## 2 Materials and Methods  
White cement of low concentration of Fe\({}_{2}\)O\({}_{3}\) (\(\leq\)0.25%, w/w) is used to facilitate the LF-NMR testing. This cement is selected instead of ordinary (gray) Portland cement because of low paramagnetic phases content in the hardened paste; which can distort the magnetic field and worsen the NMR signal. Cement paste samples were uniformly mixed according to water-to-cement ratios of 0.3, 0.4, and 0.5. Cement pastes were then poured into 2 mL autosampler vials, vibrated to remove entrapped air before sealing and then rotated horizontally to reduce water bleeding. Each cement paste was weighed and the initial water content was calculated based on the water-to-cement ratio. Samples were continuously measured in a LF-NMR analyzer for 100 h. \({}^{1}\)H NMR measurements were made on Model NMRC12-010V analyzer with a fixed magnetic field of 0.30 T and a frequency field of 20 MHz manufactured by Niumag Co., Ltd., Suzhou, Jiangsu, China, CPMG pulse sequence is applied to capture the transverse relaxation behavior of proton in water.  
Various forms of water such as chemically bound water, adsorbed water, and pore water are present in the cement paste. Protons from these forms of water are characterized by different T2. Thus, the total transverse magnetization \(M(t)\) at time \(t\) is multi-exponentially decayed  
\[M(t)=M_{0}\sum f_{i}\exp(-\frac{t}{T_{2i}}) \tag{1}\]  
where \(M_{0}\) denotes the initial magnetization at time \(t\) = 0 before resonance frequency (RF) pulse. The fraction \(f_{i}\) is proportional to the volume of fluid relaxing at a rate \(1/T_{2i}\) and is constrained to be nonnegative. Due to the extremely short relaxation time (about 10 \(\upmu\)s) of the proton in chemically bound water such as Ca(OH)\({}_{2}\), the signals produced by which were decayed rapidly after \(\pi/2\) pulse. Thus, the signals obtained practically in the experiments were indeed regarded as being from the adsorbed water and pore water, which is called evaporable water.  
It is well known that chemical reactions between cement and water transform cement pastes from fluids to rigid bodies. The materials remain workable within the induction period because further hydration is retarded by hydration products that form around the cement grains early. When these coatings are ruptured by osmotic pressure, hydration is resumed and the setting process takes place. The term'setting' has been used to describe the onset of rigidity in fresh cement paste. The setting time for the cement paste was conducted according to Vicat needle tests. The method is based on measuring the depth of penetration of a needle into the cement paste. The time between starting time and the time at which the distance between the needle and the base plate is 4 \(\pm\) 1 mm, measured to the nearest minute, is defined as the initial setting time (IST). Replace the needle of the Vicat apparatus with a needle with an annular ring, record the period elapsed between the time of adding water to the cement and the time when the annular ring fails to make the impression on the cement paste mold as the final setting time (FST).  
The early compressive strength was tested by the conventional mechanical method every 12 h. The cement pastes at different water-to-cement ratios were cast into 20 mm \(\times\) 20 mm \(\times\) 20 mm cubic samples, and cured in a standard curing environment (temperature 20 \(\pm\) 1 \({}^{\circ}\)C, RH \(\geq\) 95%). Strength measured from 12 h to 96 h after mixing.