The main reasons for low-cement castables to have no fluidity, become sticky and unable to vibrate during construction are as follows:
I. Mix ratio
1•Irrational particle grading: If the coarse and fine particles are not properly matched, such as too tight grading, too small gaps between particles, or cliff-type particle grading, some particle combinations are separated in the middle, the friction in the castable will increase, resulting in poor fluidity.
2•Ultraparticle problem: Ultrafine powder has excellent filling and lubricity. If the amount of ultrafine powder is insufficient, it cannot effectively fill the gaps between particles and provide lubrication, which will affect fluidity; on the contrary, too much ultrafine powder may increase water demand or agglomerate between particles, which will also make the castable sticky and reduce fluidity.
3•Cement dosage problem: Too much cement will increase the water demand of the castable, and may accelerate the setting speed, resulting in reduced fluidity; too little cement will make the medium-temperature strength low, affecting the overall performance and construction effect.
4• Improper water addition: Insufficient water addition will not allow the castable to be fully moistened and form a good slurry, resulting in large friction between particles and poor fluidity; excessive water addition will cause aggregate sinking and segregation, and may also increase the viscosity of the castable, resulting in poor fluidity.
2. Additives
1• Dispersant problem: Insufficient dispersant addition will not effectively disperse the particles, the water wrapped by the condensed particles cannot be fully released, the electrostatic repulsion between particles is small, and the fluidity is poor; but excessive dispersant may cause condensation and increase the viscosity of the material.
2• Retarder or accelerator: Improper use of retarder or accelerator. In summer or winter, when the temperature changes greatly, if the amount of retarder or accelerator added is not reasonably adjusted according to the actual temperature, it may lead to abnormal coagulation speed and affect fluidity.
3. Raw materials and storage
1. Aggregate shape and water absorption: When the aggregate shape is mostly flake or angular, the contact area is large, the friction is large, and it is not conducive to flow; the high water absorption of aggregates will make the amount of water used in the mixture insufficient to ensure the formation of sol and the free flow between particles.
2. Moisture or deterioration: During the storage of castables, if the ambient humidity is high, cement and other components may absorb moisture and partially hydrate, reducing the fluidity of the castables.
4. Construction
1. Mixing time and method: If the mixing time is too long, the temperature may rise, causing the castable to solidify prematurely and lose fluidity; if the mixing method is improper, such as uneven mixing, the components cannot be fully mixed, which will also affect the fluidity.
2. Ambient temperature: If the construction environment temperature is too high, the coagulation speed of the castable will be accelerated, resulting in a rapid decrease in fluidity; if the temperature is too low, the cement hydration reaction will be slow, and the castable may also become sticky and have poor fluidity.
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