Study On Sulphoaluminate Cement-Based Repair Mortar

Because the phenomenon of concrete structure with aging and disease is very common both in China and abroad, it is significant to research superior cement-based mortar. Repair mortar was produced by adopting rapid hardening sulphoaluminate cement as the main matrix material. Through aiming at the characters of the rapid setting time, the poor flexibility and easy to dry-shrinkage of the sulphoaluminate cement, a new type of retarder and redispersible polymer powders, super fine slag, fibers modified mortar were investigated to improve the setting time, the flexibility, the bonding property, the water-retention property, the dry-shrinkage property, the water loss property, the wearing property and the durability of the mortar. And the analysis of morphological structure of the cement-based modified material has been made by scanning electron microscope. Finally, with the compressive strength at 3d of the mortar as the evaluating criteria, orthogonal design and linear regression analysis was investigated. The test results show that:Aluminum sulphate can accelerate the set of the sulphoaluminate cement, and slightly decrease the compressive strength of the cement. When adding 0.3% mass fraction of boracic acid, the initial setting time of the sulphoaluminate cement is 296min and the final setting time is 361min. When adding 0.3% mass fraction of boracic acid and 0.5% mass fraction of aluminum sulphate, the initial setting time and the final setting time of sulphoaluminate cement reduce to 296min and 361min. Continueing to increase the mass of aluminum sulphate will furtherly reduce the setting time. The new type of retarder steadily delay the setting time of the sulphoaluminate cement without negative effects on the early strength and increase the later strength.The proper amount of the redispersible polymer powders can significantly improve the water-retention property, the compressive strength and the bonding strength, especially the flexural strength. With 2.0% mass fraction of polymer powders, the compressive and flexural strength of the mortar reach 49.6MPa and 8.21MPa at 28d ages; the bonding strength can reach 5.6MPa at 14d ages. With the mass fraction of the redispersible polymer powders increasing, the ratio of rupture to pressure and the flexibility of the mortar are improved. And the dry-shrinkage ratio increases constantly,but can keep in an ideal range. The wearing of the mortar is improved. When adding 2.0% mass fraction of redispersible polymer powders, a unit mass of wearing decreased by 19.6% than the based sample. The redispersible polymer powders can significantly improve the wearing of the mortar.The proper amount of super fine slag can significantly improve the anti-carbonation capability of the sulphoaluminate cement mortar. The opposite effect is produced when adding too much. When adding 20% mass fraction of super fine slag, the carbonation depth reaches to the minimum values. The carbonation depth reaches to 8.9mm at 28d. But adding 30% mass fraction of super fine slag, the carbonation depth increases. The proper amount of super fine slag can also significantly improve the wearing property of the sulphoaluminate cement mortar. The opposite effect is also produced when adding too much. When adding 20% mass fraction of super fine slag, a unit mass of wearing decreased by 47.0% than based sample. But adding 30% mass fraction of super fine slag, a unit mass of wearing increased by 4.7% than the based sample.Wood fiber and polypropylene fiber can improve the water-retention property of the mortar, and can also improve the flexural strength of different age and reduce the dry-shrinkage ratio and the rate of water loss. The polypropylene fiber can also improve the late compressive strength of the mortar. With 0.3% mass fraction of wood fiber, the dry-shrinkage ratio and the rate of water loss of the motar reach minimum values. Dry-shrinkage ratio reduces to 0.029% at 28d. The rate of water loss reduces to 0.4% at 7d. With 0.5% mass fraction of polypropylene fiber, the dry-shrinkage ratio and the rate of water loss of the motar also reach minimum values. Dry-shrinkage ratio reduces to 0.03% at 28d. The rate of water loss reduces to 0.39% at 7d.The proper amount of sludge can decrease the compressive strength and the flexural strength of the mortar, but the range is not too large. The opposite effect is produced when adding too much. The proper amount of sludge can also reduce the dry-shrinkage ratio and the rate of water loss of the mortar.With 1.0% mass fraction of sludge, the dry-shrinkage ratio and the rate of water loss of the motar reach minimum values. Dry-shrinkage ratio reduces to 0.029% at 28d. The rate of water loss reduces to 0.34% at 7d.The compressive strength at 3d of the sulphoaluminate cement mortar was used as evaluating criteria. The best experimental program is A3B2C1D1 by orthogonal design. That is to say that adding 2.0wt% of redispersible polymer, 20wt% of powders super fine slag, 0wt% of polypropylene fiber are best, and the best W/C is 0.47. This is consistent with the preious studies. By regression analysis, the equation of linear regression between the compressive strength at 3d and all factors is y = 78.24239766 + 5.833333333x₁– 0.528333333x₂– 2.429824561x₃– 32.7777778x₄±9.510525102 (MPa).