Mechanical Strength And Microstructural Study Of Cement Pastes Containing Accelerated Carbonated Hardened Cement Paste Powder

The generation of waste concrete is increasing every year at an alarming rate.The waste cement powder accounts for about 30% among the waste concrete.However,while crushing the waste concrete a percentage of 20%-30% waste cement powder is produced.The waste cement powder is a fine particle with high porosity,high water absorption characteristics,and disposal of waste cement powder is becoming a problem to the waste concrete recycling industry.Since the main chemical components of waste cement powder are calcium silicate hydrate and calcium hydroxide which are favorable for accelerated carbonation process,an experimental study is designed to improve the performence of hardened cement powder by accelerated carbonation technology.Based on the role of calcium carbonate and silica gel in the cement hydration process;this experimental study investigated the effect of carbonated hardened cement powder on mechanical property and microstructure of cement pastes.Based on the experimental results obtained in this study,the following conclusions may be deduced:(1)The main components of carbonated hardened cement paste powder are calcium carbonate and silica gel,and its physical prop erties are similar to those of cement.During carbonation,the calcium carbonate is filled between the pores,which will reduce the porosity,water requirement and improve microstructural compactness of cement paste powder.The carbonation degree of hardened cement paste powder increases with the increase of water-cement ratio.After 28 days carbonation,the calcium carbonate content of hardened cement paste powder is about 50%.(2)The flowability decreases with the increase of uncarbonated or carbonated hardened cement paste powder content.The compressive strength of cement pastes with uncarbonated powder is lower than that of Portland cement paste,and its compressive strength decreases with the increase of powder content.However,the compressive strength of cement pastes with carbonated powder is higher than that of cement pastes with uncarbonated powder.Compared to Portland cement paste,the compressive strength of the paste is improved when carbonated powder content is less than 20%.The maximum compressive strength of each mix cement pastes is reached with 15% carbonated powder content.(3)The carbonated powder with 0-75 ?m particle size is beneficial for the development of the compressive strength.The compressive strength development is higher than another mix of cement pastes when the cement pastes mixed with hardened cement paste powder containing silica fume.It can increase the fluidity of the cement pastes and contribute to the strength development when the carbonated powder and fly ash under the compounding ratio of 30% and 20%.(4)The degree of carbonation of hardened cement paste powder decreases with the increase of admixture content in primary paste.Under the condition of high admixture content,the degree of carbonation of carbonated powders containing slag group is higher than that of carbonated powders containing fly ash group.The increase of fly ash content has little effect on the early compressive strength of the same mix ratio net slurry,while the 28 compressive strength decrease s;the increase of slag content has little effect on the compressive strength of the same mix ratio pastes;the compressive strength of the pastes containing carbonated powder of slag group is slightly higher than that of the same mix ratio pastes containing carbonated powder of fly ash group.(5)The content of calcium carbonate(especially the content of high crystallinity calcium carbonate)increases and the content of calcium hydroxide decreases with the increase of carbonated powder content.When carbonated powder reacts with the aluminum phase in cement,calcium aluminate mono-carbonate will generate,which inhibits the conversion of ettringite to monosulfide calcium aluminate and increases the volume of hydration product.The calcium carbonate in carb onated powder provides a large number of nucleation sites for the hydration product,which will promote the hydration of the cement and increase the compactness and compressive strength.The silica gel and aluminium gel take part in the hydration reaction when the carbonated powder is mixed with cement.NMR also shows that the carbonated powder with the same content of mineral powder is more conducive to promoting the hydration of cement.