**1. Introduction**

One of the characteristics of South Korea's housing culture is that hot water is circulated through the piping underneath the floor. Fossil fuels, such as LNG (liquefied natural gas), LPG (liquefied petroleum gas), and coal, are mainly used as the heat sources for the hot water supply, and cement is mostly used as a flooring material. Cement, which is based on carbonate minerals, has continuously generated debates about human health risks, such as sick house syndrome and atopy, owing to heavy metal release and high pH [1]. In South Korea, the energy target management system and the emissions trading scheme were introduced based on the 21st United Nations Framework Convention on Climate Change, and efforts are being made to reduce greenhouse gas emissions [2]. Therefore, a method for reducing the use of cement, which emits 700 kg of carbon dioxide per ton, is required [3]. In construction, fly ash, which is the residue of the thermal power generation process, is used as a substitute for cement. The mixture of fly ash and cement has been used as roadbed and fill material [4–8]. When substitute materials are mixed with cement, compressive strength characteristics vary depending on the mixing ratio. To prevent the reduction in strength owing to the decreased cement content, studies have been conducted on activation

methods for increasing the reactivity of substitute materials. Furthermore, studies on eco-friendly materials and the reduction in the use of cement have been conducted of late [9–13].

With the development of nanotechnology, porous materials, for which cavities represent more than 30% of the total volume, have recently been developed. Representative porous materials include active carbon and zeolite, and studies on their use as construction materials have been reported [14–16]. However, active carbon increases environmental hazards, such as fine dust, owing to logging and heating. In the case of zeolite, only mordenite and clonoptilolite are functional among the entire zeolite mineral groups, but their reserves in South Korea are small [17].

Feldspar, a representative aluminosilicate mineral, is a commonly found mineral as it accounts for 60% of Earth's crust. It is used for the manufacture of glass in addition to various potteries and ceramics, and is also used for non-functional purposes, such as land reclamation [18,19]. In South Korea, feldspar is mainly extracted from granite and quartz bedrock, and its reserves are abundant; thus, it is available at low cost.

For feldspar, the mineral composition and surface structure are changed by the weathering process. Cavities are observed on the surface of weathered feldspar porphyry, showing a porous structure. Especially, in feldspar phenocrysts, tens of thousands to hundreds of thousands of cavities are observed. The formation of cavities is related to the specific surface area and the cation exchange capacity [20]. Therefore, feldspar with a porous structure is expected to improve the physical and chemical characteristics, such as adhesion to cement, heat transfer, and preservation capabilities.

In the floor structure of a typical Korean house, cushioning or insulation materials (more than 20 mm) and lightweight foamed concrete (more than 40 mm) are placed on the concrete slab and then hot water pipes are installed, as shown in Figure 1. The heat storage layer (more than 40 mm) composed of sand and cement is then constructed on the top [21]. In this study, a certain proportion of cement was replaced with porous feldspar in the heat storage layer to increase the thermal efficiency of floor heating. Thermal, mechanical, and chemical methods were used for the activation of natural feldspar, and changes in the density, strength, and surface structure were observed. For the utilization of porous feldspar as a flooring material, it was mixed with cement and the strength characteristics were evaluated according to the mixing ratio. In addition, the thermal conductivity and heat storage characteristics were monitored through the test construction to evaluate porous feldspar as a substitute for cement.

**Figure 1.** Construction standard of the bottom layer in Korea.

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

#### *2.1. Materials*

Table 1 shows the mineral and chemical compositions of the porous feldspar used in this study. X-ray fluorescence (XRF) analysis was conducted with samples from three areas in South Korea. The analysis results showed that the content of two components, i.e., SiO2 and Al2O3, accounted for more than 80%. Figure 2 shows the scanning electron microscope (SEM, VEGA3 SBH, TESCAN, Brno, Czech Republic) image of weathered feldspar. It can be observed that irregular cavities are present on the surface and they are connected to each other. When the pore distribution of porous feldspar was measured, a high specific surface area of 334.5 m2/g was obtained. The measurement was performed using TriStar TM as an analyzer (TriStarTM II 3020, Micromeritics, GA, USA) and the Brunauer–Emmett–Teller (BET) method. It has been reported that materials with porous structures have excellent physical and chemical characteristics owing to the high specific surface area and the cation exchange capacity. Therefore, it was judged that the pore characteristics of the feldspar used in this study satisfied the above characteristics. As for the materials used in the experiment, rocks with developed feldspar phenocrysts were collected from the granite and diorite rocks in the Chung-ju area, and were used in their powder form.


