Study On Preparation And Performance Of A Fluorine-Free And Alkali-Free Liquid Accelerator For Shotcrete

Accelerator is a chemical admixture that can accelerate the setting and hardening of cement-based materials,and is an indispensable component in shotcrete.Alkali-free liquid accelerator has become the main research direction of accelerator due to its advantages of high compressive strength,low rebound rate and low shrinkage rate.At present,the mainstream use of alkali-free liquid accelerator in the Chinese market is composed of aluminum sulfate and fluoride and other raw materials.However,fluoride not only is not conducive to the development of early strength,but also seriously endangers human health.Therefore,it is of great significance to prepare a fluorine-free and alkali-free liquid accelerator with excellent comprehensive properties.Through single-component test and orthogonal test,with setting time and compressive strength as indicators,the optimal ratio of the components of the accelerator mother liquor is determined as follows:aluminum sulfate:magnesium sulfate:amorphous alumina:triethanolamine:Water was 50:4:8:3:35.Then add 2%maleic acid to the mother liquor.The final preparation of Fluorine-free and alkali-free liquid accelerator（named FF-AF-A in this paper）has a solid content of 48%,an alkali content of 0.028%,and does not contain fluoride ions.When the dosage was 8%,the initial setting time of the reference cement paste was 2.6min,the final setting time was 6.5 min,the 1 d compressive strength of the mortar was 13.5MPa,and the 28 d compressive strength ratio was 119.2%.It is far beyond the requirements of the national standard that the initial setting time and final setting time are not less than 5 min and 12 min respectively,the 1 d compressive strength of mortar is not less than 7 MPa,and the compressive strength ratio of 28 d is not less than 90%.The application performance of FF-AF-A was tested.The adaptability between FF-AF-A and different cement,water-reducing agents and admixtures were studied.Effect of FF-AF-A on the volume stability of mortar was studied.Effect of environmental temperature and water-cement ratio on setting performance of FF-AF-A were investigated.The results show that FF-AF-A has good adaptability to different cement,and it also has a good setting effect when the dosage was 7%.Meanwhile,the 1 d compressive strength ratio of Conch Cement and Jidong Cement were 151.3%and 152.5%,respectively,and the 28 d compressive strength ratio were more than 100%.Both the naphthalene type water reducer and the large amount of polycarboxylic acid water reducer can weaken the setting effect of FF-AF-A.The shrinkage rate of mortar increases with the increase of FF-AF-A dosage.FF-AF-A has a good setting performance within 20℃～30℃.When the temperature is reduced to 10℃,the setting time will not meet the requirements of the national standard.FF-AF-A has a good setting performance when the water-cement ratio is 0.35.When the water-cement ratio is increased,the setting time will be prolonged,at this time,the target setting time can be achieved by increasing the dosage of FF-AF-A.The chemical bound water content and hydration heat release rate of cement paste were analyzed,as well as the XRD,SEM and pore structure analysis was analyzed.The results show that the SO₄^2-and Al³⁺provided by FF-AF-A will react with the Ca²⁺in the cement paste to form a large amount of ettringite,which accelerates the heat release rate of hydration,increases the number of hydration products;At the same time,the decrease of the concentration of Ca²⁺in the cement paste promotes the hydration of C₃S,and generates a large amount of C-S-H gel to fill in the pore of cement paste,so that the microstructure of the cement paste was more compact,the porosity of the cement paste was reduced,and the compressive strength of the mortar was improved.In summary,the liquid accelerator prepared in this paper is fluorine-free and alkali-free,with good setting effect,high compressive strength of mortar and good adaptability.