## 1 Introduction  
The hydration of cement takes place as soon as the cement particles meet water. A time-consuming transition of the cement paste, the so-called'setting process', from a liquid state to a porous and hardened solid occurs. However, it can still be conveniently divided into five stages in common based on the results of heat release tests during the early age, where the heat flow is proportional to the rate of reaction. The early hydration reaction process and microstructure of cement-based materials have definite impacts on the later macroscopic mechanical properties and durability. Cement paste is an important component of cement-based materials, and it is also a complex multi-phase heterogeneous composite. The chemical and physical changes of cement paste affect the setting and hardening of cement-based materials. Investigation of the rule of hydration, hardening process and strength growth of cement paste is helpful for better understanding the early performance of cement-based materials. Using original pore water as probes, the hydration and pore structure of cement-based materials can be in situ measured through the LF-NMR technique with advantages that have been shown to be non-invasive, non-destructive and fully quantitative. The first systematic analysis of cement pastes by \({}^{1}\)H NMR was by [PERSON] et al. [1]. They followed the evolution of \(T1\) and \(T2\) relaxation times of the main liquid volume from 10 minutes after mixing up to 28 days of hydration. [PERSON] et al. [2] presented for the first time an inversion of the CPMG (Carr-Purcell-Meiboom-Gill) decay into a pore volume distribution function with the help of the fast exchange model [3]. They reported two principal pore populations of mobile water that are interpreted as \"open gel\" pores and capillary pores. Since then there have been very many further studies yielding a wealth of results [4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17]. Over the years, \({}^{1}\)H NMR has managed to successfully provide valuable information about the hydration kinetics, porosity, and pore-size distributions. Especially, with the measurements of transverse magnetic relaxation being applied for hydration of cement, the amount of water consumed during the hydration process can be determined independently from the amplitude of the NMR echo signal extrapolated to the beginning of the pulse sequence. As \({}^{1}\)H from liquid water present within the cement microstructure is used as the active NMR probe, it can accurately reflect the information inside the cement-based samples, and no need to require additional processing such as drying or injecting. Therefore, by studying the nuclear magnetic signal, the change of evaporable water content can be qualitative or quantitatively obtain [8; 9; 11; 17].  
Although there have been many studies involving the relationship between LF-NMR results and cement hydration, there are very few studies on the transverse magnetization and its derivative being physically related to the different stages of cement hydration, and it is not very clear that there is some inherent relationship between the gel/space ratio and the setting time or very early strength of cement. The purpose of this work is to monitor the evolution of evaporable water throughout the early hydration process of cement paste at different water-to-cement ratios (ratios of mass of water to mass of cement) by LF-NMR technique. The degree of hydration and gel/space ratio (in volume) according to the Powers model [18] were obtained by analyzing and calculating the evolution of transverse magnetization through the CPMG pulse sequence. The results were correlated to the setting process and early compressive strength growth, which were obtained by conventional Vicat needle method and mechanical test.