# Cost-Effective Temperature Sensor for Monitoring the Setting Time of Concrete
## 1 Introduction  
Portland cement and cement-based products are the main material used in civil engineering, due to its durability and mechanical properties, which positions it as one of the most widely used materials with a global production of 4.1 billion tons in 2022 [1]. Civil engineering structures built with concrete play an important role in the socioeconomic activity of a country, and it is essential to know precisely the properties of these materials and ensure compliance with quality standards to prevent these structures from reaching the end of their useful life prematurely due to problems caused by poor curing or problemssuch as cracking, carbonation, corrosion, efflorescence, etc. [2; 3; 4; 5], thus compromising the safety of users and the preservation of public and private assets, making the ability to monitor and maintain the integrity of civil structures imperative [6]. Numerous internationally standardized tests seek to establish minimum criteria to ensure the quality of these materials; However, such tests are often expensive and take a considerable amount of time. Furthermore, these tests provide real-time data at specific times, requiring additional inspections to assess the long-term condition of the material. In structural quality inspection programs, it is still predominant to perform a periodic visual inspection [7], limited to the observation of visible damage at discrete points [8], which is insufficient to maintain the health of civil infrastructures and the safety of users [9]. In contrast, the application of techniques to monitor the properties of cement and concrete allows real-time information to be obtained, providing greater certainty about the condition of the material. A critical factor during concrete setting is temperature, which is closely related to its structural quality. The strength and durability of concrete change during the hydration process and are significantly influenced by temperature and moisture content at early ages [10; 11; 12; 13]. The exothermic chemical reaction that accompanies cement hydration, known as hydration heat, raises the temperature of the concrete, influencing thermal stress and crack formation in early stages [14]. The curing temperature, a critical parameter in the progress of cement hydration, influences the stability, transformation of hydrates, and development of concrete strength [15]. A high temperature causes a further decrease in moisture content at an early age, thus affecting the concrete properties in the short and long term [16]. Variation affects the reaction rate and produces non-homogeneous precipitation of hydrates, increasing porosity and modifying the composition of the pore solution [17; 18; 19; 20]. Temperature dependence is a significant challenge on construction sites where knowledge of the strength of concrete is required. Measuring the maturity of concrete is commonly considered to estimate its strength, as it is an excellent indicator of the development of strength in situ and is directly related to the hydration temperature. In this context, obtaining direct, in situ, and real-time measurements of concrete temperature is one of the main challenges in monitoring the structural state of civil infrastructures. In addition, it enables continuous monitoring during the curing process, allowing problems to be identified in time and appropriate corrective measures to be applied. The monitoring of temperature within the construction sector is feasible thanks to IoT, or the Internet of Things, technologies, which have emerged as a solution to connect devices and objects of daily life to the network, facilitating communication between these, as well as the collection of data for analysis and future applications. Its uses are many, ranging from remote control of home appliances to real-time health monitoring. The advancement of IoT technologies has been enabled due to the miniaturization of sensors and wireless connectivity.