Microwave Dielectric Study of Water Structure in the Hydration Process of Cement Paste Nobuhiro Miura, Naoki Shinyashiki, and Shin Yagihara Department of Physics, School of Science, Tokai University, Kanagawa, 259-12, Japan Masami Shiotsubo Measuring Engineering Division, Komatsu Engineering Corporation, Kanagawa, 254, Japan Microwave dielectric relaxation measurements, via the time-domain reflectometry method, were performed on portland cement paste for the first time, and the water structure during the hydration process was observed. The relaxation process due to the orientation of free water, which is independent of calcium silicate hydrate (C-S-H), was observed at ∼10 GHz. The relaxation strength, in pro- portion to the amount of free water, decreased rapidly as the curing time increased for the first three days. This change is in good agreement with that of a chemical reac- tion that was reported by measurements of the heat that is evolved during hydration. The free water is taken into C- S-H and is transformed to hydrated water by the hydration process. When hydration proceeds, the relaxation processes due to the orientation of the hydrated water in C-S-H occur at ∼100 MHz and 1 MHz. I. Introduction P ORTLAND CEMENT consists mainly of a calcium silicate mix- ture that contains tricalcium silicate and dicalcium silicate (C 3 S and C 2 S, respectively). After water is added to the ce- ment, the mixture gradually hardens. This phenomenon is ex- plained by the fact that the matrix of calcium silicate hydrate (C-S-H), which is suggested to be gel-like, interpenetrates the region between the adjacent cement grains. 1,2 The structure of C-S-H gel in cement paste has been investigated by various conventional methods, such as X-ray diffractometry (XRD) study, 1 transmission electron microscopy (TEM) study, 3,4 and 29 Si nuclear magnetic resonance ( 29 Si NMR). 5 In general, C 3 S is associated with the hardening reaction that occurs during the early stages of hydration that happen within the first few weeks. The slower-reacting C 2 S continuously de- velops the strength for months or more. 1 The setting and hard- ening processes are dependent on the hydration of the mixture of water and cement. However, it is difficult to directly observe the water behavior in the cement paste by using the conven- tional methods that have been described above. When the water reacts with the cement, a heat of hydration is evolved, which is directly related to the degree of hydration of the various constituents of the cement and provides infor- mation about the rate of hydration. 2 However, the thermal analysis provides little information about the structure of the cement paste. The microwave dielectric method is one of the most-reliable techniques for investigating the dynamical structure of water molecules. 6 The recent development of the time-domain reflec- tometry (TDR) method has made dielectric measurement easy and fast, and it continuously provides absorption and disper- sion data in the frequency region from 100 kHz up to 20 GHz. 7–10 The microwave dielectric measurements, using the TDR method, have been performed on water–organic-solvent mixtures, 11 water–monomer mixtures, 12 and water–polymer mixtures 13 to investigate the water structure. Pure water exhibits a dielectric relaxation process of the Debye type at a frequency of ∼18 GHz at 25°C. 6 Its mechanism is related to the rotational diffusion of water molecules that constitute clusters by hydrogen bonds. 6 A similar relaxation process was also observed in an aqueous solution of synthetic polymer, 13 because free water or bulk water results in a similar diffusion. In the case of a biopolymer solution, such as DNA, 14,15 globular protein, 16,17 and collagen, 18 another relax- ation peak, which is due to the orientation of bound water, was also found at ∼100 MHz. It is expected that the water in the cement paste can be classified roughly into two groups. One group is hydrated wa- ter, which consists of the C-S-H hydrate, and the other group is free water. The relaxation process that is due to free water will be observed at ∼10 GHz, and that which is due to hydrated water will be observed in a lower-frequency region. However, no dielectric study on the water structure in cement paste has been reported yet. In this work, the dielectric measurement by the TDR method has been performed on the cement paste to investigate the relaxation processes that are due to water mol- ecules during the curing process. The obtained results should yield important information on the relationship between the water structure and the strength of the cement paste. II. Experimental Procedure The portland cement was obtained from Chichibu–Onoda Cement Co. (Tokyo, Japan). The density of the cement was 3.16 g/cm 3 . Distilled water was added to the cement at a water: cement ratio of 0.4 (grams of water/grams of cement). Two samples were individually prepared, and they were sealed in plastic cases, to avoid evaporation at room temperature. The cases were opened for ∼1 h for each TDR measurement. The TDR method is an advanced technique for measuring the complex permittivity of dielectric materials over a wide frequency region, from 100 kHz to 20 GHz. 7–10 An incident pulse with a rise time of 35 ps, which is generated by a pulse generator, passes through a 50  coaxial line and is reflected from the top of the line, where the sample cell is attached. The form of the pulse that is reflected from the sample cell is recorded in the time domain at the sampling head (Model HP 54121A, Hewlett Packard, Tokyo, Japan) and digitized by the digitizing oscilloscope mainframe (Model HP 54121B, Hewlett Packard). The form of the pulses that were reflected from the unknown sample and from the reference sample were measured, and Fourier transforms of these forms were performed using a per- sonal computer (Model PC9800FA, NEC, Tokyo, Japan) to D. D. Viehland—contributing editor Manuscript No. 190864. Received June 27, 1997; approved October 29, 1997. J. Am. Ceram. Soc., 81 [1] 213–16 (1998) J ournal 213