Application Of Aluminosilicate Internal Curing Agent In Heat-cured Concrete

In recent years,many major projects have been carried out in China,including“The Belt and Road”,“Construction of New Urbanization”,“Ocean Development”and“Island Reef Engineering”.In addition,“13th Five-Year Plan”clearly points that the proportion of prefabricated buildings will be occupied more than 20%;the length of high-speed railways will increase by 11,000 kilometers in the next five years and 60%of which are made by concrete.In respect of underground project,the total length of national subway plan to achieve 8,600 kilometers,including more than 30 million pieces and 142 million steres tubes.Therefore,concrete is an important basic material for national projects.At present,precast concrete is mainly prepared by heat curing method,which can accelerate cement hydration at early stage,enhance early strength and improve production efficiency.Due to the high cement hydration rate during heat curing period,amount of water is consumed in early stage which is harmful to the performance of concrete at late stage.Besides,the rapid formed uneven hydrates lead to a deterioration of pore structure.To solve these issue,this paper used aluminosilicate lightweight fine aggregate?LWFA?as an internal curing agent to study the influence of its pozzolanic reaction and internal curing effect on heat-cured concrete?HCC?.The main work and achievements are as follows:?1?Ions dissolution behavior of LWFA and its regulationBased on the pozzolanic reaction of LWFA,the ions dissolution behavior in calcium hydroxide solution at different temperature was studied.Triisopropanolamine?TIPA?was used to regulate this process.The affect of pozzolanic reaction on the composition and structure of cement hydrates was discussed.It was found that not only TIPA,but also elevated temperature greatly promoted the ions(Al³⁺,Fe³⁺)dissolution of LWFA.The concentration of Al³⁺increased from 0.0209 mg/L to 1.3277 mg/L and Fe³⁺increased from 0.1349 mg/L to 2.4144 mg/L.The dissloved Al³⁺favored to the formation of C-A-S-H gels,AFm et al.?2?Pozzolanic reaction effect of LWFA on HCC propertiesThe mechanical properties,capillary water absorption,chloride ions resistance and pore structure of HCC were investigated.It was found that TIPA promoted the pozzolanic reaction of LWFA and a considerable amount of hydration products formed at interfacial transition zone?ITZ?.The structure of ITZ was dense and a“shell-like”structure was formed,which solved the problem of low early strength of HCC blended LWFA.The 1 day and 7 day compressive strength increased by 15.3%and 25.9%,respectively.The impermeability and ability to resist chloride ion of HCC were also enhanced.In addition,capillary water absorption and chloride diffusion coefficient at28 d decreased by 29.1%and 23.2%,respectively.?3?Internal curing effect of LWFA on the hydration of FA?fly ash?Different methods were carried out to determine the hydration degree of FA.HCC blended LWFA had higher relative humidity,which provided a good environment for FA hydration.Not only the selected dissolution method but also the nuclear magnetic resonance method indicated that FA hydration could be promoted by LWFA,especially at late stage.With the help of LWFA,FA hydration degrees of 1,7,28 and 60 day promoted by 6.5%,15.1%,31.0%,44.1%,respectively.The synergy of LWFA and FA reduced the 1,7,28 and 60 day portlandite content in HCC by 17.75%,16.39%,8.25%and 18.16%,respectively.?4?Internal curing effect of LWFA on HCC blended-FA performancesInternal curing effect of LWFA on mechanical properties,pore structure and duribility of HCC with and without FA were characterized.The reason why HCC blended LWFA had good chloride resistance was discussed physically and chemically.It was proved that internal curing effect of LWFA was fully developed at late stage.With the addition of LWFA,the compressive strength increased by 3.49%and 7.38%,and the chloride diffusion coefficient decreased by 38.81%and 30.54%at 28 and 60day,respectively.In summary,elevated pozzolanic activity of LWFA could greatly enhance the early properties of HCC,and its internal curing effect in favor of long term properties.This paper was a guiding for producing high durability precast concrete.