Performance And Deterioration Mechanism Of Geopolymers Under Sulfate And Acid Environment

Geopolymer is a new type of green gelling material made by the reaction of alkaline activator and activated silica-alumina material,which has the potential to partially replace traditional cement for marine concrete.However,there are a wide variety of silicon-alumina raw materials currently in use,which may show different durability in practical applications;The durability of geopolymers has not been considered in complex marine environment or coupled environment.Exploring the durability and degradation mechanism of geopolymers in different systems under the action of complex erosive ions(SO₄^2-,H⁺,Mg²⁺and Cl^-)in the marine environment has important guiding significance for the design and preparation of highly durable geopolymers.In this paper,the cement-based materials are used as a comparison to study the influencing factors and degradation mechanisms of different systems of geopolymers during the sulfate attack process;Explore the acid resistance of geopolymers in different systems and the formation mechanism of surface degradation layer;Combined with the research results,the influencing factors of the composite erosion process of Mg²⁺,SO₄^2-and HCl were explored,and the effects of single hydrochloric acid,magnesium sulfate and composite erosion on the degradation of geopolymer performance were compared and analyzed.The main conclusions drawn through the above studies are as follows:（1）Different calcium content of silicon aluminum raw material will affect the type of geopolymer reaction product.During MgSO₄ erosion,Mg²⁺will cause ion exchange with Na⁺in the N-A-S-H gel of MK-based geopolymer,resulting in deterioration of performance;The appearance of SG-based geopolymer deteriorated obviously after 120days of magnesium sulfate corrosion,and the compressive corrosion resistance coefficient was 0.73 which showing the poor durability.MK+SG-based geopolymer produces a densely structured N（C）-A-S-H gel,which has excellent resistance to magnesium sulfate corrosion,the compressive corrosion resistance coefficient is 0.96,and the dense microstructure can be maintained after 120 days of erosion;The early alkali activated of FA is not good.Products of FA+SG-based geopolymer are mainly C-A-S-H gel and unactivated FA spherical particles.Compressive corrosion resistance coefficient of FA+SG-based geopolymer mortar deteriorated during the erosion age is about 0.72,which did not meet the actual engineering durability performance requirements.（2）The presence of NH₄⁺in the attack of ammonium sulfate accelerates the reaction process of C-A-S-H gel and SO₄^2-to form gypsum,Compressive corrosion resistance coefficient of FA+SG-based geopolymer and SG-based geopolymer are 0.68 and 0.5,The structure of N-A-S-H gel is stable and is less eroded by NH₄⁺.Due to the lack of ion exchange erosion of Mg²⁺on N-A-S-H gel,compressive corrosion resistance coefficients of MK-based geopolymer and MK+SG-based geopolymer are 1.08 and 0.82,which are slightly higher corrosion of magnesium sulfate solution under the same corrosion conditions.The semi-immersion condition exacerbates the erosion and destruction of geopolymer by magnesium sulfate.The strength of the SG-based geopolymer and FA+SG-based geopolymer immersion zones decreased by 52.23%and 59.19%compared with the full immersion group.The"wick effect"accelerates the migration of erosion ions and promotes erosion damage under the condition of half immersion,MK-based geopolymer and MK+SG-based geopolymer are stable due to the product structure,and are less affected by erosion and damage which drying area and soaking area have similar appearance and compressive strength.（3）The MK-based geopolymer exhibits good durability in both pH=2 sulfuric acid and hydrochloric acid attack.During the acid attack,the MK-based geopolymer generates a well-defined surface degradation layer.After acid attack,the gel structure of the MK-based geopolymer surface layer product changed from Q⁴（3Al）structure to Q⁴and Q⁴（1Al）structure,which confirmed the formation of surface degradation layer.However,the N-A-S-H gel shrinkage during acid attack to form cracks throughout the degraded layer,which provides a channel for the acid to further attack the internal materials.The MK-based geopolymer has better acid attack resistance,which depends more on N-A-S-H gel’s better acid attack resistance than the effect of surface degradation layer.（4）After acid attack,the gel structure of the surface product of the MK+SG-based geopolymer changed from Q²（1Al）to Q³,Q⁴（1Al）and Q⁴ structures.The MK+SG-based geopolymer has fewer surface deterioration layer thickness and micro-cracks than MK-based geopolymer.The MK+SG-based geopolymer mortar specimens at different attack ages maintain the highest compressive strength and exhibit the best acid resistance.The C-A-S-H-Q² and C-A-S-H-Q²（1Al）of C-A-S-H gel is the main product of the SG-based geopolymer,which were transformed into Q³ structure after acid attack,but there wasn’t clear surface degradation layer was found.Due to the lack of"barrier effect"of the surface deterioration layer,the SG-based geopolymer exhibited the worst acid attack resistance in sulfuric acid and hydrochloric acid solutions.（5）The combined attack of HCl and MgSO₄ is dominated by MgSO₄ attack in pH=6.The H⁺has a small effect on the degradation of the N-A-S-H gel structure of the MK-based geopolymer product.It mainly reacted with NaOH in the pore solution to reduce the internal pH value of the geopolymer and destroy the stable environment of N-A-S-H gel.After 35 days of attack,the compressive strength is 36.79%lower than the MgSO₄ solution group;the high-calcium system:SG-base geopolymer’s main product C-A-S-H gel has poor durability.Both H⁺and SO₄^2-will caused damage to the product structure,and Ca²⁺is precipitated to gypsum;low-calcium system:MK+SG based geopolymer’s main product N（C）-A-S-H gel has a dense structure and has less affected by H⁺and MgSO₄.After 35 days of attack,the compressive strength can still reach93.1MPa.The concentration of H⁺is relatively high in the pH=2 environment.The compound attack of HCl and MgSO₄ is mainly based on the structure of the H⁺degraded gel.The Ca-containing product gel reacts with MgSO₄ to gypsum after the Ca was removed.The compound attack advances from the outside to inside with the deterioration of H⁺.The overall deterioration of geopolymer is same to sulfuric acid attack in pH=2.