On the Feasibility of Early-age English learning

Children's English learning in China attracts more and more people's attention and is on the tendency of starting at an early age. Under the trend of "learning English from childhood", the author has explored the Critical Period Hypothesis and discussed the younger learners' disadvantages and older learners' advantages when learning English. and concludes that early-age English learning is not feasible.

Early-age strength property improvement and stability analysis of unclassified tailing paste backfill materials

High-density tailings,small cementitious materials,and additives are used for backfill materials with poor early compressive strength(ECS),which may greatly affect the mining and backfill cycle,to prepare paste backfill materials(PBMs)with a high ECS.The effects and mechanisms of different early strength agents on the property of PBM are investigated.The action mechanism of additives on the properties of PBM is also analyzed through X-ray diffraction,scanning electron microscope,and energy dispersive spectrometry.Results show that the effects of single-component additives 1,3,and 6 are better than those of the other additives,and their optimal dosages are 3wt%,1wt%,and 3wt%,respectively.The optimum multicomponent combinations are 1wt%of additive 1 and 1.5wt%of additive 6.The ECS of the paste with additive 10 increases to a greater extent than that of the other pastes because of the synergistic action of additive 1 with additive 6.The hydration product of Ca(OH)2 is consumed,and more C-S-H gels are generated with the addition of additives to paste.Tailings particles,ettringite crystals,and gels intertwined with one another form a dense net-like structure that fills the pores.This structure can significantly improve the ECS of PBM.

Dynamic regimes of cemented backfill at early-age

In view of the mechanics characteristic of cemented tailings backfill(CTB)at early age,the separation Hopkinson pressure bar test device was used to explore the effects of curing age and impact energy.A total of 48 CTB samples with diameter of 50 mm and length of 25 mm were prepared with curing ages of 3,5,7 and 9 d.Impact tests under different impact energy(10,20,30 and 40 J)were carried out.The microstructure of CTB at different ages was analyzed by scanning electron microscopy(SEM).The results show that,the curing age mainly affects the mechanical properties and internal structure of early-age CTB.With increasing curing age,the mechanical properties of early-age CTB change from viscoelasticity to brittleness.The impact energy mainly affects the response of dynamic peak compressive strength to strain rate.Under low strain rate,the structure of CTB is broken,but still has bearing capacity,affecting the formation of later strength.It is concluded that the structural loses completely under the action of high strain rate.Therefore,the control of impact energy and the protection of curing age should be fully considered in actual production process.

Effects of Shale and CaO Incorporation on Mechanical Properties and Autogenous Deformation of Early-age Concrete

The pre-soaked shale employed as an internal curing agent and CaO employed as expansion agent were incorporated into concrete to investigate their effects on the mechanical properties and autogenous deformation of early-age concrete.We have conducted the relevant tests for setting time,mechanical properties,internal relative humidity and autogenous deformation of early-age concrete with shale or/and CaO incorporation.The results indicate that the set behavior is delayed by shale addition but is accelerated with CaO.The shale addition firstly enhances and subsequently decreases the strength,but CEA addition has a weakening effect.Additionally,shale or/and CaO incorporation deteriorates the elastic modulus.The shale and CaO incorporation significantly improve the internal relative humidity of concrete.The internal curing efficacy of shale could synergistically mitigate the autogenous shrinkage,that is,could enhance the expansion of CaO and then greatly reduce the contraction,which is significantly beneficial to impede the shrinkage-introduced cracks of early-age concrete.

Assessment of early-age cracking of high-performance concrete in restrained ring specimens

High-performance concrete （HPC） is stronger and more durable than conventional concrete. However, shrinkage and shrinkage cracking are common phenomena in HPC, especially early-age cracking. This study assessed early-age cracking of HPC for two mixtures using restrained ring tests. The two mixtures were produced with water/binder mass ratio （mw/mB） of 0.22 and 0.40, respectively. The results show that, with greater steel thickness, the higher degree of restraint resulted in a higher interface pressure and earlier cracking. With steel thickness of 6 mm, 19 mm, and 30 mm, the age of cracking were, respectively, 12 days, 8 days, and 5.4 days with the mw/mB = 0.22 mixture; and 22.5 days, 12.6 days, and 7.1 days with the mw/mB= 0.40 mixture. Cases of the same steel thickness show that the ring specimens with a thicker concrete wall crack later. With the mw/mB = 0.22 mixture, concrete walls with thicknesses of 37.5 mm, 75 mm, and 112.5 mm cracked at 3.4 days, 8.0 days, and 9.8 days, respectively; with the mw/mB = 0.40 mixture, the ages of cracking were 7.1 days, 12.6 days, and 16.0 days, respectively.

