外掺MgO碾压混凝土的膨胀效应研究

The application of roller compacted concrete（abbreviate as RCC） with magnesium oxide （abbreviate as MgO） as an admixture in water resources and hydropower engineering has achieved the purpose of simplifing construction, shortenting construction period, saving investment and enhancing concrete’s anti-crack ability. However, although the mixing amount of fly ash in RCC is greatly higher than that in ordinary concrete and the binding material has expanded from the binary system to the ternary system, the activity index and judging method of balanced content of MgO admixture in RCC are still adopting that of the ordinary concrete, which is discrepant with the RCC’s characterstic and constraints the fully effect of the MgO concrete’s superiority. To solve this fundamental problem, systematic research on autoclave expansion rate, autogenous volume deformation and pore structure of cement-flyash-rock flour paste, mortar and concrete with different rock flour amount under the effect of MgO’s activity and amount through measures such as autoclaved test, autogenous volume deformation observation, energy spectrum analysis, x-ray diffraction, mercury intrusion detection and so on are planed to be carried out to reveal its objective principle and mechanism and to propse activity index and new judging method on MgO balanced content suitable for RCC’s characterstic as a systematic and reliable fundamental date and scientific evidence for futher compilation of < Standard for test method of MgO balanced content in hydraulic concrete > or revising of < Standard for MgO technical requirement > and < Specification for hydraulic concrete construction with MgO agent > including RCC.

外掺MgO碾压混凝土在水利水电工程的应用，达到了简化施工、缩短工期、节约投资、提高混凝土抗裂能力的目的。然而，碾压混凝土的粉煤灰用量远大于常态混凝土，且胶凝材料已从过去的二元体系发展到三元体系，但掺入碾压混凝土的MgO的活性指标、安定掺量仍采用常态混凝土的标准和判定方法，这与碾压混凝土的特点不符，束缚了MgO混凝土优越性的充分发挥。为了解决这一应用基础性问题，本项目拟同时采取压蒸试验、自生体积变形观测、能谱分析、X-射线衍射、压汞检测等手段，通过系统研究MgO活性与掺量对不同石粉掺量的水泥-粉煤灰-石粉浆、砂浆、混凝土的压蒸膨胀率、自生体积变形、孔结构的影响，揭示其客观规律及机理，提出适合碾压混凝土特点的MgO活性指标和新的MgO安定掺量判定方法，为将来制定包含碾压混凝土在内的《水工混凝土MgO安定掺量判定方法》或修改《MgO品质要求》和《MgO筑坝规定》提供系统有力的基础资料和科学依据。

外掺MgO混凝土在水利水电工程的应用，达到了简化施工、缩短工期、节约投资、提高混凝土抗裂能力的目的。但当其从常态混凝土推广到碾压混凝土时，由于碾压混凝土的骨料多、胶凝材料少，粉煤灰掺量高达50%-65%，且胶凝材料已发展到三元甚至四元体系，若继续使用常态混凝土中MgO安定掺量的判定方法来确定碾压混凝土的MgO安定掺量，显然与碾压混凝土的特点不符。因此，研究外掺MgO碾压混凝土的膨胀效应，提出碾压混凝土中MgO安定掺量的判定方法，以及适用于碾压混凝土的MgO活性指标，具有重要的科学意义和实践价值。. 本项目采用压蒸实验、抗压强度实验、自生体积变形观测、电镜扫描、能谱分析、压汞检测、X-射线衍射等手段，研究了MgO掺量与活性对使用水泥-粉煤灰/磷渣粉、水泥-粉煤灰-石粉/磷渣粉、水泥-磷渣粉-石粉等拌制的净浆和砂浆的压蒸膨胀变形与相应碾压混凝土的自生体积变形及其孔隙结构的影响（其中，同申报书相比，与磷渣粉相关的研究内容为新增），主要取得了以下代表性成果。. （1）随着MgO掺量的增大，各种胶凝材料体系的水泥基材料的抗压强度C随MgO掺量M的变化曲线（C~M）均出现拐点，这个拐点位于相应的压蒸膨胀率E随MgO掺量M变化曲线（E~M）拐点的左侧或者重合。因此，碾压混凝土的MgO安定掺量，不宜像常态混凝土那样，采用水泥砂浆试件的压蒸膨胀率为0.5%或E~M曲线拐点对应的MgO掺量，而宜采用水泥砂浆试件的C~M曲线拐点对应的MgO掺量（名为“强度拐点法”），以保障外掺MgO碾压混凝土挡水坝不致发生结构强度下降破坏和体积膨胀破坏。. （2）MgO的反应活性对水泥基材料的压蒸膨胀变形、抗压强度、孔隙结构和碾压混凝土的自生体积变形、抗压强度、孔隙结构均存在影响。用于碾压混凝土的MgO活性指标，宜采用200s~280s。. （3）外掺MgO水泥基材料的压蒸膨胀变形和相应碾压混凝土的自生体积变形没有密切相关性。