Physical And Mechanical Properties And Activation Mechanism Of Tuff-based Geopolymers

The preparation of tuff-based manufactured sand by dry method produced 20%tuff powder.The disposal of this solid waste will cause environmental pollution such as atmosphere,soil and water contamination.For now,there is still a lack of effective utilization method.Using this tuff powder to prepare geopolymer cementitious materials can not only diminish environmental pollutant,but also reduce carbon emissions and consumption of energy.Previous researches focused on the fly ash,blast furnace slag,metakaolin and other silicate vitreous amorphous and artificial silicon aluminum compounds.The investigation on natural minerals such as tuff is lack.In order to promote the secondary utilization of tuff powder,the preparation method,physical and mechanical properties and reaction mechanism of tuff-based polymer were studied in this paper.The main conclusions are summarized as following:（1）The preparation methods of tuff-based polymers were proposed.The tuff powder was taken as raw material.Five different concentrations of sodium hydroxide solution and 2 wt%sodium silicate solution were taken as activator.The optimum concentration of sodium hydroxide solution was determined as 7.5 M.Then,considering 7.5 M sodium hydroxide and 2 wt%sodium silicate solution as activator,the tuff-based complex polymer was prepared with different dosages of blast furnace slag（BFS）,and the optimum dosage of blast furnace slag was determined as 20%.（2）The comprehensive characterization of raw materials were performed.The specific surface area,particle size distribution,microstructure,chemical composition,bonding characteristics and thermal stability of tuff and BFS were characterized.It was found that the tuff powder was mainly composed of crystals thus the properties were stable.The BFS was mainly composed of silicate vitreous amorphous.Then,there were differences of activation mechanism between tuff and BFS.（3）The effects of sodium hydroxide solution concentration and BFS contents on mechanical properties of tuff-based polymers were investigated.Based on the compressive and bending tests,the results indicated that when the concentration of sodium hydroxide was 7.5 M and the BFS content was 20%,the compressive strength,bending strength and Young’s modulus reached to the highest after 28 days curing.The compressive strength of tuff geopolymer after 28 days curing firstly increased and then decreased with the increase of sodium hydroxide solution concentration.The maximum values were 69.33 MPa and 71.33 MPa when the concentrations were 7.5 M and 10 M.The bending strength after 28 days firstly decreased and then increased with the increase of sodium hydroxide concentration.When the concentration was 5 M,the minimum bending strength was 6.7 MPa.The compressive strength of tuff based composite geopolymer firstly increased and then decreased with the increase of BFS contents.When the content was 20%,the maximum compressive strength was 73.33MPa.The bending strength increased with the increase of BFS contents,and the maximum was 15.57 MPa corresponding to 30%BFS.（4）The effects of sodium hydroxide concentration and BFS contents on physical properties of tuff-based polymers were investigated based on the mass and volume measurement.The density was highest when the concentration of sodium hydroxide was 10 M,and the volume shrinkage was the highest when the concentration of sodium hydroxide was 7.5 M.Adding BFS reduced the volume shrinkage,and the density and volume shrinkage were highest when the BFS content was 30%.（5）The effects of sodium hydroxide concentration on the dissolution of tuff-based polymers were characterized by ICP-OES.The results showed that the dissolution degree of tuff powder increased with the increase of sodium hydroxide solution concentration.（6）The effects of BFS contents on setting time of tuff-based polymers were revealed by the Vicat tests.The result showed that the initial setting time and final setting time of samples decreased firstly with the increase of BFS contents,and then remained nearly unchanged after the content was greater than 20%.（7）The effects of sodium hydroxide solution concentration and BFS content on the microstructure of tuff-based polymer were characterized by SEM-EDS.As the result shown,the amount of gel increased with the increase of sodium hydroxide concentration and BFS contents.The main component in the samples without BFS was N-A-S-H,and that with BFS were N-A-S-H and C-A-S-H cross-linked substances.The proportion of C-A-S-H increased with the increase of BFS content.（8）The effects of sodium hydroxide solution concentration and BFS on the products of tuff-based polymers were studied by using XRD,FTIR and TG/DTG method.The change of sodium hydroxide concentration did not influenced the component of the products,but affected the degree of crystal dissolution and the amount of cementitious materials.The addition of BFS could make the samples generate new C-（A）-S-H,Ca（OH）₂,sodium zeolite and Ca CO₃,and the number of water molecules in the samples increased with the increase of BFS content.（9）The effects of sodium hydroxide solution concentration and BFS dosage on the environmental p H value of tuff-based geopolymer were investigated.The p H testing of the leaching solution revealed that p H value increased with the increase of sodium hydroxide concentration and BFS contents.The variation of p H with curing period corresponding to strength development well.When the p H value was lowest,the strength reached to the maximum.The excessive OH^-resulted the products in depolymerize.The electrostatic repulsion at end between the lamellar particles increased,which caused the particles to point-to-surface contacts.Thus the strength of the samples reduced.With the increase of BFS contents,more sodium ions and calcium ions precipitate and more C-（A）-S-H and N-A-S-H were hydrolyzed.The moveable OH^-in the system increased,so as the p H of pore water increased.