3CaO・Al₂O₃, 3CaO・SiO₂の水和におよぼす亜硫酸ナトリウムの影響

Effective retarding agents for cement mortar have been recently required to transport the ready mixed concrete, because only the addition of gypsum is not sufficient for prolonged time owing to the traffic confusion in large cities. It was previously confirmed by the authors that the addition of 0.5 to 2.5 percent of sodium sulfite retards the setting of the cement mortar twice as long. The addition of sodium sulfite was found moreover to have a favorable effect on the strength at the early stage.<br>As a basic study of the retardation of the setting by sodium sulfite, the hydration process of tricalcium aluminate and tricalcium silicate in sodium sulfite aqueous solution has been investigated by heat measurement, electron microscopic observation and X-ray diffraction analysis. The measurement of heat liberation was made by a calorimeter devised by us on pastes of the following composition at 35°C±0.01°C for 5 hours;<br>(1) C₃A+Na₂SO₃aq.<br>(2) C₃A+CaSO₄2H₂O+Ca(OH)₂+Na₂SO₃aq.<br>(3) C₃S+Na₂SO₃aq.<br>(4) C₃S+CaSO₄2H₂O+Na₂SO₃aq.<br>Na₂SO₃aq.:0, 0.5, 1.0, 5.0, 10.0%<br>Sodium sulfite in a dilute solution depressed effectively the hydration of tricalcium aluminate, whether in the presence or in the absence of gypsum and calcium hydroxide On the other hand, in a concentrated solution tricalcium aluminate reacted directly with sodium sulfite and formed aluminum hydroside gel in the paste. This precipitated aluminum hydroxide gel, coagulating and enclosing the tricalcium aluminate grain, caused “gel set” (flash set).<br>Likewise, in the hydration of tricalcium silicate in 5.0 and 10.0% sodium sulfite aqueous solution, “gel setting” is caused by the direct reaction between calcium silicate and sodium sulfite. Sodium sulfite in a dilute solution accelerated the hydration of tricalcium silicate, which are similar to the action of gypsum on tricalcium silicate in pure water. In the presence of gypsum, the addition of a dilute solution of sodium sulfite depressed greatly the hydration of tricalcium silicate.<br>The reason why the hydration of tricalcium silicate is retarded by the dilute solution of sodium sulfite in spite of the presence of gypsum, is seemed to be understood by the following explanation. The direct reaction between tricalcium silicate and sodium sulfite will proceed with the formation of calcium sulfite and soluble sodium silicate. A subsequent reaction between tricalcium silicate and gypsum dissolved in liquid will take place precipitating silica gel or gel-like calcium silicate hydrates. These gel-like substances will temporarily enclose tricalcium silicate grains, thus the hydration is retarded. It seems that the action of dilute solution of sodium sulfite on the hydration of tricalcium silicate in the presence of gypsum is interesting in respect to as an additional retardation for the cement hydration.<br>Mortar and concrete tests which were carried out concurrently, gave results which agreed with the above-mentioned experiment.