The chemistry of the aqueous phase of ordinary portland cement paste at early ages (<2 h) has been analyzed in terms of the concentrations of the elemental components in the pore fluid. The concentrations of calcium, sulfur, aluminum, and silicon are rationalized by plotting the data on ''phase diagrams.'' To simplify the analysis, the portland cement system is described using two subsystems: (i) CaOAl 2 O 3 -CaSO 4 -H 2 O, modified by the presence of sodium and potassium, and (ii) CaO-SiO 2 -H 2 O. During the first 10 min of hydration, the calcium, sulfur, and aluminum concentrations all decrease, roughly in proportion, which suggests a precipitation process, a conversion of calcium sulfate hemihydrate to gypsum, and the initial formation of ettringite. The CaO-Al 2 O 3 -CaSO 4 -H 2 O subsystem seems to move from a phase assemblage of gypsum, Al 2 O 3 ⅐3H 2 O, and ettringite to an assemblage of gypsum, calcium hydroxide, and ettringite during the first 15-30 min after the water and the cement are mixed. The silicate equilibrium is approached more slowly. The intensity of mixing has relatively little effect on the concentrations beyond the first few minutes.