Influence of Superplasticizer Type and Dosage on Early-age Drying Shrinkage of Cement Paste with Consideration of Pore Size Distribution and Water Loss

We introduced a parameter r_s(the radius of the pores where the meniscus forms),which is composed of two factors,i e,water loss and cumulative pore size distribution(PSD),to provide a better explanation of the influence of superplasticizers(SPs)on early-age drying shrinkage.In our experiments,it is found that the addition of three types of SPs leads to a significant increase in the early-age drying shrinkage of cement paste,and drying shrinkage increases with the dosage of SPs.Based on the results above,we further studied the mechanism of the effects of SPs on the early-age drying shrinkage of cement paste by PSD and water loss,which are two components of r_s.The experimental results indicate that r_s can be a better index for the early-age drying shrinkage of cement-based materials with SPs than a single factor.In addition,the effects of SPs on other factors such as hydration degree and elastic modulus were also investigated and discussed.

Early-age Hydration Characteristics of Composite Binder Containing Graphite Powder

The early-age hydration characteristics of composite binder containing graphite powder(GP)with two different finenesses were investigated by determining the hydration heat,thermo gravimetric,morphology of hardened paste as well as the compressive strength of mortar.The experimental results show that:replacing 2%-6%of cement with graphite powder significantly improves the piezoresistive effect of early age mortar,can be used to monitor accidental loads caused by dropped objects,collisions,or other accident events,and thus avoids initial damage.Some GP provides additional nucleation sites that lead to a fast formation of hydration products(nucleation-site effect).However,due to the almost hydrophobic water contact angle,most of the GP causes a large number of micro-cracks in the hydrated paste(gap effect).Because of the lamellar shape and high surface energy,GP is easily balled and can not be uniformly distributed in the composite,resulting in clumping together and wrapping some of the cement particles(barrier effect).Due to nucleation-site effect,when the dosages of coarse and fine GP reached 2%and 4%,1 d strength were increased by 9.1%and 9.6%,respectively.At 3 days,as the interior damage caused by the gap effect gradually increased,and the retarding effect on cement hydration caused by barrier effect was enhanced.GP has an obvious negative effect on compressive strength.However,micro-cracks caused by fine GP are less,so its negative effect on 3 d compressive strength is lower.

Effect of Additional Surfaces on Ordinary Portland Cement Early-Age Hydration

Early-age hydration of Ordinary Portland Cement (OPC) was studied in the presence of two additional surfaces. Additional surfaces are known to accelerate the early-age hydration of OPC. Autocatalytic reaction modelling was used to determine acceleration mechanism of additional surfaces. Heat development of the hydration was measured with semi-adiabatic calorimetry and the results were modelled with an autocatalytic reaction. Autocatalytic reaction modelling was able to determine number of initially active nucleation sites in early-age hydration. OPC hydration followed autocatalytic reaction principles throughout induction period and accelerating period. Both of the added surfaces, limestone filler and calcium-silicate-hydrate (C-S-H) coated limestone filler accelerated the early-age hydration. According to autocatalytic modelling, the C-S-H coated filler increased the number of initially active nucleation sites. Pristine limestone filler accelerated the early-age hydration by providing the additional nucleation sites throughout the early-age hydration. The difference was explained with common theories of nucleation and crystal growth. Autocatalytic model and measured calorimeter curve started to significantly deviate at the inflection point, where the reaction mode changed. The reaction mode change depended on the average particle distance. Early-age hydration, modelled as autocatalytic reaction was able to improve understanding of OPC early-age hydration and quantify the number of initially active nucleation sites. Understanding and quantifying the acceleration mechanisms in early-age hydration will aid larger utilization of supplementary cementitious materials where understanding the early-age strength development is crucial.

