Experimental Study On MgO-CaO Mixtures Activated Ground Granulated Blastfurnace Slag And The Applicaticpn In Soil Stabilization

It is now accepted that new binders are needed to replace ordinary Portland cement(OPC)for its high energy consumption,high CO2 emission and poor durability performanee.Alkali-activated slag,among other industrial by-products,has shown its superiority in low energy consumption,low CO2 emission,high strength and good durability and is an alternative binder to partially replace OPC.Using conventional alkali activators are suffering issues like over fast setting time,large shrinkage,safety conceems in using strong alkali materials and uneconomical efficiency,which urges the development of novel activators,including reactive magnesium oxide(MgO)and calcium oxide(CaO).This paper investigates the mechanical and hydration properties of MgO-CaO mixtures activated ground granulated blastfurnace slag(GGBS).The MgO-CaO mixtures are from both artificial mixing and calcined dolomite.In addition,the durability of novel alkali-activated slag in chemical solutions and the application of activated slag in soil stabilization are also explored.Based on the results obtained by lab experiments,the following statement can be made:1.The mechanical and hydration properties of slag pastes activated with MgO-CaO mixtures of 7 different ratios were investigated.The effect of MgO/CaO ratio on the strength,hydration extent,hydration products and microstructure were discussed.The results indicated that the addition of CaO in reactive MgO could greatly accelerated the hydration of slag in the early age.In general,the extent of hydration decreased with increasing MgO/CaO ratio and increased with increasing curing periods.The main hydration products of MgO-CaO mixtures activated slag were C-S-H gel and hydrotalcite-like phases.Portlandite and calcite were also observed in samples with high CaO content.The strength of slag was greatly dependent on the curing conditions and the water to binder ratio.2.Two kinds of calcined dolomite(D800 and D1000,calcined at 800oC and 1000oC for 1 hour,respectively)were prepared in lab conditions and conducted to characteristic analysis.The mechanical and hydration properties were investigated.The results revealed that:Calcined dolomite could effectively activate the slag.The use of D1000 could induced higher hydration extent at each curing period than D800 did.The poring filling effect of calcite from D800 enhanced the early strength of D800 activated slag.By increasing the dosage of calcined dolomite,the strength of slag can be increasing and the increase was more significant using D800 than using D1000.The main hydration products of calcined dolomite activated slag were C-S-H gel and hydrotalcite-like phases while D10OO also induced the presence of portlandite.Calcined dolomite activated slag showed its superiority in carbonation resistance.3.The durability of reactive MgO activated slag(MAS)and CaO activated slag(CAS)in 5%Na2SO4,5%MgSO4,5%H2SO4 and 5%CH3COOH solution was studied.By analyzing the development of strength,weight change,corroded depth and reaction products,the performance of MAS and CAS under chemical attack was compared.The enhanced resistance of MAS to chemical attack was mainly attributed to the more compacted structure and the absence of portlandite in MAS.4.The durability of D800 activated slag(D800AS)and D1000 activated slag(D1000AS)in 5%Na2SO4,5%MgSO4,5%H2SO4 and 5%CH3COOH solution was studied.The results revealed that the calcination temperature of dolomite was the controlling factor of the resistance of slag to chemical attack.D800AS showed better resistance to all used chemical solutions than D1000AS except the long term durability in CH3COOH solution.The presence of calcite was found to increase the resistance of calcined dolomite activated slag to chemical attack in this study.5.Initial investigation of the application of MgO-CaO mixtures activated slag in soil stabilization was carried out.Two model soils were prepared in the lab:one clayey soil and the other sandy soil.The effect of MgO/CaO ratio and curing period on the stabilization was examined.Moreover,the resistance of stabilized clayey soil to water corrosion was also investigated.The results revealed that:the strength of stabilized clayey soil increased with the MgO/CaO ratio.However,the use of MgO-CaO mixtures in sandy soil stabilization induced lower strength than using individual MgO or CaO.In order to keep the resistance of stabilized clayey soil to water corrosion,the MgO/CaO should not be higher than 1 in clayey soil stabilization.6.Initial investigation of the application of calcined dolomite activated slag in soil stabilization was carried out.Calcined dolomite blended with slag is an altermative soil stabilizer.The efficiency greatly depended on the dosage of stabilizer,the type of soil and the calcination temperature of dolomite.