Study On Carbonation Resistance And Mechanism Of Alkali Slag Concrete

Alkali-activated slag concrete（AASC）is the green building material with good mechanical properties,frost resistance and impermeability.However,the understanding of the anti-carbonization performance of alkali-activated slag concrete is not uniform at home and abroad,and the study on carbonization of alkali-activated slag concrete is less and single.In this paper,alkali-activated slag concrete NC,CNC and CSC were prepared by using three kinds of activators:“sodium hydroxide”,“calcium oxide+sodium carbonate”and“calcium hydroxide+sodium sulfate”,and three kinds of content of alkali were considered（4%,6%,8%）.In addition,OPC,the compressive strength of which is same as CNC4 and NC4,is used as a comparison.From the macroscopic performance changes,combined with the microscopic test results,studying the carbonation resistance of NC,CNC and CSC in the natural carbonization and accelerated carbonization in 3%and 20%CO₂concentrations.The main conclusions are as follows:1.The carbonization rate（carbonization depth growth rate）of AASC and OPC is faster in the first 14 days,and gradually flattens later in the accelerated carbonization（3%and 20%CO₂concentration）and natural carbonization environment.The difference is that the carbonization rate of AASC is significantly greater than that of OPC in accelerated carbonization environment.While the difference of the carbonization rate of AASC and OPC is small in the natural carbonization environment.The carbonation resistance of AASC is seriously underestimated in an accelerated carbonization environment.2.The carbonization rate of AASC in the accelerated carbonization environment with 3%and 20%CO₂concentration is much faster than that in the natural carbonization environment.And the carbonization rate under accelerated carbonization with 20%CO₂concentration is faster than that under the accelerate carbonization with3%CO₂concentration.The depth of carbonization of AASC in three carbonization environments is proportional to the square root of the age.When the content of alkali in AASC is the same,the compressive strength before carbonization is negatively correlated with the carbonization rate of concrete in the three carbonization environments.Carbonization rate decreases with the increase of compressive strength.3.When the content of alkali is the same,after carbonization,the degree of declining of apparent p H is CSC4>CNC4≈NC4,and the carbonization rate is CSC4>CNC4>NC4,and the degree of declining of cement stone microhardness is CSC4>CNC4>NC4,so the ability to resist carbonization is NC4>CNC4>CSC4.When the content of alkali increases,the apparent p H increases.With the content of alkali increasing,the degree of declining of apparent p H decreases,and the carbonization rate slow down,the degree of declining of cement stone microhardness reduces,else the ability to resist carbonization improves.4.The low calcium to silicon ratio C-S-H（I）gel is the main carbonizable substance in the hydration products of AASC.When carbonization occurs,C-S-H after decalcification produces new pores,resulting in an increase in the number of large and middle pores（≥50nm）.However,Ca CO₃formed by carbonization can fill small pores,reduce the number of small pores（≤50nm）,and finally reduce the total pore volume of NC,CNC and CSC after carbonization.With the content of alkali increasing,the degree of declining of the total pore volume decreases.In the natural carbonization environment,NC,CNC and CSC only produce calcite,but they would produce aragonite and vaterite in an accelerated carbonization environment,and the content of aragonite and vaterite increases with increasing CO₂concentration.In the process of carbonization,the linear chain of C-S-H is decalcified to form a more branched silica gel,and the degree of polymerization of the silicon chain is increased.