Experimental Study Of SAP-hydrogel In Alkali Environment And Its Influence On Cement Mortar Behavior

Superabsorbent polymers(SAP)used in Portland cement to control the early age shrinkage of concrete.The basic mechanism is when SAP being added into fresh concrete mix,it absorbs water form SAP-hydrogels and slow the hydration process.Then,SAP-hydrogels release water in a later time to extend the hydration period.Such a mechanism of retaining and then releasing water will smooth hydration and can reduce shrinkage in concrete.However,this mechanism is not well understood.The water absorption capacity of SAP,water releasing capacity of SAP and the duration of water is SAP-hydrogels are still unknown and need to verify experimentally.From experimental point of view,it is not feasible to break concrete specimens to analyze SAP-hydrogels after concrete is hardened.Considering concrete is an alkali medium,and so is silicate-sodium(S-S),they both have almost same p H values,whereby it is proposed here that the mix of SAP-hydrogels-S-S is used to simulate the behavior of SAP in hardened concrete.This study further employs a novel experimental filtering scheme to measure SAP’s capacity in retaining/releasing water versus time up to 30 days in the mix of SAP-hydrogels-S-S that involves four types of SAP(a,b,c,d)based on the molecular chains.A patent is under preparation for the experimental filtering scheme.The mesh for SAP-a,SAP-b,SAP-c and SAP-d is 30-60,60-100,120-180 and 200-240,respectively,and the smaller the mesh,the longer the molecular chain.This means that SAP-a is of the longest chain,and SAP-d is the least.The primary finding of this study is that SAP can take about 80-200 times of water.SAP-a takes the most,and SAP-d is the least.Considering the fresh concrete will exert the pressure on SAP-hydrogels,two pressure levels(0.75 KPa and 1.5KPa)are applied in the filtering scheme and the finding is that such pressure does increase the water release rate for SAP-hydrogels.Another case is to add two types of water reducers into SAP-hydrogels,which again accelerates the water releasing.The explanation is that the ions in water reducers will deplete the integrity of SAP-hydrogels,and expel the water in SAP-hydrogels.The presence of both(S-S)and water reduces in SAP-hydrogels will cause server water release foreseeably.From the view of keeping water in SAP-hydrogels,it is found that SAP-a is the worst and SAP-d is the best.In addition,three temperatures(-5°C,20°C and 60°C)is also evaluated that the finding is such temperatures have limited influence on SAP-hydrogels.Despite that SAP will quickly absorb a large amount of water at the beginning of mixing,but if SAP-hydrogel cannot keep it long enough,contribution of SAP is low.Therefore,this study recommends SAP concrete should have a high watercement ratio and SAP-d should be selected.Based on the above progress of SAPhydrogel work,a further study of SAP added cement mortar is conducted with two SAP-a and SAP-d being added at 0.1%,0.2% and 0.3% dosage for two water cement ratios of 0.29 and 0.33.Also,dry-process and wet-process are used in preparing SAP cement mortar specimens.The flexural strength and compressive strength are measured and dry shrinkage test is carried out as well as the test of measuring the cracking time by the external squared eccentric ring method.The results show that SAP-d does better than SAP-a in all aspects.In summary,this study covers a systematic experimental work of quantifying the absorption,maintaining and releasing of water in the mix of SAP-hydrogel-S-S by a filtering scheme.