Study On Preparation And Properties Of Seed Crystal Based Accelerator

Shield construction is the pioneer of prefabricated construction,of which shield segments are prefabricated in the factory.Due to the low temperature in winter,the strength development of precast concrete segments is slow,which not only reduces the efficiencies of production and construction progress,,but also increases the number of defects during the demould process.These problems increase the durability risk of concrete.Therefore,it is important to develop a kind of crystal based accelerator that can meet the requirements(free steaming,rapid stripping and good durability)of precast concrete segments which applied in subway..In this paper,the seed crystal based accelerator was prepared by means of pre-hydration method and chemical synthesis method,respectively.The effects of preparation conditions on the promotion impact for the early strength of motor cured at a low temperature and early hydration degree of the cement paste were studied.By analyzing the hydration product composition,microstructure and composition of the seed crystal based accelerator,the factors affecting the effect of the seed type early strength agent and its influence mechanism were discussed,and the suitable conditions for preparing the early strength agent were obtained.The preparation of crystal based accelerator with different pre-hydration comditions showed that,with the increase of prehydration temperature from 20? to 60?,the more prehydration products,the more "crystal seeds",the more effective nucleation sites,,and the greater the accelerating effect on cement hydration.As the pre-hydration time prolonged,the amount of pre-hydrated product exceeded the demand of nucleation site in the process of maximum dissolution in cement paste,and the hydration reaction no longer be accelerated.When the water-to-cement ratio of pre-hydration was larger than 0.5,the growth space and seed crystal size of the pre-hydrated product were larger,which reduced the nucleation site and decreased the acceleration effect on the hydration reaction When the water-to-cement ratio is 0.5,pre-hydration temperature was 60?,and curing time was 7 days,the crystal based accelerator worked best,compared with the blank sample,the hydration degree of cement increased by 62.5%,and the strength of mortar increased by 140% after curing for 8 h at temperature of 15 °C.The preparation of crystal based accelerator with different grinding conditions showed that the accelerator had the finest particle(D50=308nm)and the largest surface energy when it ground for 4.5h,which were benefit to the hydration reaction.When the grinding time exceeded 4.5h,the seed particles began to agglomerate and the early strength effect decreased.When the accelerator was prepared by using superplasticizer with different solid content ranged from 6% to 12%,the results showed that the accelerator prepared by superplasticizer with a solid content of the 9% has the smallest D50(298nm)and best acceleration effect.When the superplasticizers exceeded a certain amount,the slurry concentration increased,the grinding force became weak,and the excess superplasticizers reduced the electrostatic repulsion between the particles to cause agglomeration,which makes the particle size in the accelerator increased and the acceleration effect decreased.When the solid content of superplasticizrs was 9% and grinding time was 4.5 hours,the crystal based accelerator worked best,compared with the blank sample,the hydration degree of cement increased by 92.6%,and the strength of mortar increased by 240% after curing for 8 h at temperature of 15 °C.The preparation of C-S-H based accelerator with different chemical synthesis conditions showed that the synthesis efficiency and polymerization degree of C-S-H increased as the reaction time increased from 8h to 24 h,the temperature increased from 20°C to 60°C and the reactant C/S ratio increased from 1.0 to 1.5,which were beneficial to accelerate the hydration reaction.However,the increment of particle size of C-S-H crystal seeds leaded to a more compact stureture of C-S-H,which was unfavorable for accelerating the hydration reaction.When the reactant C/S ratio increased from 1.5 to 2.0,the particle size of the C-S-H seed crystal sharply increased,which was also disadvantageous for accelerating the hydration reaction.When the reactant C/S ratio was 1.5,synthesis time was 24 h,and synthesis temperature was 40 °C,the crystal based accelerator worked best.With the addition of this crystal based accelerator,the compressive strength of the mortar reached 2.5 MPa after curing for 8 h at temperature of 15 °C,which satisfies the strength requirements of demolding condition of precast concrete segments in winter.The compressive strength of concrete with addition of C-S-H based accelerator was studied.The results showed that the compressive strength of concrete was 10.3 MPa after curing for 8 h at temperature of 15 °C,which meets the requirements of rapid demolding.Because the early hydration of cement was accelerated,the 3 days' strength of concrete is obviously improved.The strength of concrete slightly decreased after curing for 28 d and 90 d,because the hydration process of cement was influenced.However,the 180 days' strength did not change much,indicating that the accelerator has no negative impact on the final long-term mechanical properties of concrete.Furthermore,the working performance of the fresh concrete meets the molding requirements and does not affect the impermeability of the concrete.