Sprinkle the yeast over 1 cup of warm water in a large bowl. The water should be no more than 100 degrees F (40 degrees C). Let stand for 5 minutes until the yeast softens and begins to form a creamy foam. Combine the flour, sugar, and baking soda in a small bowl. Stir half of the flour mixture into the yeast mixture until no dry spots remain. Stir in the remaining flour, a 1/2 cup at a time, mixing well after each addition. When the dough has pulled together, turn it out onto a lightly floured surface and knead until smooth and elastic, about 15 minutes.
Lightly oil a large bowl, then place the dough in the bowl and turn to coat with oil. Cover with a light cloth and let rise in a warm place (80 to 95 degrees F (27 to 35 degrees C)) until doubled in volume, about 1 hour. Divide the dough into 6 equal pieces and form into rounds.
Place a steamer insert into a saucepan, and fill with water to just below the bottom of the steamer. Cover, and bring the water to a boil over high heat. Add the buns, recover, and steam for 20 minutes. Serve hot.