Preface: In accordance with the core product standards and test method standards for refractory mortar, there are defined technical ranges for water addition of refractory mortars with different materials. Issues including rapid water evaporation under high temperature and slow setting at low temperature can be resolved via process optimization together with dedicated admixtures. Detailed solutions are listed as follows:
I,Standard Water Addition Ratio of Refractory Mortars of Different Materials (Mass Ratio)
Core Criteria for Determining Water Addition: After mixing, the refractory mortar shall reach qualified plasticity. Its physical state should be described as forming a ball when squeezed by hand and breaking apart once dropped. During masonry construction, a uniform thin layer of 2–3 mm can be extruded from mortar joints. No water bleeding or dry lumps shall occur, and all performance requirements specified in the corresponding product standards shall be satisfied.
| Type of Refractory Mortar | Standard Water Addition Range | Core Performance Constraints | Applicable Current Valid Standards |
| Fireclay Refractory Mortar | 18% - 22% | Fullness of mortar joints ≥ 90%; cold modulus of rupture bond strength ≥ 2.0 MPa | GB/T 14982-2008 Fireclay refractory mortars |
| High-Alumina Refractory Mortar | 15% - 20% | Apparent Porosity ≤ 35%; cold bonding flexural strength after 28 days of curing ≥ 2.5 MPa | GB/T 2994-2021 High alumina refractory mortars |
| Siliceous Refractory Mortar | 12% - 16% | Standing time after mixing ≤ 10 min to avoid cracking induced by polymorphic transformation of free silica | YB/T 384-2023 Silica refractory mortars |
| Magnesia/Magnesia-Alumina Mortar | 10% - 15% | Alkaline system. Excessive water shall be avoided to prevent hydration expansion of magnesia clinker; acidic admixtures are prohibited | YB/T 5009-2011 Magnesia, magnesia-alumina and magnesia-chrome refractory mortars |
II,Countermeasures for Rapid Evaporation in Hot Weather (Water Retention Without Performance Loss)
Core Principle: The adjusted water addition shall not exceed the upper standard limit, so as to guarantee the later strength and erosion resistance of refractory mortar and comply with the requirements of test method standards.
1,Optimization of Water Adding Process
Adopt staged water addition and low-speed mixing: firstly add 80% of the standard water quantity and mix for 3 minutes. After standing for 2 minutes, supplement the remaining water according to the dryness and wetness to prevent rapid drying of the surface layer.
Water addition can be increased by 2%–3% (not exceeding the upper limit of the range):For example, the maximum water addition for fireclay refractory mortar is 24%. Meanwhile, 0.2%–0.5% Hydroxypropyl Methyl Cellulose (HPMC), a water-retaining agent, shall be added to bind free water and extend the working open time to more than 30 minutes.
2,Construction Environment Control
Avoid high-temperature period from 10:00 to 16:00 at noon and arrange construction in cool morning and evening hours. Spray and moisten the kiln substrate before construction (no standing water on the surface) to reduce water absorption of the base material and cut down water loss of refractory mortar.
Cover mixed refractory mortar with wet cloth for moisture retention. Limit each batch mixing volume to an amount consumable within 20 minutes to prevent crusting caused by water loss from exposure.
3,Addition of Compliant Retarders
Mix in 0.1%–0.3% sodium citrate or sodium gluconate (special admixtures for refractory materials) to retard hydration reactions and prevent rapid initial setting under high temperatures.
Admixtures shall be dissolved in water prior to incorporation. Dry direct addition is strictly prohibited. After addition, the performance shall meet the requirements of GB/T 22459.4-2008 Refractory mortars - Part 4: Test method for cold modulus of rupture bond strength.
III,Countermeasures for Slow Setting Rate in Cold Weather (Anti-freezing Without Performance Deterioration)
Core Principle: Accelerate setting via temperature rise and formula adjustment. No extra water shall be added to avoid impairing the density of refractory mortar.
1,Temperature Control of Water and Raw Materials
Mix with warm water at 20–30°C; cold water or ice water is prohibited. Before winter construction, preheat refractory mortar in an indoor environment of ≥15°C for no less than 4 hours to raise material temperature and accelerate hydration reactions.
Water addition can be reduced by 2%–3% (not lower than the lower limit of the range). For example, the minimum water addition for high alumina refractory mortar can be reduced to 13% to lower the free water content and shorten the setting time.
2,Thermal Insulation of Construction Area
Set up thermal insulation sheds, and raise the construction ambient temperature to 5–10°C by hot blast stoves or electric heaters. After masonry work is completed, cover the masonry with rock wool blankets or fireproof cloth. The temperature shall be maintained at no less than 5°C during curing, and the curing period shall be extended to 48–72 hours (only 24 hours required for curing at normal temperature).
3,Addition of Special Accelerators
Cement-bonded refractory mortars (fireclay and high alumina types) may be mixed with 0.5%–1.0% sodium aluminate accelerator to accelerate setting and hardening without impairing high-temperature properties.
Calcium chloride and other acidic accelerators are strictly prohibited for magnesia refractory mortar to prevent neutralization reaction with magnesia. Special composite accelerators for magnesia systems shall be adopted, and the dosage shall follow the manufacturer's instructions.
IV,General Precautions (To Guarantee Application Performance)
Water addition shall only be adjusted within the specified standard range. Excessive water addition will cause the linear change rate of refractory mortar at high temperature to exceed the limit, resulting in spalling and cracking during service. The mortar shall meet the high-temperature performance requirements specified in GB/T 22459.7-2019 Refractory mortars - Part 7: Test methods for other properties.
The dosage of admixtures must be precisely controlled. Overdosage will lead to performance defects (e.g., excessive retarder causes insufficient strength; excessive accelerator induces cracking).
Masonry shall not be moved or vibrated during curing. The curing period lasts 24–48 hours under high-temperature conditions and 48–72 hours under low-temperature conditions. Upon completion of curing, the bond strength can be tested in accordance with the corresponding product standards.