High Temperature Properties Of Concrete Blocks Cured With CO₂

Due to the properties of light weight,energy saving,and good thermal insulation,concrete blocks are widely used as main building materials.Generally,concrete blocks have poor resistance of high temperature,however,the reactions between carbon dioxide and unhydrated cement particles or cement hydration products in the process of CO2 curing can form calcium carbonate and silica gel,which experience higher decoposition temperature than that of calcium silicate hydrate and calcium hydroxide products.This paper investigates several factors on the high-temperature resistance of concrete blocks with CO2 curing,including water-cement ratio,fly ash content,and slag content.Compared to the control sample with natural curing,the variation of exterior color,mass loss,and strength of blocks under different temperature levels is used as parameters to evaluate the high temperature performance of concrete blocks after CO2 curing.The microstructure of CO2 cured concrete blocks are analyzed by X-ray diffraction,thermogravimetric analysis and scanning electron microscopy(SEM).The results show that the color of the CO2 cured concrete blocks is similar to the natural samples at the different target temperature.It is interesting to note that CO2 curing can reduce the external damage of concrete blocks under high temperature.The cracks of blocks are less than natural cured samples at 300-500?,and the external corners of the blocks are destroyed slightly at 600-800?.The ratio of water to cement has a great influence on the high temperature performance of concrete blocks.The increase of excess moisture in the block affects the diffusion of CO2 in the block,but it is also bad effect of low water-cement ratio on CO2 reaction.In this paper,when the W/C ratio is 0.3,the compressive strength of the concrete block is highest and well performed at defferent target temperatures.The amount of fly ash and slag also has a great influence on the CO2 curing degree,the external damage of the concrete blocks reduce gradually at high temperatures compared with natural samples,the strength increases with increasing dosage.The results of SEM show the cement matrix is covered by numerous of calcium carbonate crystal after CO2 curing,the denser internal structure and small crack damage are discovered at the temperature of 600 ?.Besides,mixed with fly ash and slag,the concrete blocks show less pores inside and higher intensity of connection between calcium carbonate.Compared with natural curing,the quality of calcium hydroxide in CO2 curing block is less,the quality of calcium carbonate increases,and the decomposition temperature of calcium carbonate will increase.The addition of mineral powder and fly ash will react with calcium hydroxide in block,further reduce the content of calcium hydroxide in block,but increase the content of calcium carbonate,which indicates that the addition of mineral powder and fly ash will promote the reaction of carbon dioxide.In addition,the X-ray diffraction found that the intensity and quantity of the peaks(including tricalcium silicate,calcium hydroxide and ettringite)are decreasing,while the peak of ettringite are increasing.After the concrete blocks containing fly ash and slag are treated with CO2,the number of characteristic peaks of calcium carbonate is more than that of non-admixed blocks,indicating that the addition of fly ash and slag can promote the degree of carbonization and crystallization of blocks and improve the calcium carbonate crystals.It is also one of the reasons that the strength of the block increases with the increase of the amount of the additive.