**Table 1.** Characteristics of experimental materials (porous feldspar).

**Figure 2.** SEM image of weathered feldspar.

#### *2.2. Experimental Conditions*

To examine the reactivity of porous feldspar with cement, the uniaxial compressive strength according to the mixing ratio was measured first and the results were compared with the compressive strengths of other substitute materials. Furthermore, when cement is replaced with porous feldspar, a reduction in strength is expected. To compensate for the reduction, mechanical activation for reducing the particle size of materials and chemical activation for improving chemical reactions by mixing a solidifying agent were employed. In addition, mixing tests were conducted to evaluate the applicability of porous feldspar to the heat storage layer by replacing cement and sand. The purpose and method of each test are as follows.

#### 2.2.1. Characteristics of Strength

The purpose of this test was to investigate the strength characteristics according to the weight ratio of porous feldspar powder. The mixing ratio of cement and porous feldspar was varied, and three specimens were fabricated for each mixing ratio in accordance with KS L ISO 679 [22]. The specimens were cured at room temperature (20–24 ◦C) and humidity (50–60%) for 7 days. The average compressive strength of the three specimens was used as the compressive strength under each condition (EXP-R1 to EXP-R10). Table 2 shows the reactivity test of cement and feldspar.


**Table 2.** Test conditions for reactivity of cement and feldspar.

Industrial minerals that can be widely utilized as construction materials are clay minerals generated from weathered silicate minerals [23]. Among silicate minerals, silica fume, metakaolin, illite, and dolomite, which are representative pozzolanic materials containing a large amount of silica and alumina and are highly reactive with cement, were selected as substitutes for cement. When cement was replaced with porous feldspar, the compressive strength was compared with those of these materials (EXP-RM to EXP-RF). Five specimens were fabricated under each condition in accordance with KS L ISO 679, and the average compressive strength was used. After the fabricated specimens were cured at room temperature for three days, the uniaxial compressive strength was measured. Table 3 shows the mixing ratios of the materials under the above experimental conditions.

**Table 3.** Test conditions for reactivity of cement and silicate minerals.


#### 2.2.2. Method of Activation and Experimental

To compensate for the strength reduction when porous feldspar was used as a substitute for cement, mechanical and chemical activation methods were used. The particle size of the material causes changes in the physical characteristics, such as the unit weight and compressive strength. The unit weight of a material can change its thermal diffusion and heat storage characteristics. In this study, the unit weight of materials for each particle size was measured in accordance with ASTM C 128 (KS F 2504) for mechanical activation [24,25]. In this instance, all the tests were conducted five times to improve objectivity. The compressive strength test was then conducted on mortar, in which porous feldspar with various particle sizes was substituted for sand. The specimens used in the experiment were fabricated and the compressive strength test was conducted in accordance with KS L ISO 679. Section 1 in Table 4 shows the test conditions for mechanical activation.


**Table 4.** Test conditions for activation methods (mechanical and chemical activation).

\* PC: Portland cement (%), FS: Feldspar (%), S: Solidifying agent of 0. 1% by weight of cement (%).

When feldspar was used as a material to replace cement, a chemical activation method involving mixing a solidifying agent was used for preserving strength. A developed liquid-type inorganic product was used as a solidifying agent in the test. Section 2 in Table 4 shows the test conditions for chemical activation. The fabrication of specimens and the compressive strength test were conducted in accordance with KS L ISO 679, and changes in the surface structure were observed using SEM imaging (EXP-A1 to EXP-A3). The particle size of the feldspar used for specimen fabrication was 80 μm, which was selected based on the strength change results obtained via mechanical activation.
