Research On The Formation Mechanism Of Thaumasite In Saline Area Of Xinjiang

Xinjiang is located in the hinterland of Eurasian continent,which is one of the most arid areas in China,with the widest distribution of soil salinization and the most types of salinization.In many erosion and destruction,thaumasite form of sulfate attack causes the main source of cement-based material strength,calcium silicate gel decomposition,and cementitious materials lose some cementation ability or even overall collapse,which is more serious than traditional sulfate attack.In recent years,with the explored study of thaumasite,it is found that carbonate mineral admixtures in cementitious system,low temperature and salt rich environment are the key to the formation of thaumasite.Xinjiang is temperate continental climate,and the annual average temperature is only about 10 ℃,which provides the necessary conditions for the formation of thaumasite.It is great significance to carry out research on thaumasite formation of sulfate attack for the durability of infrastructure projects in Xinjiang.This paper mainly studies the influence of carbonate mineral admixtures and erosion media on the formation of thaumasite in cement-based materials,analyzes the phase composition of cementitious system by thermodynamic principle,and quantitatively analyzes the erosion products by X-ray diffraction to explore the influence of the formation of thaumasite on the strength of cement-based materials.The main research contents and achievements are as follows:The chemical composition of two kinds of cement on the market was analyzed by X-ray fluorescence.The hydration model and thermodynamic model of the two kinds of cement were established based on cement clinker and a small number of oxides.The influence of cement components on hydration products and sulfate erosion products was analyzed,the low temperature sulfate erosion test was used to analyze the strength degradation law of two kinds of cement mortar specimens and the evolution of erosion phase in cement paste.Compared with limestone,dolomite has weak reactivity and weak promotion effect on the early strength of cement mortar.However,with the progress of erosion reaction,dolomite reacts with alkali carbonate to form calcite,a small amount of brucite and hydroxyl hydrotalcite,which enhances the compressive strength of cement mortar on the basis of dense pores.Compared with sodium sulfate solution,magnesium sulfate solution causes more serious corrosion damage.The cement paste specimens with 20% or more dolomite form a small amount of thaumasite in the sixth month,while the dolomite specimens soaked in sodium sulfate solution form a small amount of thaumasite after 18 months,the specimens immersed in magnesium sulfate solution at the same age had typical sulfate attack of thaumasite form.The specimens were completely softened and lost cementation ability.The results show that the cement mortar samples under double salt erosion have serious crystallization and the strength degradation speed is obviously accelerated.The results of quantitative analysis of cement paste show that a small amount of ettringite is formed in the sample mixed with dolomite in the early stage of erosion,and a relatively violent alkali carbonate reaction occurs,which leads to the decomposition of dolomite,while the volume of cement paste expands obviously in the late stage of erosion,resulting in a large amount of paste.At room temperature,the cement-based materials will be eroded by ettringite and gypsum sulfate,and the cement mortar strength will deteriorate rapidly.At low temperature,the cement-based materials will be eroded by ettringite and gypsum sulfate,and the cement mortar strength will deteriorate slowly.The increase of temperature will accelerate the erosion to a certain extent.