1. Introduction
The construction industry is responsible for 41% of total CO
2 emissions. In particular, CO
2 emissions from the cement firing process account for approximately 8.8% [
1]. Methods to reduce greenhouse gas emissions have been widely investigated since the Kyoto Protocol [
2]. In the construction industry, cement substitutes, such as fly ash, blast furnace slag, and red mud, have been investigated to reduce CO
2 emissions due to the cement firing process [
3,
4,
5,
6,
7,
8]. According to data provided in EN 197-1:2011 [
9], approximately 0.49 tons of carbon dioxide is generated when producing 1 ton of Portland cement. It can be seen that the amount of carbon dioxide reduced varies depending on the replacement ratio of 0.34 to 0.48 tons when producing 1 ton of cement using fly ash and blast furnace slag, which are existing cement mixing materials. In addition, in this experiment, the study was conducted because when replacing hwangto as a cement binder, the same carbon dioxide reduction as that of the existing binder can be expected. Research to reduce carbon dioxide is also underway in Europe, and recent research on using clay as a cement replacement material has been continuously conducted in Europe. Clay is a cement replacement material for hwangto and is expected to have similar mechanical properties. It is believed that through research on clay and hwangto, the construction industry will be able to reduce costs by reducing cement, and it will be an opportunity to change people’s perception of it as an eco-friendly material. In addition, this study is expected to be helpful in research on early age related to soils such as clay and hwangto.
In particular, hwangto has been studied since the 2000s. Hwangto is easy to obtain because it is found in approximately 10% of the ground surface worldwide and has a chemical composition similar to existing cement admixtures, such as fly ash and blast furnace slag [
10,
11,
12]; moreover, hwangto offers numerous advantages, such as temperature/humidity control and deodorizing functions [
13]. Hence, hwangto has attracted increasing attention as an eco-friendly building material. However, the use of hwangto has been limited to low-rise residential buildings owing to low strength development and durability degradation. Compared to concrete using only cement, as the ratio of hwangto in concrete mixed with hwangto increases, cracks increase and workability, air volume, and setting speed decrease [
14,
15,
16,
17,
18]. The low strength development of concrete mixed with hwangto is attributed to the approximately 6% decrease in volume due to the cohesion between particles when hwangto reacts with water, which leads to cracks [
19].
Low strength development may have a considerable impact on the construction period and quality in the early construction stage. In the early construction stage, it is important to evaluate the timing of form removal, and if a form collapse accident occurs during the construction process, it can result in significant human and property damage. In particular, the low strength of hwangto can have a significant impact on the construction period and quality in the early construction stages. Because concrete mixed with hwangto has lower strength compared to concrete mixed with 100% cement, evaluation at the time of form removal, such as with existing concrete, may cause problems such as form collapse. The vertical formwork demolding point is specified as 5 MPa in KASS and JASS and as 12 h after concrete pouring in the ACI and BS standards [
20,
21,
22,
23]. In each specification, the evaluation for demolding concrete vertical formwork is divided into compressive strength and curing time.
Figure 1 shows the estimations of strength development over time for concrete with target strengths of 30 and 45 MPa [
22,
24].
The red shaded area is the part that did not reach the strength of 5 MPa and the time of 12 h, which is the point at which the vertical form was demolded, and the blue shaded area is the part that only reached 12 h. In addition, the blue dashed circle is the part where the form demolding strength and time have been reached, and the red dashed circle is the part where the form demolding strength has not been reached. After 12 h, which is the formwork demolding point, only the ACI estimation equation reached 5 MPa for concrete with a target strength of 45 MPa, whereas the other estimation equation did not. Since the difference in strength over time is significant in evaluating the formwork demolding point of concrete, confirming a developed strength of 5 MPa is considered a safe and quantitative decision criterion. Therefore, the formwork demolding point was evaluated using strength in this study [
25].
The compressive strength of concrete is part of the quality index and is also an index that predicts various strengths of concrete. It is believed that overlooking the compressive strength of concrete can have a direct impact on cost, construction period, and quality. It is necessary to evaluate the compressive strength at an early age in an accurate and quantitative manner. Fracture tests on specimens are necessary for an accurate evaluation of strength development in concrete. However, identifying the accurate strength development point is challenging because the mechanical properties of concrete can be affected by environmental factors, such as temperature and weather, and monitoring the concrete strength development mechanism is difficult. To address this problem, studies have investigated the identification of strength development point using ultrasonic pulse velocity (UPV), which is a non-destructive method [
4,
26,
27,
28]. UPV is an easy and rapid technique to obtain results with high accuracy and, thus, facilitates the monitoring of density change and cracking in specimens during concrete setting and curing. An accurate evaluation of the strength development is necessary when new materials, such as non-sintered hwangto (NHT), are used in concrete.
In existing studies, the correlation between the compressive strength of concrete and UPV was expressed as an exponential function or a linear function. Looking at the correlation between compressive strength and UPV, the degree of increase in compressive strength and UPV is minimal at first, but as time passes, compressive strength increases rapidly compared to UPV. When the correlation between the compressive strength of concrete and UPV was expressed as a linear function and as an exponential function, the correlation coefficient of the exponential function was high. Additionally, because the correlation graph between the compressive strength of concrete and UPV is similar to a quadratic equation, it is expressed as an exponential function. Therefore, in this study, hwangto, an eco-friendly material, was used as a construction material and compressive strength and UPV were measured from the initial age over time according to the replacement rate of hwangto as a cement substitute material. We would like to propose a strength prediction equation in the form of an exponential function by analyzing the compressive strength of concrete mixed with hwangto and the UPV results through correlation.
4. Conclusions
In this study, the strength of concrete mixed with NHT as a cement substitute was evaluated at early ages using UPV, which is a non-destructive method. In addition, the correlation between compressive strength and UPV was analyzed.
The compressive strength of HTC41-15 was 37% higher than that of NC41 after 8 h, but it was 27% lower with a value of 6.58 MPa after 24 h. The compressive strengths of HTC33-15 and HTC33-30 were similar to that of NC33 until 15 and 10 h, respectively. The difference between the strengths of NC33 and HTC33 increased over time.
At a water-to-binder (W/B) ratio of 41%, mortar mixed with NHT showed high UPV in contrast to concrete. However, at a W/B ratio of 33%, the UPVs of mortar and concrete were similar. As the NHT substitution proportion and W/B ratio increased, UPV decreased.
The correlation between compressive strength and UPV was analyzed. The correlation coefficients (R2) for HTC41-30 and HTC33-30 were 0.88 and 0.94, respectively. In contrast, a high correlation coefficient of 0.95 or higher was observed for other specimens. Since the strength prediction formula showed a high correlation coefficient in this experiment, it is judged that it is possible to predict strength at an early age using the strength prediction formula.
As a result of comparing the strength prediction formula proposed in this study with the strength prediction models of existing researchers, the trend lines for NC and HTC were similar, but the results were different from the strength prediction models of existing researchers. In addition, it can be seen that the strength prediction models of existing researchers lack research on early age.
In this study, Equations (1)–(3) were derived through the correlation between concrete compressive strength and UPV. It is believed that it can be applied to quantitatively evaluate the compressive strength at the initial age of concrete mixed with NHT.
In the scope of this study, a prediction model using compressive strength and ultrasonic speed was presented for the development of initial strength using NHT, but the analysis of hydration mechanism and tissue composition were somewhat lacking. We plan to confirm these aspects through additional research in the future, and through this, we believe that the use of eco-friendly materials such as hwangto will contribute to reducing the damage caused by current global warming.