Research On Activity Evaluation And Activation Of Natural Volcanic Powder In Cement-Based Materials

As mineral admixtures,apart from economic and environmental benefits,the incorporation of high-activity natural volcanic powders（NVPs）can improve the properties of cement-based materials.However,the differences in physical properties and chemical characteristics render NVPs exhibit different pozzolanic activity,and thus lead to the difference in macro-performance of their application.In this thesis,the activity evaluation methods suitable for NVPs with different pozzolanic activity levels were systematically studied.A modified R³ method which can quickly and accurately evaluate the activity of NVPs was proposed.The activation effects of different activation methods on NVPs were compared and their influences on the setting time,rheological properties,and volume stability of NVP-cement composite systems were also investigated.These provide a theoretical basis for the scientific research and engineering application of the NVPs-cement system.（1）The activity assessments of six common activity evaluation methods for NVPs were compared systematically.It was found that the electrical conductivity method is suitable for the pozzolan activity at early age and the strong adsorption effect of NVPs would enhance its activity performance.The accuracy of the pozzolanic test is limited by the subjective judgment and objective factors such as the slow dissolution rate of active ingredients and the type of cement.The modified Chapelle test is more suitable for low pozzolanic activity materials.The thermal analysis method can evaluate the activity of pozzolanic materials with different activity levels quickly and accurately.（2）The modified R³ method was proposed,which can effectively simulate the hydration environment of the composite system and have a good correlation with the strength activity index.To ensure the formation of ettringite in early hydration,improve the activity of pozzolan and promote the pozzolanic reaction,the p H range of the mixed solution of sulfate and alkali was determined 13.0～13.3,and sufficient Ca（OH）₂ can ensure the pozzolanic reaction to be major.The mix proportion is 1.45g Na₂SO₄,0.9g Na OH,37.5g Ca（OH）₂,12.5g pozzolanic materials and 60g H₂O.（3）Both mechanical and chemical activation methods had excellent activation effect on NVP.The residual of 45μm sieve of undisturbed NVP was 16.45%and its strength activity index was 80.4%,which mean qualified pozzolanic activity.When the residual of 45μm sieve of NVP was less than 10%,the 28d compressive strength of composite system can be increased by 10%～20%.Among the activators,Li₂SO₄,Na₂SO₄ and TEA had a better activation effect,among which 0.5%Li₂SO₄ had the highest improvement in 28d compressive strength,up to 18%.The addition of activators can reduce the porosity and refine the pore distribution of the composite system.The incorporation of the compound activators decreased the strength of the composite system,and even led to the strength reduction at late age,while the combined effect of physical-chemical can improve the strength of the composite system at different ages.（4）The NVP had a significant acceleration effect on the setting of cement paste.The increase of fineness and the incorporation of single activators could promote the setting of composite system while compound activators could prolong the setting time.Due to the enhancement of TEA on the late strength and its good compatibility with other activators,TEA was used as the first activator in the orthogonal test.The influence of the three factors in the orthogonal test on the initial setting time was the second activator>fineness>TEA content,and on the final setting time was fineness>the second activator>TEA content.（5）The Modified Bingham model was suitable to decriable the rheological behavior of activated-NVPs-cement composite system.The addition of sodium salt can increase the dynamic yield stress,plastic viscosity and the structural build-up rate,but the addition of TEA would delay the structural build-up rate of the composite system.In the orthogonal experiment,the order of influence of three factors on setting time,dynamic yield stress and plastic viscosity was fineness>the second activator>TEA dosage.（6）The results of volume stability tests showed that the addition of Li₂SO₄ and Na OH increased the drying shrinkage of composite system,while Na₂SO₄ behaved the inhibition effect,calcium formate and TEA had little effect on the drying shrinkage.The inorganic activators promoted the early hydration,reduced the free water content of the composite system,and thus decreased the mass loss,while the organic activators would increase the mass loss.There are 87 figures,21 tables,and 142 references in this paper.