Research On Influencing Factors Of Chemical Shrinkage Of Composite Cement Paste

As the technology of double mixture of mineral admixture material and admixturebecomes mature in concrete, nano-scale ultra-fine components are adopted, the volumedeformation at early ages is one of the critical infrastructure problems of modern concrete,chemical shrinkage is undoubtedly more complex. The theme of this paper is research on theinfluence factors of chemical shrinkage of composite paste using ASTM C1608test method（absolute volume method with sealed small container in constant temperature water bathenvironment）, the influence factors of chemical shrinkage of composite cement paste includes:the variety and fineness of cement, water binder ratio, mineral admixture materials, additives,nano-materials and curing temperature, the main conclusions are as follows:1. The content of cement clinker plays a decisive role on chemical shrinkage of cementpaste, among the four kinds of mineral clinker, C₃A has the largest shrinkage, C₂S have asmaller one so the chemical shrinkage reduced accordingly at early ages. The cement has aquicker early age hydration as the fineness become smaller, which causes larger early agechemical shrinkage.2. The chemical shrinkage of cement paste increases as the water-cement ratio/watercement ratio increased, for example chemical shrinkage of cement paste with w/c of0.38increase of48%than the paste with water-cement ratio of0.26in7d. The differences ofchemical shrinkage caused by water-cement ratio are very significant.3. Adopt mineral admixture material such as fly ash （FA）, slag （S）, silica fume （SF） andflu gas desulfurization gypsum （FGD） which replacing the same quantity of cement, mixingcomposite paste, research on the effect of mineral admixture on chemical shrinkage:（1）The composit paste with the fly ash express high inbibitional effect on chemicalshrinkage because of the micro-aggregate filling effect of fly ash, the chemical shrinkage ofcomposite paste with45%FA I is reduced by41%compared to cement paste, the inbibitionaleffect decreases at the later age due to the secondary hydration of fly ash with CH, theinbibitional effect of FA II is not obvious as FA I at early ages. （2） The chemical shrinkage of composite paste with silica fume increase slightlycompared with cement paste, and shrinkage increases as the dosage increase,for example thechemical shrinkage with12%Silica fume increase of12%compared to the cement paste on3d. However, the chemical shrinkage of composite paste with8%nano-SiO₂is decrease of19%compared with cement paste on28d. Obviously, the growth rate of chemical shrinkageof composite paste with nano-SiO₂is significantly reduced at later ages.（3） The chemical shrinkage of composite paste with slag reduced significantly thancement paste, and the shrinkage decreased with the increasing of the content of slag, thechemical shrinkage of composite paste with45%slag decreased by53%compared withcement paste at3d, the inbibitional effect of slag power decreased at later ages.（4） When double mixed with FA and FGD, chemical shrinkage of composite cementwith30%FA decreased as the increases of FGD. Because of the crystallization of theself-expanding in the cement hydration process of the gypsum.the chemical shrinkage ofcomposite paste with30%FA I and FA II double mixed20%FGD are reduced by11%and37%respectively compared to cement paste.（5） The chemical shrinkage of composite paste double mixed with fly ash and slagpower is significantly reduced, and the change of mixing proportion does not much effect onchemical shrinkage. The chemical shrinkage of composite paste with25%fly ash and15%slage decreased35%compared with the cement paste on3d, the growth rate of chemicalshrinkage accelerated at middle and later ages, the chemical shrinkage is3.15ml/100g on28d.4. Three types of nanomaterials were used: active material nano-SiO₂, functional formmaterial nano-TiO₂, inert material nano-CaCO₃, with the same dosage level and the samewater-cement ratio, the order of chemical shrinkage of composite cement with nano materialis SiO₂> TiO₂> CaCO₃. Among that, low dosage of nano-SiO₂（1-3%） can increases chemicalshrinkage, and shrinkage grows as the dosage increase; high dosage of SiO₂（4-8%） cansignificantly inhibited chemical shrinkage at later ages. When mixed the nano material withmineral admixture, the chemical shrinkage of composite paste decreased significantly. Mixed5%nano-CaCO₃with the composite cement with fly ash and slage, the chemical shrinkage in3d reduction of46%than cement paste, reduced by11%to composite cement withoutnanomaterials. That is because of the addition of nanomaterials promote composite powder packed more closely.5. The admixtures in their dosage range, chemical shrinkage of paste with retarder of0.15%dosage decreased by63%compared with the paste without retarder in3d, butchemical shrinkage of paste with retarder has a rapid development at later ages, the shrinkageeven larger than the shrinkage of cement paste without retarder; The paste with naphthaleneformaldehyde water reducer appears subtle swelling phenomena at early ages because of thecontaining of expansion agent, and the shrinkage is very low in the entire ages; chemicalshrinkage of cement paste with air-entraining agent or early strength agent within the dosagerange show little change on chemical shrinage.6. Curing temperature affect the hydration rate significantly, than affect the chemicalshrinkage of cement paste. Under the high temperature curing conditions, the chemicalshrinkage of cement paste grows rapidly at early ages, the chemical shrinkage of paste under40°C increased by more than50%compared with the paste under15°C in7d, the differencescaused by the different curing temperatures decreases at later ages.In addition, this paper research on particle accumulation state of different particle sizedistribution of the composite powder and its influence on filled water and adsorbed water,using bulk density test and fluidity test, analyze the effect of particle packing to the chemicalshrinkage of paste, draw preliminary study conclusions.