Optimal Design And Mechanism Study Of Blended Portland Cement Based On The Autogenous Shrinkage

The blended cement with the addition of blast furnace slag（BFS）and fly ash, whichexhibits the advantages of energy conservation and emissions reduction, good workability,uniform strength development, is widely used in the construction industry. The application ofhigh performance superplasticizer will significantly reduce the water to cement ratio ofblended cement pastes, decrease the pore sizes of blended cement pastes, and increase theautogenous shrinkage of cementious materials at early age. The autogenous shrinkage plays avital role in the volumetric stability and durability of blended cement pastes. The investigationon the characteristics of autogenous shrinkage of blended cement pastes is fundamental to thepractical interests of blended cements. This study focuses on the influence of the particle sizeof cementitious materials (ordinary Portland cement, blast furnace slag, and fly ash) on theautogneous shrinkage of blended cement pastes. The outcomes optimize the mixture design ofblended cements.The basic properties and shrinkage properties of different fractions of ordinary Portlandcement, blast furnace slag and fly ash were studied. The results indicate that the chemicalcomposition of ordinary Portland cement, blast furnace slag and fly ash is influenced by theparticle size, respectively. The chemical composition of fly ash is significantly changed withits particle size, and the carbon content of fly ash increases with the decrease of particle size.The water requirement of normal consistency of ordinary Portland cement, blast furnace slagand fly ash increases with the decline of particle size. The chemical shrinkage and theautogenous shrinkage of ordinary Portland cement and blast furnace slag increase with thedecrease of particle size, respectively. With decrease of particle size, the chemical shrinkageof fly ash increases first and then decreases.In order to investigate the chemical effect of blast furnace slag on the autogenousshrinkage of blended cement pastes, the blast furnace slag cements are prepared by twomethods (the fractional replacement blast furnace slag cements and the ordinary blast furnaceslag cements). The basic properties and shrinkage properties of two types of blast furnace slagcement were investigated. The results indicate that: the water requirement of normalconsistency of two types of blast furnace slag cement is larger than that of the ordinaryPortland cement. The initial setting time and final setting time of blast furnace slag cementpastes are longer than that of the ordinary Portland cement paste, respectively. Thecompressive and flexural strength of two blast furnace slag blended cement pastes are lowerin comparison with that of the ordinary Portland cement pastes at3days. But the addition of fine blast furnace slag increases the compressive strength and flexural strength of blastfurnace slag blended cement mortar at28d (even higher than that of ordinary Portland cementmortar). The early chemical shrinkage of the blast furnace slag cement pastes is lower thanthat of the ordinary Portland cement paste. The chemical shrinkage of the blast furnace slagcement pastes at later age is higher than that of the ordinary Portland cement paste. Theautogenous shrinkage of the fractional replacement blended cement pastes is lower than thatof the ordinary addition blended cements.In order to investigate the chemical effect of fly ash on the autogenous shrinkage ofblended cement pastes, the fly ash cements are prepared by two methods (the fractionalreplacement fly ash cements and the ordinary fly ash cements). The basic properties andshrinkage properties of two types of fly ash cements were investigated. The results indicatethat the blended cements with fine fly ash shows lower water requirement of normalconsistency and longer setting time in comparison with the ordinary Portland cement. Theblended cements with middle fly ash and coarse fly ash exhibits higher water requirement ofnormal consistency and lower setting time in comparison with the ordinary Portland cement,respectively. For the ordinary fly ash cements, the mortars show lower strength in comparisonwith the ordinary Portland cement. The difference of particle size of the added fly ash showinsignificant influence on the strength of the fly ash cement mortars at3days. However, thestrength of the fly ash cement mortars at28days increase with the decrease of particle size ofthe added fly ash. For the fractional replacement fly ash cements, the mortars with the coarsefly ash show highest strength at28days, the next are the mortars with the fine fly ash. Themortars with the meddle fly ash shows the lowest strength at28days. In comparison with theordinary Portland cement paste, the fly ash cement pastes show lower chemical shrinkage atearly age, and show higher chemical shrinkage at later age. The autogenous shrinkages of flyash cement pastes is relatively low.In order to optimize the mixture design of blended cements, the blended cement(GCF)was prepared with the fine blast furnace slag, middle ordinary Portland cement and coarse flyash. The mechanical properties and shrinkage properties of blended cement (GCF) wereinvestigated. The results indicate that GCF blended cement shows lower strength at3daysand higher strength at28days (45MPa). GCF blended cement shows higher strength. Incomparison with the ordinary Portland cement paste, the GCF blended cement paste exhibitslower chemical shrinkage and lower autogenous shrinkage at early age, and shows higherchemical shrinkage and higher autogenous shrinkage at later age. In conclusion, the mixtureof fine blast furnace slag, middle the ordinary Portland cement and coarse fly ash with the volume ratio of1:1:1, which shows similar particle size distribution with the ordinaryPortland cement, sufficient compressive strength at28days and volumetric stability, is areasonable mixture design.