Study On The Application Of Admixtures For Adjusting Workability And Early Strength Of Concrete

Adjusting the workability and accelerating the early strength growth of concrete are important projects in the concrete industy.Applying self compacting concrete in prefabricated concrete was taken as an example to study the chemical admixtures for finely tunining the rheological properties,and for accelerate the early strength growth of concrete.The prefabricated concrete(PC)technology is the critical sector in the assembly-house contruction industry,which determines the safty and service performances of the houses.Self compacting concrete is one of options for casting and molding the elements,which requires proper rheological behavior of the fresh concrete and is benefical to the quality guarantee.One the other hand,in order to increase the production efficiency,high termperature curing at 60?C and/or high temperature and high pressure steam curing at 120?C are often adopted in the production of prefabricated concrete to achieve higher early strength of concrete.Therefore,two groups of chemical admixtures,respectively for rheology control and for promotion of early strength are systematically investigated in order to guide their applications in self-compacting concrete and prefabricated concrete.In the part of rheology control,the influences of water to cement ratio(w/c),PCEs(LNSP with no side chain and 24 SP,40 SP with different side chain length)and viscosity modifiers(VMAs)on the rheological properties of mortar were carried out through rheological test.It was found that increasing w/c can simultaneously decrease yield stress and plastic viscosity.The side chain in the molecular structure of PCEs had a significant effect on reducing the yield stress,and the effect of reducing the yield stress is in the order of 24 SP 40SP>LNSP.The effect of PCEs on the yield stress is significant,but it had little influence on the plastic viscosity.Two types of admixtures,superabsorbent(SAP)and a specific thickener(Y-64)for cementitioius system are selected as the VMAs in this study.It was found that SAP increased the yield stress and plastic viscosity by reducing the content of free water,and the effect of Y-64 on yield stress and viscosity varies with the dosage due to the adsorbation form of Y-64 on the surface of the cement particles.Within the appropriate dosages,Y-64 can increase yield stress by forming flocculation and bridging particles.The competitive adsorption between Y-64 and PCE was observed.After summarizing the effect of the admixtures,a rheograph of their influence on rheological properties was given in this paper.Effects of three types of early strength enhancers,including high-earlystreng polycarboxylate(PCE)superplasticizers,alkanoamines and nano-C-S-H seed on on cement hydration and concrete strength were investigated using isothermal calorimetry and compressive strength tests.It was found that the retarding effect of PCEs on cement hydration with the same workablily is in order of 40-5 PC(PCE with side chain length of 4000 and anionic charge to side chains ratio of 5)< 24-4 PC < YJ(commercial PCE).TEA,MDEA,DEIPA significantly changed the shapes of isothermal calorimetric curve indicating they promoted the reaction of aluminum phase.The synthesized C-S-H particles significantly shortened the induction period and increase the peak rate of heat release.Furthermore,the C-S-H particles imporved the hydration rate during 12~24h in a large scale.The C-S-H particles imporved the 12 h compressive strength while had no effect on 24 h.The C-S-H particles optimized the capillary pores of hardened cement paste by changing the precipitation position of hydration products.The mechanism of C-S-H particles on accelrating hydration degree without improving the compressive strength is that C-S-H particles loosed the hydration products,which reduced the strength of the matrix.