# Experimental Evaluation of Thermal Performance and Durability of Thermally-Enhanced Concretes
## 2 Materials and Methods  
### Materials  
In order to improve the thermal performance of concrete, various insulating materials were selected in this study. Hollow micro-spheres, micrometer-sized glass bubbles with a mean diameter of 65 \(\upmu\)m and a specific gravity of 0.125, are used as a replacement of the coarse aggregate. A micro-foam agent, an acrylic polymer-based foam agent, is added as a chemical admixture in order to develop stable foams 10-100 \(\upmu\)m in diameter. Diatomic powder, which is categorized as one of the well-known natural pozzolans, is used as a supplementary cementitious material for a partial replacement of cement in order to compensate the compressive strength loss and reduce the manufacturing cost of thermally-insulated concrete. In addition, diatomic has been used as one of the insulating materials for refractories due to its internal pores. Since its thermal conductivity is measured 0.014 W/m\(\cdot\)K at 400 \({}^{\circ}\)C [22], diatomic powder also can be expected to improve the thermal performance of thermally-enhanced concrete. In this study, diatomic powder with more than 90% SiO\({}_{2}\) content and a mean particle diameter of 30 \(\upmu\)m was used. The physical properties of the micro-foam agent and diatomite powder are summarized in Tables 1 and 2, respectively. A lightweight aggregate made of expanded slate was used, since not only do its internal pores contribute to the reduction of thermal conductivity of concrete, but it also has a low thermal conductivity of 0.106 W/m\(\cdot\)K in totally dry conditions. Density and the maximum size of the lightweight coarse aggregate was 1460 kg/m\({}^{3}\) and 12.5 mm. Crushed granite was used as a normalweight coarse aggregate with a specific density of 2.61 and maximum aggregate size of 25 mm. Cleaned sea-sand with a specific density of 2.60 and a fineness modulus of 2.83 was employed as a normalweight fine aggregate for all mixtures. Table 3 summarizes the physical properties of these fine and coarse aggregates. Type I Ordinary Portland Cement (OPC) with a Blaine surface area of 3325 cm\({}^{2}\)/g and a specific density of 3.15 was selected in this study. Polycarboxylate-based high-range water reducing admixture with a dosage of 0.7% of cement weight was added. An air-entraining agent 0.001% of cement weight, for a target air content of more than 4.5%, was applied to achieve the freeze-thaw resistance [23]. The concrete mixtures were categorized in two groups according to the different types of coarse aggregate (normalweight and lightweight aggregates). Three different insulation materials produced a total of eight specimens, labeled as NN and LN (e.g., the reference specimen for normalweight and lightweight aggregates concrete, adding none of the insulation materials), ND and LD (e.g., NWAC and LWAC replacing 15% of coarse aggregate volume with diatomic powder), NH and LH (e.g., NWAC and LWAC replacing 40% of the coarse aggregate volume with hollow micro-spheres instead), and NM and LM (e.g., adding the micro-foam agent in a measure of 4% of the cement binder as a chemical additive). The detailed classifications are tabulated in Table 4. The mixtures were designed for achieving the designed cylindrical compressive strength of 24 MPa at 28 days, as listed in Table 5. To design the structural concrete, comprehensive studies are needed, including recycled constituents [25]. However, structural concrete is defined as a concrete that has the compressive strength of more than 24 MPa in Korea. Thus, the target designed cylindrical compressive strength \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Density (g/cm\({}^{3}\))** & **pH** & **Viscosity (cPs)** & **Solid Content (\%)** & **Color** \\ \hline
2.15 & 6.97 & 80 & 15.6 & White Translucent \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Physical properties of the micro-foam agent.