Any aluminate cement clinker, mainly composed of calcium aluminate, is ground into a hydraulic cementing material called aluminate cement, and its code name is CA. According to needs, an appropriate amount of αAl₂O₃ powder can be added when grinding cement with an Al₂O₃ content greater than 68%. Aluminate cement can be divided into four categories according to the content.
CA-50 50%≤Al₂O₃ <60%
CA-60 60%≤Al₂O₃<68%
CA-70 68%≤Al₂O₃<77%
CA-80 77%≤Al₂O₃
The physical properties of aluminate cement are as follows.
(1) Fineness
The specific surface area shall not be less than 300m²/kg, or the 0.045mm sieve residue shall not be more significant than 20%, as agreed upon by the supply and demand parties. In the event of a dispute without agreement, the specific surface area shall prevail, providing a fair resolution process.
(2) Setting time (mortar)
It shall meet the following requirements: The initial setting time of CA-50, CA-70, and CA-80 shall not be earlier than 30 minutes, and the final setting time shall not be later than 6 hours; the initial setting time of CA-60 shall not be earlier than 60 minutes, and the final setting time shall not be later than 18 hours.
Aluminate cement is an early-strength cement. Its 1d strength can reach more than 80% of the 3d strength, which can reach the level of 28d ordinary Portland cement. The later strength growth is not significant. It is mainly used for projects with urgent construction periods (such as road and bridge construction), emergency repair projects (such as plugging leaks), and winter construction projects.
Unlike ordinary hardened cement, Aluminate cement does not contain tricalcium aluminate or precipitate-free calcium hydroxide. This unique composition results in a dense structure after hardening, making it highly resistant to the erosion of mineral water, a key advantage in various construction projects.
Hydration and hardening of aluminate cement
The hydration of aluminate cement, particularly the hydration process of calcium aluminate, is a fascinating area of study. It’s intriguing to note that its hydration reaction varies significantly with temperature. At different temperature ranges, we observe the formation of distinct hydration products, each with its unique properties and implications.
The hydration of CA₂ in aluminate cement is the same as that of CA, but the hydration rate is slower. The hydration reaction of C₁₂A₇ is swift and also produces C₂AHg. However, the response between C₂AS and water is feeble and can be regarded as an inert mineral. A small amount of C₂S produces hydrated calcium silicate gel.
Hydrates CaHiO or C₂AHg are needle-shaped or flake crystals that form a solid crystal bond to create a crystal skeleton. At the same time, the aluminum hydroxide gel generated fills the skeleton space to form a relatively dense structure so that the initial strength of cement can be rapidly increased while the subsequent strength increase is not significant.
CaH₁O and C₂AHg gradually transform into relatively stable C₃AHs over time. This transformation process accelerates with the increase of ambient temperature. As a result of crystal transformation, free water precipitates from the cement paste, increasing the pores. At the same time, the strength of the transformation product C₃AH₆ itself is relatively low, and the bonding between crystals is poor, so the strength of the cement paste is significantly reduced. The transformation of crystals will cause a long-term decrease in strength, especially in a hot and humid environment, where the strength decreases significantly (the later strength may be reduced by more than 40% compared to the maximum strength value). However, if it is used correctly, treated cautiously, and specific measures are taken, its undesirable properties can be improved somewhat. Such as adding gypsum or anhydrous gypsum to cement, reducing the water-cement ratio, lowering the curing temperature, and other measures.
3. Application scope and precautions of aluminate cement in cement mortar
Aluminate cement is mainly used in dry-mixed mortars that require fast hardening properties or high temperature stability, such as self-leveling dry-mixed mortar. This is because under normal hardening conditions, the cement paste of aluminate cement does not contain tricalcium aluminate and calcium hydroxide, and its density is also relatively large, so it has a high resistance to the erosion of mineral water and sulfate. Notably, the acid resistance of aluminate cement makes it suitable for preparing dry-mixed mortars with high requirements for acid corrosion resistance.
The standard consistency of aluminate cement does not require much water, but it needs more bound water during hydration, reaching about 50% of the cement mass, almost twice that of silicate cement. This is an important reason why aluminate cement has a higher density and impermeability after hardening. Taking advantage of aluminate cement’s high density and impermeability, it can also be used to prepare rigid plugging and build waterproof dry mortar.
