Ordinary Portland cement mixed with various amounts of absorbent polymer in the form of sodium acrylate ((-CH-)(n)COONa) have been studied. As the content of absorbent polymer increased, heat evolution of samples decreased up to 1.15 wt % of absorbent polymer addition and conversely increased over 1.75 wt %. Flexural strength of cement paste with absorbent polymer was improved more than 20%. As the content of absorbent polymer increased, the porosity values decreased and mean pore diameter shifted to small pore diameter region. Flexural strength of ordinary Portland cement paste had a linear correlation with non-evaporable water content but, that of cement paste with absorbent polymer deviated from a linear correlation with non-evaporable water content. The chemical difference between cement pastes with and without absorbent polymer was found by the inductively coupled plasma-atomic emission spectroscopy and the infrared spectroscopy. For the infrared spectra of absorbent polymer, bands at 1416 and 1560 cm(-1) were assigned to C-O single bond and C=O double bond respectively, namely, unidentate complex. As the curing time increased, the absorption bands near 1416 cm(-1) shifted to longer wave number and the absorption bands near 1560 cm(-1) to shorter wave number and finally bidentate complex was formed. Absorbent polymer released sodium ions to pore solution under the basic condition of pH 12.5-13.5 and became polyacrylic acid. Then some of these polyacrylic acid were crosslinked with others by calcium ions leached from cement grains. Calcium ion was regarded as a central charge connecting the negative parts in carbon-oxygen polarization of absorbent polymer' functional groups.