Hydration Mechanism And Durability Of Binary And Ternary Composite Cement Incorporating Ferronickel Slag And Lithium Slag

Ferronickel slag（FNS）is a kind of magnesium-rich industrial waste produced in the process of smelting ferronickel,which can be divided into blast furnace FNS and arc furnace FNS according to the difference in production process,and the majority of FNS in China is arc furnace FNS.At present,the annual emission of FNS in China is nearly 30 million tons,only12%of which is used to prepare glass ceramics or used as mine backfill materials and concrete aggregate,the utilization rate is still low.Lithium slag（LS）is a gypsum-rich byproduct derived from the production of lithium carbonate by sulfuric acid process.Producing 1 ton of lithium carbonate produces about 10 tons of LS,now the annual discharge of LS in China is as much as 800,000 tons,among which,only a small part are used in construction projects,the rest are stacked in the open air or used for road landfill.It is an effective way to use FNS/LS as concrete mineral admixture.In order to determine whether the introduction of FNS in concrete can cause the volume expansion and whether the introduction of LS in concrete can cause the delayed ettringite formation（DEF）,firstly,the phase composition and characteristics of FNS/LS were analyzed in this paper;secondly,the influence of hydration environment on the hydration properties of FNS/LS was studied;then,the sulfate resistance of FNS/LS blended cement cement mortar was studied under both early age steam and standard cured conditions;finally,the hydration properties of FNS blended cement,LS blended cement and blended cement incorporating FNS and LS were studied.The main results are as follows:（1）Large size FNS sand is mainly composed of mineral crystalline phase（forsterite and enstatite）,while small size FNS sand is mainly composed of amorphous phases,therefore,MgO in FNS exists in two forms,one is mineral crystalline phase,the another is amorphous phase.The use of FNS sand can be classified according to its particle size:the large size FNS can be used to replace part of concrete aggregate,while the small size FNS can be ground and used as concrete mineral admixture.（2）LS is an acid and porous material,whose pores are mainly mesopores of 2⁵0nm,caused by layered leached spodumene,carbon black,porous diatomite,etc.The main elements in LS affecting cement hydration are S,Ca,Si,Al,K,etc.,while the influence of Li can be neglected.In addition to gypsum,the reason why LS has relatively high activity is also related to sodium sulfate,sodium carbonate and lithium carbonate in LS.（3）The amorphous MgO in FNS can react to form hydrotalcite and other products under standard curing,steam curing and autoclave curing conditions.The amorphous Fe₂O₃ in FNS can partially replace Al₂O₃ and participate in the formation of hydrated aluminate phase.However,the crystalline forsterite and enstatite in FNS are still relatively stable under autoclave curing condition.（4）Most of Al2O3 and SiO2 in LS mainly exist in LiAlSi2O6 while only a few in amorphous phases.The reaction of LiAlSi₂O₆ in cement is slow under standard curing condition,but steam curing and autoclave curing can accelerate this reaction.Due to the high SO₃ content in LS,DEF is observed in standard cured LS cement paste,however,initial steam curing has been proved to be an effective way to eliminate this risk.（5）The hydration products of FNS activated by CH and gypsum are porous C-S-H gels rich in Mg and Al with low Ca/Si ratio as well as AFt,monocarbonate,CaCO₃ and hydrotalcite,etc.The role of FNS in cement mainly consists of filling effect and pozzolanic reaction.（6）Hydration products of LS activated by CH mainly include reticular C-A-S-H gels with low Ca/Si ratio,AFt,monocarbonate and CaCO₃,however,with the increase of curing time and the decrease of alkalinity in pore solution,the hydration product AFt will decompose to form AFm and gypsum,etc.In addition to filling effect and pozzolanic reaction,the hydration mechanism of LS in cement still include adsorption of free water from fresh cement paste by lithium slag and the early age chemical activation of sulfate and carbonate.（7）Under both the early age steam curing and standard curing,20%FNS addition can improve the sulfate resistance of cement mortar under dry-wet cycles and partial immersion,20%LS addition can improve the sulfate resistance of cement mortar under partial immersion but decease the sulfate resistance under dry-wet cycles.This is because the high temperature and high humidity of the dry-wet cycles accelerated the reaction of LiAlSi2O6（especially for Al₂O₃）in the LS,which led to the formation of a large number of ettringite under the condition of sulfate attack.（8）When the replacement level of cement is less than 20%,the coupled effect of FNS and LS is greater than that of single FNS or LS:the cumulative hydration heat of blended cement containing FNS and LS in 72 hours is higher than that of FNS blended cement or LS blended cement with the same replacement level.The addition of both FNS and LS in cement can improve the early strength of FNS blended cement mortar,decrease its drying shrinkage,and reduce the risk of DEF in LS blended cement paste.