Since aluminate cement releases a large amount of heat when hydrating, it is necessary to pay attention to its dosage and application thickness when preparing thick mortar such as self-leveling mortar; according to the temperature reaction performance of aluminate cement, in addition to paying attention to the construction temperature of its products, it should also be considered during preparation; aluminate cement has poor alkali resistance, and when setting the scope of use of dry-mixed mortar based on aluminate cement, it is necessary to list the precautions for use and take appropriate protective measures; products prepared with aluminate cement are not suitable for long-term load-bearing structures and in high temperature and high humidity environments; aluminate cement reacts quickly when it comes into contact with water, and dry-mixed mortar prepared with it should pay more attention to moisture-proof than dry-mixed mortar prepared with silicate cement.
Some dry-mixed mortar products, such as self-leveling mortar, are prone to whitening and powdering on the surface because of aluminate cement’s crystal transformation. Add dihydrate gypsum, zeolite, and other materials to aluminate cement to prepare commercial mortar to improve product performance. The modified product has a faster hardening rate, no later-strength shrinkage, and a stable crystal form under hydrothermal conditions. The 28d compressive strength ratio and alkalinity can determine the adaptability of the mortar formula.
4. Sulphoaluminate cement
Sulphoaluminate cement is a type of hydraulic cementitious material obtained by calcining raw materials with appropriate composition and mixing cement clinker with anhydrous calcium sulphoaluminate and dicalcium silicate as the main mineral components with different amounts of limestone and the proper amount of gypsum. Sulphoaluminate cement includes fast-hardening, low-alkalinity sulphoaluminate cement, and self-stressed sulphoaluminate cement.
A type of hydraulic cementitious material with high early strength, which is made by grinding sulphoaluminate cement clinker with appropriate composition, a small amount of limestone (the amount added should not exceed 15% of the cement mass), and a proper amount of gypsum, is called fast-hardening sulphoaluminate cement, and its code name is R·SAC. It is divided into four strength grades of 42.5, 52.5, 62.5, and 72.5 according to the 3d compressive strength.
A type of hydraulic cementitious material with low alkalinity, which is made by grinding together appropriate sulphoaluminate cement clinker, a large amount of limestone (its addition amount should be no less than 15% of the cement mass and no more than 35% of the cement mass), and an appropriate amount of gypsum, is called low-alkalinity sulphoaluminate cement, and its code name is L·SAS. It is divided into three strength grades of 32.5, 42.5, and 52.5 according to the 7d compressive strength. Low-alkalinity sulphoaluminate cement is mainly used to make glass fiber-reinforced cement products. When used to prepare concrete products and structures with steel fibers, bars, mesh, and embedded parts, stainless steel should be used.
A type of hydraulic cementing material with expansion properties, which is made by grinding sulphoaluminate cement clinker with appropriate ingredients and adding the proper amount of gypsum, is called self-stressed sulphoaluminate cement, and its code name is S·SAC. Self-stressed sulphoaluminate cement is divided into four self-stress grades of 3.0, 3.5, 4.0, and 4.5 according to the self-stress of 28d.
Sulphoaluminate cement, with its unique properties, is a versatile material that can be used in a variety of cement mortars. It is particularly beneficial for applications that require fast hardening and early strength, such as fast-setting engineering repair dry mortar, dry mortar for winter construction, dry mortar for ground engineering, and mortar suitable for plugging engineering.
Sulphoaluminate cement is a fascinating material with unique properties. It can harden normally at 5℃, thanks to its lack of C₃A minerals and high-density cement paste. This results in good sulfate resistance, minimal shrinkage in the air, and excellent antifreeze and anti-seepage properties. The pH value of the cement paste liquid phase is 9.8-10.2, making it a low-alkali cement. This unique property allows it to be mixed with fibers with low alkali resistance, such as glass fibers.
The early strength of sulphoaluminate cement develops quickly, and the later strength develops slowly but does not shrink. The dry mortar prepared with sulphoaluminate cement has the same rule. The setting time of sulphoaluminate cement is relatively fast, and the interval between the initial and final sets is also relatively short.
