The Hydration Mechanism And Tne Improvement Of Early Properties Of Ternary Cement Blends With Limestone Powder And Metakaolin

More and more industrial by-products have been applied in cement industry to reduce its carbon footprint and environmental impact.Limestone powder（LM）is a by-product of the quarry crushing,stone processing and aggregate production process,which has been used as an inert filler in cement products for a long time.Although recent studies have found that a small amount of limestone powder is reactive,but its application is still limited.Meanwhile,as the requirement of precast components is increasing,the precast products are expected to be able to be demolded as soon as possible,which means the higher early strength of concrete is required.In order to solve the problem of low-activity of limestone powder in cementitious materials,this paper will apply metakaolin（MK）with high-activity will be used as another mineral admixture with LM,to improve the properties of composite cementitious materials.The early properties improvement method of composite cementitious materials is studied as well.The main research contents and results of this paper are as follows:Firstly,the workability,mechanical properties,shrinkage performance and pore structure of the binary and ternary systems of limestone powder-metakaolin-cement composite were studied.The results show that:The addition of LM can improve the workability of blended cement,but it will prolong the setting time and reduce its strength from the 1d-28d.With LM and MK,the water requirement of ternary blend increases and setting time is shortened,and its mechanical properties are obviously improved.The compressive strength of mortar with 5%LM and 10%MK at 1d is almost epaul to that of the control group,and the strength after 3d is higher than that of the control group.In addition,the early autogenous shrinkage reduces and the drying shrinkage increases in binary blend with LM,while the trend is opposite in ternary blend with LM and MK.In terms of pore structure,LM playes a filling role in the matrix in the early stage,which reduces the porosity.After28d of hydration,the dilution effect is dominant,thus the pore structure is slightly coarsened.Ternary blend shows more refined pore structure at both 1d and 28d.Secondly,isothermal calorimetry and low-field nuclear magnetic resonance technology were applied to study the early hydration reaction process of binary and ternary blend,and XRD and TG-DTA were used to explore the changes of its hydration products.The study found that the early hydration of blend can be divided into five stages.The addition of LM or LM&MK both can shorten the hydration induction period of blend cement paste.However,the hydration of ternary blend paste shows obvious aluminate reaction peak.It noted that a small amount of water is released during the aluminate reaction in ternary blend paste.The results of XRD and TG-DTA also show that carboaluminates（Hc and Mc）can be found after 1 day of hydration in ternary blend,indicating that the addition of metakaolin in this system provides a large amount of Al-source and can stimulate limestone powder dissolution and participates in the reaction.As the reaction time passing,there is mutual conversion between Hc and Mc,and AFt can exist stably for a long time.In addition,due to the pozzolanic effect of MK,the amount of chemically bound water in blend cement increases,and the degree of reaction is higher in the ternary blend.Finally,the method of improving the early properties of ternary blend with LM and MK was explored.It found that pre-mixing LM and MK and adding the slurry to the cement material can significantly improve the workability of the cementitious material and reduce the requirement of polycarboxylate superplasticizer.Meanwhile,compared with mixing in powder term,the strength of cement mortar made by the pre-mixed slurry has increased,and the addition of 2%Ca（OH）₂ into the slurry can further increase the 1d compressive strength,which is higher than that of pure cement.Besides,the influence of the ratio of the premixed slurry on the strength of cement mortar was also studied,and it was found that when the slurry ratio is 10-15%,the strength is the highest;and the resting time after mixing the slurry of LM and MK has little effect on the strength.This method can effectively improve the strength of ternary blend without adding other additives,which has good economic benefits.