Experimental Study On The Mechanism And Application Of Alkali-free Liquid Concrete Accelerators

The accelerators is able to accelerate the setting and hardening procedure ofcement or concrete, which has become one of the most important materials used inmodern shotcrete construction. According to its nature and status, the concreteaccelerators can be divided into alkaline powdered accelerators, alkali-free powderedaccelerators, alkaline liquid concrete accelerators and alkali-free liquid concreteaccelerators. Gradually traditional alkaline accelerators has being substituted byalkali-free liquid concrete accelerators based on its comparative advantages onfollowing aspects: lower alkali content, almost no effect on the late compressivestrength of cement&concrete, better suitable for wet spraying process construction ofshotcrete, conducive to maintaining a good working environment, harmless to humanhealth and not causing alkali aggregate reaction of shotcrete. Up to now, most ofresearches on alkali-free accelerators are focusing on the product development ratherthan its accelerating mechanism as well as its application.Based on the combination of macroscopic and microscopic techniques includingXRD and SEM, compatibility test, shotcrete process performance test andmicroscopic test on alkali-free accelerators is employed here in this paper toinvestigate its performance and its compatibility with cement, fly ash andpolycarboxylate-type superplasticizers, to explore its influence on shotcrete processperformance and to reveal its accelerating mechanism. The purpose for this paper is topropose a suitable quality-test method for alkali-free liquid accelerators by comparingcurrently existing quality-test methods for accelerators.Here the results showed that alkali-free liquid accelerators are significantlysuperior to the traditional accelerators in setting times, early compressive strength,growth rate of late compressive strength and compressive strength ratio. And both foralkali-free accelerators and traditional accelerators, its compressive strengths in eachages are satisfied with the logarithmic growth relationship.By comparing testing results on accelerators performance when applying withJC477—2005(building materials standards of China) and TGPST31—2005(threegorges group standards with reference to the European standards), it is shown thatJC477—2005had failed to meet the alkali-free accelerators performance testingrequirements, while TGPST31—2005can better reflecting the essential attributes ofalkali-free accelerators. Here in this paper the author revised new performance-testingstandards as―Shotcrete with Alkali-free Concrete Accelerators‖by making reference to the standards both home and abroad.There are some discrepancies between the compatibility of alkali-freeaccelerators with cement, with fly ash and with polycarboxylate-type superplasticizers.According to cement, the setting time of new concrete accelerator is mainlyinfluenced by the mixing materials content, gypsum content as well as C3A and C3Scontent. Speaking with fly ash, there is great impact on alkali-free accelerators cementmortar’s setting times and compressive strength in each age. When fly ash content ofcement is about30%, the accelerator’s setting time is the shortest and the compressivestrength ratio of alkali-free accelerators and fly ash content is satisfied with thelinearly growth relationship. The compatibility between alkali-free accelerators andpolycarboxylate-type superplasticizers is good.As testing results of Cement hydration heat、XRD and SEM tests shown, theperformance and mechanism of inorganic-organic composite alkali-free liquidaccelerator is mainly controlled by its inorganic constituents rather than organicingredients, and the impact of accelerators on cement hydration mainly works in theearly phase and affects little on cement hydration within28days. When acceleratorscontent is8%，there are much more stick ettringite in the structure of cement pastehydrated in30minutes. And those generated ettringite interleaved and filled in thepore of cement paste，which makes the structure of cement paste more compact andenhance its early strength. The accelerating mechanism of alkali-free liquidaccelerator is to generate more ettringite to make cement setting. According to SEMpictures, ettringite was produced by liquid chemical reaction-precipitation exhalation,then those short columns oriented randomly and had been distributed disorderlythroughout the whole cement paste, which made the setting times shorter and thecompressive strength increased.Based on testing results on shotcrete both in laboratory and construction process,it is concluded that alkali-free liquid accelerators has little affect on the performanceof shotcrete, by which its late compressive strength could be effectively ensured.Furthermore, when mixing the shotcrete with alkali-free liquid accelerator, theconstruction rebound rate is slightly higher than with alkali-low liquid accelerators.