*Materials* **2020**, *13*, 4204

**Figure 13.** Temperature change of heat storage layer (heating cycles).

#### 3.6.2. Energy Efficiency

Figure 15 shows the heat storage layer temperature and power consumption during the experiment period under the two conditions. PT-2 exhibited a higher temperature increase rate than that of PT-1 until 100 min after heating, and the increase rate decreased after 30 ◦C. PT-1 showed a temperature increase rate similar to that of PT-2 until 20 ◦C, but its temperature increase rate became lower than that of PT-2. During 180 min of heating, the maximum temperature was 34.4 ◦C for PT-2 and 31.6 ◦C for PT-1, showing that the value for PT-2, which used porous feldspar, was 2.7 ◦C higher. When the target temperature of the heat storage layer was set to 25 ◦C, the boiler operation was stopped after approximately 40 min and the power consumption was 1.84 kWh for PT-2. For PT-1, the boiler operation was stopped after approximately 70 min and the power consumption was 3.22 kWh. In other words, the boiler was operated 30 min longer for PT-1 than for PT-2, and PT-1 required the additional power of 1.38 kWh, confirming that PT-2 could save approximately 57% power. If the boiler restart temperature is set to 20 ◦C after the cooling process, it is expected that the boiler will restart after 380 min for PT-2 and after 310 min for PT-1; thus, PT-2 will delay the boiler operation by approximately 70 min. Through the experiment, the rapid temperature increase effect in the heating process owing to the high thermal conductivity caused by the specific surface area characteristics of porous feldspar and the energy-saving effect in the cooling process owing to the delay effect caused by the heat storage characteristics of porous feldspar could be confirmed.

**Figure 15.** Change of temperature and electric consumption.

#### **4. Conclusions**

In this study, feldspar, which is found in South Korea in large quantities and has a porous structure owing to weathering, was used as a substitute for sand in cement. When sand was replaced with porous feldspar and four other silicate minerals in the cement mortar, the specimen that used the porous feldspar exhibited approximately 16–63% higher compressive strength, thereby confirming a higher reactivity with cement than other minerals.

When the particle size was reduced via mechanical activation to increase the specific surface area of porous feldspar, the unit weight decreased by approximately 30%, but the uniaxial compressive strength increased by up to 90%, confirming that the physical characteristics were improved.

A solidifying agent was mixed in to compensate for the strength reduction caused by the addition of porous feldspar. When 70% of the sand weight was replaced with porous feldspar and the solidifying agent was mixed in, the compressive strength was improved by approximately 30% compared with when only cement was used.

When chemical activation (2:8) was performed after reducing the cement content by 5% and replacing the sand with porous feldspar, the compressive strength at 3 days improved by approximately 43% compared to that of the specimen in which cement and sand were mixed in a ratio of 1:3. The compressive strength at 7 days was approximately two times higher.

As for the thermal diffusion, the mortar in which the sand was replaced with the porous feldspar exhibited approximately 6–7 ◦C higher temperatures than that of the standard concrete in the heating process. In addition, it maintained approximately 1–3 ◦C higher temperatures in the cooling process. This was because the mechanical activation increased the thermal diffusion by reducing the density of the porous feldspar mortar and the heat storage effect of the feldspar was relatively better owing to its porosity.

In a large-scale model experiment, porous feldspar exhibited excellent thermal diffusion and heat storage characteristics in the heat storage layer as well as an approximately 57% energy saving effect, thereby confirming its high applicability as a heat storage layer material.

The use of porous feldspar as a substitute material for aggregate can reduce the cement content, thereby decreasing the CO2 emissions. In addition, porous feldspar is an economical option owing to its easy availability and inexpensive characteristics.

**Author Contributions:** J.-G.H. and J.-W.C. provided the idea and applied for funding to support this paper. J.-W.C., S.-W.K., and Y.-S.P. performed these experiments. J.-W.C. and J.-Y.L. contributed to the analysis of experiment data. J.-Y.L. and J.-W.C. wrote this paper. J.-G.H., J.-W.C., and J.-Y.L. revised and put forward opinions for this paper. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was supported by the MSIT (Ministry of Science and ICT), Korea, under the ITRC (Information Technology Research Center) support program (IITP-2020-2020-0-01655) and the MSIP (NRF-2019R1A2C2088962), the X-mind Corps program (2017H1D8A1030599) from the National Research Foundation (NRF) of Korea, the Human Resources Development (No.20204030200090), and the of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korean government.

**Conflicts of Interest:** The authors declare no conflict of interest.
