Purpose To investigate the prevalence and related factors for myopia in school-aged children in the Economic and Technological Development Zone of Qingdao, Eastern China. Methods A total of 4890 (aged 10 to 15 years) students were initially enrolled in this study. 3753 (76.75%) students with completed refractive error and questionnaire data were analyzed. The children underwent a comprehensive eye examination. Multiple logistic regression models were applied to assess possible factors associated with myopia. Results The prevalence of myopia increased as the children's grade increased (χ2 = 560.584, P < 0.001). Low myopia was the main form of myopia in adolescent students (30.22%). With the growth of age, students spent significantly more time on near work (P = 0.03) and less time on outdoor activity (P < 0.001). In multivariate models, only the following variables were significantly associated with myopia: age, two myopic parents, outdoor activity time, and continuous near work without 5 min rest. Conclusions The prevalence of myopia increased as the grade increased. Age, two myopic parents, and continuous near work time without 5 min rest were risk factors for myopia. Outdoor activities had protective effect for myopia.
The influence of anhydrite on mechanical property and alkalinity of sulphoaluminate cement clinker was investigated. The alkalinity was examined by mixing powdered samples with a solvent and measuring the pH of the suspension. The hydration mechanism was analysed by XRD analysis. The results indicated that with the increase of anhydrite addition, both the alkalinity and compressive strength decreased. Yeelimite completely converted into ettringite with the dosage of anhydrite reaching to 50% by weight of the paste. With the dosage of 30%, the 7 d pH could be reduced to 10.43 while the 7 d compressive strength was 48 MPa.
Influence on activation property of coal gangue by calcining temperature was discussed. The experimental results show that C12A7 and C2S with hydraulic-activity are created in coal gangue calcined at 900 - 950°C which has reasonable pore distribution after hydrating.
In this research, the influencing factors of oil well cement on the rheological property were experimentally studied. Based on the experiments and surveys of other class G oil well cement plants, the main influencing factors in the routine inspection were sample storage and processing condition and inspection equipment accuracy. The experimental results illustrate that sample storage and processing condition had a highly effect on the rheological property of drilling cement slurry. In addition, temperature positive deviation increased the rate of cement hydration by causing more rapid nucleation of hydration products, leading to earlier thickening time; conversely, minus deviation delayed cement nucleation, causing later thickening time. Compared with temperature deviation, the effect of pressure deviation was not obvious.
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