Analysis of High Temperature Strength Retrogression of High Water/Cement Ratio Set Cement with Silica Powder
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摘要: 实验发现，通过增加水固比为0.74获得的密度为1.65 g/cm3的低密度水泥浆（加有40%粗细搭配的硅粉），在185℃、21 MPa养护48 h，抗压强度衰退率为20.3%，超声波强度衰退率高达50.6%。对该配方低密度水泥浆不同养护龄期的水泥石进行物相分析及微观形貌分析，认为其强度高温衰退的原因为：高水固比的水泥石机体内微孔隙本身较大，在高温养护过程中，随着水泥石机体内部物相的结晶化，微孔隙进一步增加，导致了强度衰退现象的发生，而且由于微孔隙对声波传输速度影响很大，这种衰退现象在声波强度上体现更加明显。通过外掺25%粒径为0.154 mm、密度为1.35 g/cm3的碳粉C-filler配制1.65 g/cm3低密度水泥浆，水固比降低为0.51，固相体积分数由32.0%升为46.0%，高温养护后水泥石密实、机体内微孔隙较少，强度衰退现象可得到改善。Abstract: It has been found in laboratory experiment that, a low-density cement slurry of 1.65 g/cm3 (containing 40% sized silica powder) prepared by increasing water/cement ratio to 0.74, had its compressive strength reduced by 20.3%, and the strength measured by ultrasonic method reduced by 50.6% after aging for 48 hours under 185℃ and 21 MPa. This low-density cement slurry was aged for different curing periods, and the set cements obtained were analyzed for their phases and microstructure. It was concluded that the set cements obtained from high water/cement ratio slurry had large micro-pores inside them. During aging, the volume of the micropores was increasing with crystallization of the set cements, thereby leading to strength retrogression. Since micro-pores in set cement greatly affect the sonic transmission speed, the retrogression of the strength of the set cement measured by ultrasonic method is much more severe, as shown in the laboratory experiment. To mitigate the strength retrogression of set cement, a low-density cement slurry (1.65 g/cm3) was prepared by adding 25% of C-filler (a carbon powder with particle size of 0.154 mm and density of 1.35 g/cm3) and reducing water/cement ratio to 0.51. The volumetric fraction of the solid phase in the cement slurry was correspondingly increased from 32.0% to 46.0%. This cement slurry, after aging at elevated temperature, had less micro-pore left inside, and the strength of the set cement was improved.
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