. The temperature remained at over 23 ◦C even after 180 min of cooling, confirming the excellent heat storage characteristics.

These results can be divided into the influence of the material properties of the heat storage layer and that of the bottom storage layer structure (Figure 1). First, in terms of material properties, the concrete containing the porous feldspar (PT-2) was favorable for thermal diffusion because its density was lower than that of PT-1, as shown in the thermal diffusion and heat storage test in Section 3.5. In contrast, PT-1 exhibited slow thermal diffusion because the density of its heat storage layer was higher compared to that of PT-2. Owing to the high thermal resistance, more heat flow occurred in the bottom direction (lightweight foamed concrete + slab), thereby causing relatively low thermal diffusion to the heat storage layer.
