Abstract: A fast-cure process of manufacturing concrete products of critical shapes and strength requirements such as block, tile and pipe provides substantially maximum strength in a few hours as compared with the many months it would take in an ambient atmosphere. Critical shape is maintained without cracks, etc., by control of temperature, humidity and atmospheric CO.sub.2 within a kiln in which the uncured concrete products reside. The humidity is kept at saturation until substantially all the concrete reacts chemically with the water. The temperature is controlled to prevent excessive temperatures at critical times and to accelerate cure when the products can stand higher temperatures. Free hydrated lime is fast cured by a CO.sub.2 rich atmosphere at elevated temperature.