Effects Of Limestone Powder On CO₂ Curing And Subsequent Water Curing Of Concrete

Recently,it is becoming increasingly recognized that carbon dioxide can be beneficially used to replace steam curing for cement and concrete productions to achieve deep cuts in energy consumption and CO2 emissions from cement industry and power plant.In comparison with traditional steam curing,CO2 curing of concrete consumes CO2,reduces energy consumption,improves early strength and produces more stable products.However,low water-to-binder ratio coupled with high compaction pressure and supercritical carbon dioxide curing made this technology very difficult to achieve in industry.Prof.Shi recently proposed the dry pre-conditioning technology and achieved a maximum CO2 curing degree of 30%,but there were a large portion of cement remaining unreacted.Using calcium carbonate powders to replace a portion of cement,as long as comparable properties can be achieved,can significantly reduce the amount of cement used in concrete,increase the carbon dioxide storage rate,and open great prospects regarding the curing of cement and concrete productions.However,very few studies have examined the influence of calcium carbonate powder on the reaction products,microstructure and mechanism of CO2 curing of concrete.This research attempted to improve the knowledge about the microstructure and mechanism of CO2 curing of concrete containing calcium carbonate powder and subsequent water curing.Therefore,the CO2 curing degree,compressive strength,reaction products and microstructure of the CO2-cured concrete containing calcium carbonate were studies in this research.The parameters studied include CO2 curing process parameters,calcium carbonate powder content,remained water-to-binder ratio and subsequent water curing.The results and findings are as follows:(1)The remained water-to-binder ratio was proposed to quantitatively characterize the moisture content of the samples during pre-conditioning curing.It was found that there is a quadratic function relationship between the two factors,and the remaining water binder ratio was reduced quickly at the beginning,and then slowed down.(2)A microwave dry pre-conditioning technology was proposed.Compared with the concrete samples preconditioned in the dry environment,the pre-conditioning time of the microwave pre-conditioned concrete samples sharply decreased from 2-4 hours to about 5 minutes.After 3 hours of CO2 curing,the concrete samples with 5 minutes of microwave pre-conditioning demonstrated similar CO2 curing degree and compressive strength with the concrete samples with 4 hours of pre-conditioning in the dry environment.(3)The addition of calcium carbonate powder in cement paste resulted in a significant decrease in C3S and C2S,but experienced an increase in the precipitation of calcium carbonate and polymerized C-S-H and/or amorphous silica gel.It was suggested that the polymerized calcium silicates were mainly formed in the eary age,and ultimately reacted with carbon dioxide to produce calcium carbonate and amorphous silica gel.Quantitative XRD and TG-DSC results confirmed the formation of both a large amount of amorphous calcium carbonate and calcite.(4)It was found that the remained water-binder ratio was critical for the CO2 curing of both mortar and concrete speciemns incorporating with calcium carbonate powder.When the remained water-to-binder ratio was 0.18,the crystallization degree of the precipitated calcium carbonate was slightly higher than that of other specimens.(5)During the subsequent water curing process,the compressive strength of the CO2-cured concrete incorporated with calcium carbonate powder increased with the curing age.The higher the compressive strength of concrete after CO2 curing,the higher the ultimate compressive strength.During the subsequent water curing process,crystallization degree of the produced carbonates increased.In addition,the calcium carbonate powder,as crystal nucleus,promoted the formation of ettringite.