Effect Of CSH Nano-crystal Nucleus And Na₂SO₄ On The Performance Of Non-steam Curing Precast Concrete

The rapid development of prefabricated building structures has led to a sharp increase in demand for precast concrete components.However,when a large amount of fly ash is added,the early strength development of concrete is slow,and the turnover efficiency of formwork is low.High temperature steam curing can significantly improve the early strength of concrete,but it not only increases production costs,increases energy consumption and carbon emissions,but also easily causes thermal damage to concrete and damages its performance.To achieve non-steam curing,early strength agents must be added.But the improvement of traditional early strength agents on the early strength of concrete is not enough to meet the requirements of prefabricated components,and there are problems such as poor workability of concrete,insufficient or even reduction of later strength during use.This article focuses on the insufficient development of early strength of precast concrete without steam curing mixed with fly ash.A new type of nano-material early strength agent CSH nano-crystal nucleus is used,and a relatively inexpensive traditional early strength agent sodium sulfate is selected for comparative research.The effects of single addition of CSH nano-crystal nucleus,single addition of sodium sulfate,and the combination of CSH nano-crystal nucleus and sodium sulfate on the performance of concrete are studied separately.Mainly including the impact on the workability,early and late strength development,water absorption performance,and durability（impermeability,frost resistance,and anti-carbonation）of concrete.The early strength mechanism of CSH nano-crystal nucleus and sodium sulfate are studied using microscopic techniques such as hydration heat,X-ray diffraction analysis（XRD）,scanning electron microscopy（SEM）,and synchronous thermal analysis（TG-DTG）.The main research conclusions are as follows:（1）CSH nano-crystal nucleus improves the workability of precast concrete.When maintaining a certain workability,the dosage of polycarboxylate superplasticizer decreases with the increase of CSH nano-crystal nucleus content.and the effect on C25concrete is more significant.Furthermorte,the law is more pronounced in C25 concrete.Adding sodium sulfate reduces the workability of precast concrete,and it is necessary to increase the dosage of polycarboxylate water reducing agent to meet the required workability.（2）Single addition of CSH nano-crystal nucleus significantly promotes early hydration of cement.The amount of hydration products is more abundant,and the morphology and contour of the products are more regular and clear.Needle shaped ettringite and hexagonal plate-like Ca（OH）₂ are interspersed or inlaid in C-S-H gel,overlapping each other to form a three-dimensional structure.When CSH nano-crystal nucleus are mixed with sodium sulfate,the early promotion effect of cement hydration is more significant,while the promotion effect of sodium sulfate alone on cement hydration is not significant.（3）When 15%and 35%fly ash are added separately,a suitable amount of CSH nano-crystal nucleus or a suitable amount of CSH nano-crystal nucleus and sodium sulfate compound can make C25 precast concrete meet the demolding strength requirement（≥15 MPa）within 24 hours.And C40 precast concrete meet the demoulding strength requirement（≥15 MPa）at 12 hours.A single addition of an appropriate amount of sodium sulfate can meet the demolding strength requirements at a 15%fly ash content.At a 35%fly ash content,the 24 hour strength of C25 precast concrete is 13.9 MPa,and the 12hours strength of C40 precast concrete is 12.9 MPa,both of which cannot meet the demolding strength requirements.Severe shrinkage occurs at 28 days of strength,and the greater the dosage,the more severe the strength shrinkage.However,both addation CSH nano-crystal nucleus and CSH nano-crystal nucleus mixed with sodium sulfate did not exhibit strength reversal at 28 days.（4）The addition of CSH nano-crystal nucleus improves the impermeability,frost resistance,and anti-carbonation of precast concrete.The addition of sodium sulfate reduces the impermeability,frost resistance,and anti-carbonation of precast concrete,and the larger the dosage,the more severe the decrease.When mixed with CSH nano-crystal nucleus and sodium sulfate,except for impermeability,the improvement of frost resistance and anti-carbonation of prefabricated concrete is not as good as that of only addition of CSH nano-crystal nucleus.In addition,the improvement effect of CSH nano-crystal nucleus on the above-mentioned properties of prefabricated concrete with higher fly ash content is more significant.When 15%and 35%fly ash are added separately:Single addition of an appropriate amount of CSH nano-crystal nucleus,combined addition of an appropriate amount of CSH nano-crystal nucleus and sodium sulfate,the frost resistance grade of C25 and C40 precast concrete can reach F200.Moreover,C25and C40 precast concrete with an appropriate amount of CSH nano-crystal nucleus can still maintain a low mass loss rate and a high relative dynamic elastic modulus after undergoing 200 freeze-thaw cycles.However,the C25 and C40 precast concrete mixed with sodium sulfate did not reach F200.