Hydration Of Special Cement And The Performances Of The Concrete In Altitude With Harsh Service Conditions

Low-heat Portland cement(PLH)has characteristics of lower hydration heat and shrinkage,higher corrosion resistance and durability,and much better long-term mechanical performances compared to that of ordinary Portland cement(OPC).It has been successfully applied in hydropower engineering and other fields,and is expected to have a broad prospect in the harsh plateau environment of long-life major infrastructure construction projects.In spite of this,the "short plate" with low strength in the early stage of low-heat Portland cement is also obvious,especially in the harsh service environment areas such as low pressure,low humidity and large temperature difference.The problems of low heat Portland cement and concrete early hydration rate decline and low mechanical properties will be further highlighted,which will not only affect the construction progress and increase the difficulty of construction maintenance.And durability can not be fully guaranteed.Moreover,the durability of the infrastructure cannot be fully guaranteed.To overcome these challenges,this study proposes the reconstruction and activation of the key clinker minerals of PLH as well as the optimization and recombination of the clinker mineral system to design and produce low-heat,early strength,low-shrinkage special cement suitable for use in a harsh service environment at high altitudes.The hydration hardening law,structure formation,development,deterioration mechanism,and performance of special cement and concrete at a simulated high altitude in a harsh service environment were studied.The main research results were as follows.Special cement clinker and cement with low heat,early strength and low shrinkage were obtained through industrial production by means of mineral composition regulation and crystal activation of clinker.The three-dimensional(3D)compressive strength of low-heat,early strength,low-shrinkage special cement was 18.8 MPa at 3 d,and it is 22.4% higher Compared to low-heat Portland cement;The dry shrinkage rate at 28 d was 0.065%,and the lower thermal Portland cement was reduced by 13.3%.The 3 d and 7 d hydration heat values were 193 and230 k J/kg,respectively,all indicators meet the requirements of the PLH hydration heat specified in GB/T 200-2017.The hydration hardening law of special cement at a simulated high altitude and in harsh service environments was studied.It was found that the degree of hydration of the special cement in a low pressure and low humidity environment was reduced compared with that in the standard environment;however,the development law of the two environments was basically the same.When considering low pressure,low humidity,and large temperature difference conditions,the degree of early hydration of special cement is considerably improved;however,the 90 d hydration is not as good as in the standard environment and low-pressure and low-humidity environment.The total porosity and average pore size of special cement hardened bodies show an increasing trend with increasing harshness of the environmental conditions.The rate of development of the mechanical properties of special cement matches the degree of hydration,and its mechanical properties are less affected by environmental impact than those of OPC.Special cement exhibits better toughness and a rapid growth with respect to its early toughness coefficient.Further,its shrinkage rate is considerably lower than that of OPC.The mechanical properties,durability,and pore structure characteristics of C40 concrete under standard,low pressure and low humidity conditions,and low pressure and low humidity conditions with large temperature differences were studied.On simulating the development of the mechanical properties of concrete over a long term,the compressive and splitting tensile strengths of special-cement concrete decreased by 6.7% and 4.2%,respectively,in lowpressure and low-humidity environments and by 14.3% and 11.6%,respectively,in lowpressure and low-humidity environments with large temperature differences.The concrete spacing coefficient increases while the percentage of 0–100 μm pores decreases with the harshness of the curing environment and the freeze–thaw cycle,and accelerate the deterioration of the durability of concrete.The characteristics of special-cement-concrete pore structures have a good correlation with their mechanical properties and durability and specialcement concrete has clear advantages in terms of impermeability and frost resistance.Field trials were conducted at high altitudes.Special-cement concrete exhibits better working performance,mechanical properties,and durability.The temperature-rise control effect of special-cement concrete is better,the temperature rise is slower,and the temperature peak is lower.In addition,the shrinkage deformation and volume stability of special-cement concrete are considerably better than those of OPC concrete.Further,the deformation change range is0 at a frequency of >40 με while that of OPC is 10%.The frequency of the change range of20～40 με is 22.9%,and the frequency of OPC is 36%.Special-cement concrete has a smaller pore diameter and a denser structure,which are more conducive to the improvement of concrete durability.The fitting results of temperature field calculated by ABAQUS finite element model are in good agreement with the measured values and the temperature stress is less than the measured concrete splitting tensile strength,which indicates that the special cement concrete has excellent cracking resistance.