Effect Of Magnesium Oxide On Shrinkage Behavior Of Alkali-activated Slag Mortars

Alkali-activated slag binder(AAS)is a kind of hydraulic cementitious material made of alkali metal compound to activate ground granulated blast furnace slag.Compared with ordinary Portland binder,AAS has excellent mechanical properties and resistance to chemical corrosion.Development of AAS is of great significance to enrich the types of binder products and promote the structural adjustment of cement industry.Nevertheless,there are also many disadvantages of AAS including high autogenous and drying shrinkage,which significantly hinders its widespread useage.It is necessary to study the methods about reducing the shrinkage of AAS concrete for enhancing the AAS concrete applications.In this paper,the effects of magnesium oxide on the shrinkage behavior of alkali(water glass)activated slag was studied.And the effects of magnesium oxide activity and content on autogenous shrinkage and drying shrinkage of alkali slag mortar under different alkali equivalents were discussed.In addition,the effects of SCEA(CaO expansion agent)and calcined dolomite on the shrinkage behavior of alkali activated slag mortar was studied,and the reasonable dosage range of these materials for reducing the shrinkage of alkali activated slag mortar was preliminarily determined.The hydration,hardening and structure formation of AAS pastes with and without magnesium oxide and SCEA and burnt dolomite were investigated by thermogravimetric analysis,X-ray diffraction and scanning electron microscopy techniques.The results show:(1)The shrinkage of the alkali slag binder mixed with MgO is affected by the activity and content of MgO and sodium concentration of activator.When the sodium concentration is 4%,the autogenous shrinkage of the AASM(alkali activated slag binder mortar)increases with the MgO content,the drying shrinkage is not affected by the MgO,and no siginificant effects of MgO activity on the shrinkage of AASM were observed.When the sodium concentration is 5%,the greater the amount of MgO added,the lower the activity of MgO,the higher drying shrinkage and autogenous shrinkage of AASM were achieved.(2)The autogenous shrinkage and drying shrinkage of alkali activated slag mortar decreased with the calcium(the dolomite calcined at 1000 ? for 1h).Add 2.5% to 5.0% of SCEA or D1000 can effectively reduce the shrinkage of alkali activated slag mortar.When the amount of calcium cementious materials is more than 5%,alkali activated slag mortar has the risk of cracking caused by excessive expansion of SCEA or the initial flow is significantly reduced caused by the LBD 1000.(3)The effect of MgO on the hydration and hydration products of alkali activated slag binder was different under different sodium concentration and activator.When the sodium concentration is 4%,the MgO decreases the hydration rate of decay period of water glass activated slag binder,and the hydrotalcite-type phase in the hydration product increases slightly with the increase of the MgO content.When the sodium concentration is 5%,the MgO prolongs the hydration induction period and the hydration acceleration peak is delayed,the type and amount of hydration product does not change with the amount of MgO.When the activator is NaOH,the hydrotalcite-type phase of alkali activated slag binder is significantly increased.(4)The hydration and hydration products of magnesium oxide in alkali avtivated slag binder are affected by activators and slag.The degree of hydration of magnesium oxide in the same time in different liquid phase components is: water>NaOH solution> water glass solution.In the magnesium oxide-pure water-slag system,the presence of slag inhibition the hydration of magnesium oxide,and with the magnesium oxide content increase,the magnesium oxide-related hydration products from a single hydrotalcite-like phase transition to hydrotalcite-like and brucite coexistence.(5)The addition of SCEA or D1000 shortens the hydration induction period of the alkali activated slag binder,and the hydration acceleration peak appears in advance,which the impact of D1000 is more significant?The Ca(OH)2 increased with the increase of SCEA content in the hydration product of the alkali activated slag binder.A lower degree of polymerization and an increase in the Ca/Si ratio in the C-S-H gel structure are achieved in the alkali activated slag binder with the D1000.These factors are the main reason for the decrease in shrinkage of alkali activated slag mortar mixed with calcium cementicious materials.