Bear in mind that calcium carbonate is taken in large doses to act as an antacid - thousands of milligrams. Soylent contains maybe 800 mg (exact formula unavailable), and that’s split into three or four meals, so no more than 266 mg or 200 mg per meal.
Tums Utlra 1000 tablets contains 1000 mg calcium carbonate each. Dosage instructions: take two to three tablets as symptoms occur.
Tums Regular contain 500 mg each, with instructions to take two to four tablets as symptoms occur.
So as an over-the-counter antacid, you take 1000 to 3000 mg of calcium carbonate per dose. Soylent contains 266 mg or less per meal.
Your stomach acid is not “neutralized” by Soylent. Your stomach produces additional acid as necessary for digestion, maintaining acidity at a pH of around 4 or 5. The pH in your stomach is more acid when empty, around 2.0, but ranging 1.5 to 3.0; when you eat a meal, the acid works on the food, but the food raises the pH of the stomach. The body secretes more acid as necessary to keep the pH around 4 or 5 until the food has passed; when the food is gone and the pH has dropped to the 2.0 range again, secretion reduces or stops.
The amounts of alkaline substances in Soylent and other normal foods - including, but not limited to the calcium carbonate - should not overwhelm the normal function of the acid system in the stomach for healthy people.
Incidentally, after the food passes the stomach and goes to the intestine, your body produces bicarbonate itself, in order to neutralize the remaining acid - it produces as much as necessary to get the pH up to about 8 (on the basic side; 7 is neutral); the pH in your small intestines is maintained at around 8.0.