Application of Calcium Oxide in the Production of Concrete Expansive Agents

In the construction of modern concrete projects, shrinkage cracking is a core problem affecting the durability, tightness and stability of structures. As a key admixture, expansive agents effectively solve this issue. Featuring large hydration expansion volume, stable performance, controllable cost and accessible raw materials, calcium oxide serves as a high-efficiency traditional expansion base material. It has become a core raw material for the production of concrete expansive agents and is widely applied in engineering fields such as waterproof concrete, mass concrete and grouting materials. Relying on its unique hydration reaction characteristics, calcium oxide-based expansive agents can effectively compensate for the shrinkage and deformation of concrete during hardening, inhibit cracks from the source, and improve the overall construction quality of projects.

The core principle of applying calcium oxide to produce expansive agents lies in its specific hydration chemical reaction. Pure calcium oxide (quicklime) undergoes a vigorous hydration reaction with water to generate calcium hydroxide, with the chemical equation: CaO+H₂O=Ca(OH)₂. This reaction not only releases a large amount of heat but also causes significant volume variation. The volume of the hydration product calcium hydroxide is approximately twice that of the original calcium oxide, producing a stable volume expansion effect. Different from calcium sulfoaluminate expansive agents, calcium oxide-based expansive agents feature a gentle and sustained expansion process with mild early expansion and stable late expansion. They can accurately meet the demand for dry shrinkage and cold shrinkage compensation in the middle and late hardening stages of concrete, making up for the deficiency of insufficient late shrinkage compensation of traditional expansive agents.

In industrial production, the raw material selection and pretreatment process of calcium oxide directly determine the performance of finished expansive agents. Calcium oxide used in production is required to have high activity and high purity. It is generally prepared by calcining high-quality limestone at a temperature above 1200℃, with the effective calcium oxide content no less than 93%. The content of impurities such as magnesium oxide and silicon dioxide must be strictly controlled. Excessive impurities will reduce the expansion efficiency and cause unbalanced expansion stability, further leading to local cracking of concrete. In addition, raw calcium oxide has an excessively fast hydration rate, and direct application will result in excessive early expansion and insufficient late expansion. Therefore, modification treatment is essential in production. Processes including surface coating, powder refinement and proportion compounding are adopted to regulate its hydration rate, so that the expansion effect can precisely match the hardening rhythm of concrete.

At present, mainstream calcium oxide-based expansive agents on the market are divided into two categories: pure calcium oxide type and composite modified type. Pure calcium oxide expansive agents feature simple production technology and high expansion rate, and are mostly used in scenarios requiring large expansion capacity such as equipment grouting and foundation reinforcement. Composite expansive agents take calcium oxide as the core base material and are compounded with calcium sulfoaluminate, gypsum, active mineral powder and other raw materials, integrating the advantages of two types of expansion materials. Specifically, calcium oxide provides stable late expansion, while calcium sulfoaluminate realizes rapid early expansion. The dual expansion system can comprehensively compensate for plastic shrinkage, hardening shrinkage and temperature shrinkage of concrete, suitable for most civil engineering projects including building construction, tunnels and water conservancy projects, making it the most widely used type at present.

Compared with other types of expansive agents, calcium oxide-based expansive agents have prominent application advantages. Firstly, they possess high expansion efficiency and excellent stability. Their hydration products fill the capillary pores of concrete, which not only compensates for shrinkage but also improves concrete compactness, as well as enhances impermeability and frost resistance. Secondly, with abundant raw material reserves and low production costs, they deliver high cost performance in industrial production and are suitable for large-scale engineering applications. Thirdly, they have strong compatibility with various cement systems such as ordinary Portland cement and slag cement, and will not cause alkali-aggregate reactions, ensuring the long-term stability of concrete structures.