Mechanism Analysis And Engineering Application Of Early Strength Agent On Strength Reinforcement Of Fast Hard Cement

With the announcement of China’s carbon neutral target in September 2020,cement as an important source of CO₂ emissions,the development and application of low carbon cement is particularly important in order to reduce CO₂ emissions.White anti-crack fast-setting and fast-hardening high-belite sulfate cement（referred to as white anti-crack double-fast cement）is a low-carbon cement with high strength and low cost.Therefore,the development of white anti-cracking double-fast cement can promote the green development of cement industry.White anti-cracking double-fast cement has excellent characteristics such as early strength and high strength,fast setting and fast hardening,micro-expansion,low dry shrinkage,anti-freezing and anti-seepage,and anti-corrosion.However,it has a relatively fast hydration rate and short setting time,which sometimes cannot meet the construction time requirements of actual projects,so it often delays the setting time by adding retarding agents,but adding retarding agents will destroy its early strength,so it is of great significance to explore the early strength of white anti-cracking double-fast cement to complement the cement to reduce the production cost and improve the performance of the product.For the problems of early strength and setting time of white anti-cracking double-fast cement due to the addition of retarder,this paper chooses to compound retarder and early strength agent into white anti-cracking double-fast cement mortar,and investigates the effect of retarder compounded with early strength agent on the performance of white anti-cracking double-fast cement mortar by studying the setting time of white anti-cracking double-fast cement slurry,the strength and hydration product of each age of the sand specimen,and the reaction mechanism by X-ray diffraction and scanning electron microscopy.In this paper,through the cement mortar setting time,strength and hydration products of the study of early strength agent and retarder compounding mechanism of action,and the application of research results to the cement-based self-leveling,the results of the study are as follows:1)C₄H₆O₆,as a retarder for white anti-cracking double-fast cement,has a good retarding effect on the cement,and the suitable admixture of C₄H₆O₆ in this cement is0.35%to 0.55%.Compared with the blank group,the setting time of this cement was extended by 60 min compared with the blank group when the amount of C₄H₆O₆ was0.55%,which had a significant retarding effect.The addition of C₄H₆O₆ had a greater effect on the early strength of the cement,while the effect on the strength after 28days was smaller.2)It was found that both Li₂SO₄ and Li₂CO₃ could be used as early strength agents for white anti-cracking double-fast cement,and the compensation effect on the early strength of this cement was similar.When Li₂SO₄ and Li₂CO₃ are dosed in appropriate amounts,the compensating effect on the early strength of the cement both achieve good results.The flexural strength of this cement at 1d,7d and 28d increased by 79%,22%and 23%relative to the control group when the amount of Li₂SO₄ was0.55%.The compressive strength of this cement increased by 21%and 54%at 1d and7d relative to the control group,and its compressive strength at 28d was comparable to that of the control group.When the amount of Li₂CO₃ was 0.55%,the flexural strength of this cement increased by 63%,25%and 14%at 1d,7d and 28d relative to the control group,and the compressive strength increased by 43%and 21%relative to the control group,and its 28d compressive strength was comparable to that of the control group.However,when Li₂SO₄ and Li₂CO₃ were mixed with more than 0.55%,their strengths were not higher than those when they were mixed with 0.55%,so the mixing of excessive early strength agent would not cause the cementitious sand strength to increase continuously.This is due to the fact that the Li⁺in the Li₂SO₄ and Li₂CO₃ act to destroy the cement surface particles,accelerate the hydration process of cement,and generate more calcium alumina,which is beneficial to the early strength.The incorporation of C₂H₂O₄Ca did not weaken the retarding effect of C₄H₆O₆ on the cement,and it was not obvious to promote the hydration of the cement and compensate for the early strength of the cement,so it was not suitable as an early strength agent for the cement.3)The compounding of C₄H₆O₆ and early strength agent makes the cement have suitable setting time and at the same time the strength is also guaranteed.Applying the results of this paper to the white anti-cracking double-fast cement-based self-leveling,it is found that its fluidity and setting time meet the actual construction requirements.And the suitable dosing of Li₂SO₄ in this cement-based self-leveling is 1.5 g/Kg,at this time the setting time of this cement-based self-leveling is reduced by 48.8%compared with the control group,and its 1d,7d and 28d flexural strengths are increased by 36%,32%and 50%,and compressive strengths are increased by 4%,21%and 11%,and the application of this cement-based self-leveling to the actual project.The achieved results are good.