Experimental Study On Mechanical Properties And Chemical Resistance Of Xiashu Soil Modified By Magnesia-based Alkali-activated Ground Granulated Blastfurnace Slag

Portland cement is widely used in the field of soil modification,which has problems such as high energy consumption,heavy pollution,and poor chemical resistance.In recent years,the selection of soil modified materials is gradually changing out of consideration of environmental protection and sustainable development.Ground granulated blastfurnace slag(GGBS)as an industrial by-product has shown great potential in soil modification.Reactive magnesia(MgO)has been proved to be able to effectively activate GGBS and has many advantages such as high strength and good stability,but its application in soil modification is still less.In this paper,laboratory experiments were conducted to study the mechanical properties,hydration properties and chemical resistance of MgO activated GGBS modified soil and calcined dolomite activated GGBS modified soil,and analyzed its modification principle.Simultaneously,the erosion mechanism of sulfate and acid solution on the modified soil was analyzed.The main work and research results of the paper are as follows:1.The effects of MgO activated GGBS(MgO-GGBS)content,curing conditions,and soil particle aggregate size on the unconfined compressive strength(UCS)of modified Xiashu soil were studied,combined with the microstructure and hydration product analysis expound the influence mechanism of various factors on strength characteristics.The results show that MgO can effectively activate GGBS and obtain higher early strength.The UCS can generally reach 70%to 90%of cement modified soil after 7 days of curing,and the highest can reach 2.41MPa;after curing for 120 days,UCS is 4.43MPa,only 56%of cement soil.Properly increasing the curing temperature of the modified soil can effectively improve the strength of the modified soil.The strength of the sample at 35?curing temperature is generally 130%to 170%of the sample at 20?curing condition,and the highest can reach 240%.In addition,reducing the size of the soil particle aggregates will effectively increase the strength of the modified soil soil.The strength of the small aggregate samples is generally about 130%of the large aggregate samples,and the highest can reach 165%.The main hydration products of MgO-GGBS modified Xiashu soil are calcium silicate hydrate(CSH)and hydrotalcite-like;the dense hydration products are evenly distributed around the soil particles in the later period of curing,providing a good structure for the soil.At the same time,the hydrotalcite-like content increases in the later period of curing,which can effectively fill the pores and increase the strength of the soil.2.The effects of calcined dolomite activated GGBS(D800-GGBS)content,curing conditions and soil particle aggregate size on the UCS of modified Xiashu soil were studied,and explained the influence mechanism of different influencing factors on strength characteristics.The results show that calcined dolomite can effectively activate GGBS,and the influence of each influencing factor on the strength of D800-GGBS modified soil is similar to that of MgO-GGBS modified soil.But its early hydration rate is relatively slow,UCS is generally not high,the highest is 0.34MPa which is only14%of the MgO-GGBS modified soil samples at the same curing days.A stable strength growth rate was maintained in the middle and late stages of curing for D800-GGBS modified soil.UCS can reach 130%of the MgO-GGBS modified soil with the same curing condition when the curing reaches 120 days.D800-GGBS modified soil has a poor activation effect at 20?curing temperature,UCS is only 52%of MgO-GGBS modified soil under the same conditions and 31%of D800-GGBS modified soil cured at 35?for the same curing days.In the early stage of curing,the hydration product was mainly vanadinite with a little amount,and no obvious hydration product C-S-H was observed.In the later period of curing,products such as C-S-H can better fill pores,connect soil particles,and increase soil strength.3.The chemical resistance of MgO-GGBS modified Xiashu soil under the conditions of 3wt.%sulfate solution(3wt.%Na₂SO₄,3wt.%Mg SO₄)and acid solution(3wt.%CH₃COOH)were researched,and combining the analysis of reaction products and microstructure,the mechanism of mechanical properties changes in different solution environments is explained.The results show that the low content of MgO-GGBS modified soil is prone to disintegration in a salt solution environment.After being soaked in Na₂SO₄ solution for 7 days,the strength drop is concentrated in45%?60%,and the strength drop after 28 days is 50%?75%,the decrease in soil strength decreases with the increase in the amount of curing agent;The strength decrease is concentrated in 50%?65%after 7 days of Mg SO₄ soaking,and the strength decreases in 50%?76%after 28 days of soaking.The decrease in strength generally increases with the increase of the curing agent content,and vice versa in the later period.The intrusion of sulfate will cause the originally dense products to be eroded and produce swelling minerals such as hydrotalcite-like(Ht)and gypsum(Ca SO₄)or non-cemented M-S-H and other products,causing the strength of the soil to decrease or even disintegrate.Acetic acid solution can quickly dissolve the hydration products and make the strength of the modified soil decrease rapidly.Since the erosion of acetic acid does not produce expansive minerals,there is no swelling and squeezing between the particles inside the soil,and the overall preservation of the soil is relatively intact.There will be a phenomenon of partial destruction or overall disintegration of the soil.4.The chemical resistance y of Xiashu clay modified by D800-GGBS under the conditions of resisting 3wt.%sulfate solution(3wt.%Na₂SO₄,3wt.%Mg SO₄)and acid solution(3wt.%CH₃COOH)was explored.The results show that the samples with low-temperature curing and low curing agent content are easily disintegrated in the sulfate solution.The strength decrease of different samples in Na₂SO₄ solution is quite different(18%?70%after soaking for 7 days;38%?90%after soaking for 28 days);strength decreases by 25%?80%after soaking for 7 days in Mg SO₄and decreases by 35%?100%after soaking for 28 days.The soil strength decreases with the increase of the curing agent content.Due to the relatively high calcium content in D800-GGBS modified soil,it is easy to produce a large amount of calcium vanadium in the salt solution environment,which can fill the pores of the soil to a certain extent,and its salt solution corrosion resistance is stronger than MgO-GGBS modified soil under best conditions.But in acid solution,the decrease of D800-GGBS modified soil was significantly larger(the decrease was concentrated in 60%?70%after soaking for 7 days;after 28 days of soaking,the decrease was 70%?90%),which was slightly higher than that of MgO-GGBS under the same conditions.Modified soil.The residual Ca CO₃ in the soil after soaking can improve the chemical resistance of the soil in salt and acid solutions to a certain extent.5.Experiments prove that MgO-GGBS and D800-GGBS can effectively increase the strength of soil:the former can provide better early strength,while the latter can provide better long-term strength;under the conditions of this study,the lower curing temperature is not suitable for D800-GGBS modified soil.The salt-tolerant of D800-GGBS modified soil performs better than MgO-GGBS modified soil under suitable conditions,and vice versa under acidic solutions.