Effects of Portland cement addition on the early-age properties of solid-waste-based sulphoaluminate cement

This paper investigated the effects of different proportions of Portland cement addition(0-20%)on the setting time and early-age strength of solid-waste-based sulphoaluminate cement.The mechanism of hydration process was analyzed by X-ray diffraction(XRD)technology.It was found that the incorporation of Portland cement could reduce the early-age strength of the composite cement,and shorten the setting time.When the Portland cement reached 20%,the early-age strength of the composite cement was the lowest and the setting time was the shortest.The XRD analysis demonstrated that the addition of Portland cement could accelerate the hydration process of the composite cement,and the more the Portland cement was added,the faster the hydration process of the composite cement was observed.The main reason is that the addition of Portland cement could increase the alkalinity of the composite cement and promote the hydration of anhydrous calcium sulphoaluminate.This study provides an important reference for the production of solid-waste-based composite cement.

Experimental study on temperature stress calculation and temperature control optimization of concrete based on early age parameters

Temperature control curve is the key to achieving temperature control and crack prevention of high concrete dam during construction,and its rationality depends on the accurate measurement of temperature stress.With the simulation testing machine for the temperature stress,in the present study,we carried out the deformation process tests of concrete under three temperature curves:convex,straight and concave.Besides,we not only measured the early-age elastic modulus,creep parameters and stress process,but also proposed the preferred type.The results show that at early age,higher temperature always leads to greater elastic modulus and smaller creep.However,the traditional indoor experiments have underestimated the elastic modulus and creep development at early age,which makes the calculated value of temperature stress too small,thus increasing the cracking risk.In this study,the stress values of the three curves calculated based on the strain and early-age parameters are in good agreement with the temperature stress measured by the temperature stress testing machine,which verifies the method accuracy.When the temperature changes along the concave curve,the law of stress development is in consistent with that of strength.Under this condition,the stress fluctuation is small and the crack prevention safety of the concave type is higher,so the concave type is better.The test results provide a reliable basis and support for temperature control curve design and optimization of concrete dams.

Colorectal cancer in patients under 50 years of age:A retrospective analysis of two institutions' experience

AIM:To investigate the epidemiological characteristics of colorectal cancer(CRC)in patients under 50 years of age across two institutions.METHODS:Records of patients under age 50 years of age who had CRC surgery over a 16 year period were assessed at two institutions.The following documents where reviewed:admission notes,operative notes,and discharge summaries.The main study variables included:age,presenting symptoms,family history,tumor location,operation,stage/differentiation of disease,and post operative complications.Stage of disease was classified according to the American Joint Committee on Cancer TNM staging system:tumor depth;node status;and metastases.RESULTS:CRC was found in 180 patients under age50 years(87 females,93 males;mean age 41.4±6.2years).Young patients accounted for 11.2%of cases during a 6 year period for which the full data set wasavailable.Eight percent had a 1stdegree and 12%a 2nd degree family CRC history.Almost all patients(94%)were symptomatic at diagnosis;common symptoms included:bleeding(59%),obstruction(9%),and abdominal/rectal pain(35%).Evaluation was often delayed and bleeding frequently attributed to hemorrhoids.Advanced stage CRC(Stage 3 or 4)was noted in 53%of patients.Most tumors were distal to the splenic flexure(77%)and 39%involved the rectum.Most patients(95%)had segmental resections;6 patients had subtotal/total colectomy.Poorly differentiated tumors were noted in 12%and mucinous lesions in 19%of patients of which most had Stage 3 or 4 disease.Twenty-two patients(13%)developed recurrence and/or progression of disease to date.Three patients(ages 42,42and 49 years)went on to develop metachronous primary colon cancers within 3 to 4 years of their initial resection.CONCLUSION:CRC was common in young patients with no family history.Young patients with symptoms merit a timely evaluation to avoid presentation with late stage CRC.