Expansive cement can expand to a specific volume during the hydration process, while silicate cement often shrinks during the hardening process, causing cracks and water permeability in cement components. Thus, expanding cement overcomes the shortcomings mentioned above of silicate cement and improves the compactness and integrity of cement concrete components.
When expansive cement is used in reinforced concrete, the expansion of concrete will cause a specific tensile stress on the steel bars, and the concrete will be subjected to corresponding compressive stress. This compressive stress can prevent the concrete from producing internal microcracks. When its value is significant, it can also offset part of the tensile stress caused by external factors, thereby effectively improving the defect of low tensile strength of concrete. This pre-existing compressive stress comes from the hydration of the cement itself, which is called self-stress, and the “self-stress value” (MPa) is used to represent the magnitude of the compressive stress generated in the concrete.
Expansive cement with different expansion values can be obtained by adjusting the proportion of various components. Expansive cement can be divided into two categories according to self-stress size. When the self-stress value is ≥2MPa, it is called self-stress cement; when the self-stress value is <2MPa (usually around 0.5MPa), it is called expansive cement.
Expansive cement, a versatile material, can be divided into four categories according to its main components: silicate, aluminate, sulfoaluminate, and calcium aluminoferrate. Its expansion mechanism is the expansion of calcium sulfonate formed in cement paste. Silicate-type expansive cement sets and hardens slowly, while aluminate-type expansive cement sets and hardens quickly.
Silicate-type expansive cement is made of silicate cement as the main component, with aluminate cement and gypsum added. Its expansion value is adjusted by changing the content of aluminate cement and gypsum. If aluminate cement in silicate expansive cement is replaced by alumite, it is called alumite expansive cement. The main component of alumite is [K₂SO₄·Al₂(SO₄)₃·4Al(OH)₃], which can generate calcium sulfonate and is currently the best expansive cement. If aluminate expansive agent and aluminate expansive agent are added to silicate cement, the effect of expansive cement can also be obtained.
Aluminate expansive cement is made by mixing aluminate cement clinker and dihydrate gypsum or grinding them separately and then mixing them. It has the advantages of high self-stress value, good impermeability, and air tightness. Sulphoaluminate expansive cement is made of anhydrous sulphoaluminate and dicalcium silicate as the main components, plus gypsum.
Calcium ferroaluminate expansive cement consists of an iron phase, anhydrous calcium sulphoaluminate, dicalcium silicate, and gypsum as the main components.
The following types of expansive cement, each with its unique properties and practical applications, are currently commonly used.
1. Alumite expansive cement
Any hydraulic cementitious material with expansive properties made by grinding natural aluminate, gypsum, and granulated blast furnace slag (or fly ash) in appropriate proportions with silicate cement clinker as the main material is called aluminate expansive cement.
Alumite expansive cement is divided into three grades: 425, 525, and 625. Its technical performance requirements are as follows.
(1) Sulfur trioxide
The sulfur trioxide content in cement shall not exceed 8.0%.
(2) Specific surface area
The specific surface area of cement shall not be less than 420m²/kg.
(3) Setting time
The initial setting shall not be earlier than 45 minutes, and the final setting shall not be later than 6 hours.
(4) Strength
Each grade’s cement strength at each age shall equal the value in Table 2-19.
(5) Expansion rate
The free expansion rate of cement paste specimens cured in water at various ages shall meet the following requirements: 1d shall not be less than 0.15%; 28d shall not be less than 0.35% but not greater than 1.20%.
(6) Waterproofness
1:3 After the soft mortar specimen is cured in water for 3 days, it shall be impermeable under constant pressure of 1.0MPa for 8h (Note: Optional indicator, applicable to anti-seepage projects. If the cement is not used in anti-seepage projects, the water permeability test may not be performed).
2. Low-heat micro-expansion cement
Any hydraulic cementing material with low hydration heat and micro-expansion performance made by adding the appropriate amount of silicate cement clinker and gypsum to granulated blast furnace slag as the main component is called low-heat micro-expansion cement, code-named LHEC.
3. Self-stress ferroaluminate cement
Self-stress ferroaluminate cement is a strong, expansive hydraulic cementing material made by adding an appropriate amount of gypsum to the clinker obtained by calcining raw materials with an appropriate composition, anhydrous calcium sulfoaluminate, iron phase, and dicalcium silicate as the main mineral components, and the code-named SFAC. The product is divided into three grades according to the 28d self-stress value: 30, 40, and 50.
4. Self-stressed silicate cement
Self-stressed silicate cement, or silicate expansion cement, is an expansive hydraulic cementitious material made by grinding ordinary silicate cement, alumina cement, and natural water gypsum in appropriate proportions.
Self-stressed silicate cement concrete will shrink when placed in the air, but the shrinkage rate is only 0.03% to 0.06%. After re-wetting and curing, the expansion and self-stress values lost by shrinkage can be restored. Self-stressed silicate cement has good frost resistance. After 150 freeze-thaw cycles, the expansion degree of 1:1 self-stressed cement mortar is the same as that of long-term curing in normal temperature water, but the strength can be continuously improved during the freeze-thaw process.
