Research On The Mechanism And Road Performance Of Slag-based Geopolymer In Improving Coastal Saline Soil

The coastal area is the main carrier of Chinese population and economy,as well as the important support point of the three national strategies of"One Belt,One Road",the coordinated development of Beijing-Tianjin-Hebei and the Yangtze River Economic Belt,with a large amount of coastal saline soil.In particular,in the coastal areas of Bohai Bay,under the action of high salt-bearing strata composed of Marine sediments,seawater intrusion caused by groundwater overexploitation,and warm temperate sub-humid climate,coastal saline soil not only has a wide distribution range and high salinization degree,but also suffers from seasonal alternations of dissolution and salt swelling,and seasonal replenishment of salt and water.It is difficult to guarantee the curing stability of coastal saline soil in the region.Previous studies have shown that sodium chloride can play an auxiliary activation role on the slag-based geopolymer and at the same time constrain Cl~-in the insoluble crystal hydration products.By fully exploring the ability of slag-based geopolymer binding sodium chloride,the curing agent can be prepared to reduce the"degree of salinization",which is expected to solve the curing problem of coastal saline soil completely.The gelling property is the basis of the performance of hardener.Therefore,this study firstly started from the study of the hydration reaction mechanism of slag-based geopolymer gelling material under the activation of sodium chloride,gypsum and quicklime.Based on the hydration product analysis,thermogravimetric analysis and pore structure analysis of geopolymer paste,and the tests of mortar’s mechanical properties,shrinkage and cracking resistance,the composition was preliminarily optimized.Coastal saline soil contains high salt due to the action of tide and evaporation,and usually has the characteristics of collapsibility and salt expansion,which is very harmful to engineering construction.On the basis of the study on the gelling performance of the hardener,according to the salt-bearing characteristics of the saline soil,Na Cl is removed from the gelling material to make a slag-based geopolymer hardener for the coastal saline soil.By using the chloride salt and sulfate contained in the saline soil,the slag-based geopolymer hardener is conducted to cure the coastal saline soil.The curing effect on medium chlorine saline soil and strong chlorine saline soil was studied and compared.The main contents and conclusions of the study are as follows:(1)Sodium chloride,gypsum and quicklime show a synergistic effect in the combined activation of slag,which can obtain higher early strength,good stability and crack resistance,and less drying shrinkage than cement.Under the combined activation of sodium chloride,gypsum and quicklime,ettringite,Friedel’s salt,Kuzel’s salt and sodium-bearing zeolite were formed in the slag-based geopolymer.The cross-linking of these crystal products reduces the porosity and improves the compactness of the hardened paste.The addition of sodium chloride also generated Na OH in geopolymer,which improved the alkalinity of the system and further promoted the hydration of slag.(2)For the medium chlorine saline soil without sulfate,the content of gypsum and quicklime is adjusted when the sodium chloride component in the curing agent is removed.The unconfined compressive strength test shows that the optimal content of gypsum and quicklime in the curing agent is 7.5%and 15%respectively.The composite activated slag-based geopolymer curing agent has reached or exceeded the slag cement curing agent in unconfined compressive strength,softening coefficient and water stability against fresh water and sea water.With 12%curing agent content,the 7-day unconfined compressive strength can meet the strength requirements of heavy traffic expressway and primary highway subbase.(3)For the highly chlorinated saline soil containing more chlorine salts and sulfate,the sodium chloride component in the curing agent was removed,and the gypsum content was adjusted.Through the unconfined compressive strength test,the optimal gypsum content in the curing agent was 0%,indicating that the saline soil had enough chloride salts and sulfate to react with geopolymer.For the strong chlorine saline soil with very poor engineering properties,the composite activated slag-based geopolymer curing agent shows good curing performance.At 12%content,the 7-day unconfined compressive strength of solidified soil reaches the base strength standard of secondary and sub-secondary highways in medium and light traffic,and 2.33 times of PSA cement curing agent.And its softening coefficient,water stability resistance to fresh water and sea water are more than slag cement curing agent.(4)The composite activated slag-based geopolymer curing agent can use the chloride salt and sodium sulfate salt in the strongly chlorinated saline soil to generate crystal products such as ettringite,calcium hydroxide,Friedel’s salt and Kuzel’s salt.These crystal products cross connect and fill the pores,forming a tight structure with C-S-H gel and soil particles.It plays a similar role of"reinforcement"and"aggregate",and also reduces the salt content of the strong saline soil by 4.24%,showing a good curing effect.This is also an important reason why its curing property is better than slag cement.(5)The composite activated slag-based geopolymer curing agent does not use sodium hydroxide and other strong alkali,and can use a large number of slag produced in the iron and steel industry center in the region,has the advantages of low energy consumption,low carbon emissions and low price,its production energy consumption,carbon emissions and price are 70%,87%and 10%lower than PSA cement curing agent.