Coupling Effects Of Inorganic Ions In Sea Water On Cemnet Hydration

In the background of the increasing shortage of freshwater resources,population growth leads to an increase in infrastructure construction and the construction of China's Maritime Silk Road.As an important component of concrete,the mixing water will be consumed.Therefore,the study of seawater as mixing water is a very attractive exploration,which is consistent with significance of sustainable development and national strategic significance.The preparation of concrete by sea water instead of fresh water is different from that of marine concrete.Seawater is directly involved in cement hydration in seawater mixed concrete.The hydration process of cement directly determines the workability,durability,mechanical properties and other key properties of concrete structures.Seawater is rich in many kinds of inorganic salt ions,especially Na⁺,Mg²⁺,Cl^-and so on.In this paper,a series of studies have been carried out on cement hydration under the coupling action of inorganic salt ions in seawater.Firstly,the basic properties of cement paste under the action of NaCl and MgCl₂ are studied,and its influence on the macroscopic properties of cement paste is analyzed.Secondly,the hydration-related properties of cement under the action of NaCl and MgCl₂ are studied,and the hydration mechanism of C₃S under the action of NaCl and MgCl₂ is explored.Finally,the hydration properties of C₃A+gypsum system under the action of NaCl and MgCl₂were studied,and the hydration mechanism was explored.The conclusions are as follows:?1?When NaCl is added into cement paste,the fluidity has no obvious effect;setting time is shortened;compressive strength of cement paste is increased in early stage?1 d,3 d,7 d?,but not in later stage?28 d?;flexural strength of cement paste is increased in early stage?1 d,3 d?,decreased in 7 d,and no obvious effect in later stage?28 d?;porosity of cement paste is reduced.When MgCl₂ is added into cement paste,the fluidity of cement paste will be significantly reduced with the critical concentration of 0.28 mol/L;the setting time decreases with the increase of MgCl₂concentration;the early compressive strength increases with the critical concentration of 0.28 mol/L,the later compressive strength has no obvious effect;the early flexural strength increases with the critical concentration of 0.28 mol/L;The flexural strength of cement paste decreases by 7 d and 28 d with the low concentration of MgCl₂?<0.28 mol/L?,while the flexural strength of cement paste increases slightly by high concentration MgCl₂ for 7 d and 28 d;The porosity of cement paste decreases.?2?When NaCl is added to cement,the induction period shortens,the acceleration period speed increases,and the main peak appears earlier.When MgCl₂ is added to cement,the induction period shortens,the acceleration period speed increases,and the main peak appears earlier.Moreover,with the increase of MgCl₂ concentration,its influence on the hydration process of cement becomes more obvious.Na⁺and Mg²⁺have different effects on the hydration process of cement.Compared with Na⁺the effect of Mg²⁺on cement hydration is more obvious,and the effect of Cl^-is more obvious when Mg²⁺is added.?3?When NaCl is added to cement,the hydration products are C-S-H,CH and Friedel salts.The structure of hydration products is loose and C-S-H is clustered like fibers.When MgCl₂ is added to cement,the hydration products are C-S-H,CH and Friedel salts,and there are a litter MH.The structure of hydration products is compact and similar to that of rocks.The clustered distribution of C-S-H is almost weakened,and has geometrically shaped hole.In the hole found some spherical accumulation of hydration products and plate-like hydration products.The addition of NaCl and MgCl₂ in cement can significantly affect the distribution of CH.Mg²⁺and Na⁺will affect the formation of Friedel salt,adding NaCl will form more Friedel salt.?4?NaCl accelerates the hydration process of cement and changes the hydration products.The reason is that the adsorption of Na⁺on the surface of C-S-H affects the growth of CH.Cl^-participates in the reaction to form Friedel salt,which is adsorbed on the surface of C-S-H.MgCl₂ accelerates the hydration process of cement and changes the hydration products.The reason is that Mg²⁺participates as MH.Mg²⁺adsorbs on the surface of C-S-H.Some Mg²⁺dissolves in C-S-H and affects the crystal growth of CH.Cl^-participates in the reaction to form Friedel salt and adsorbs on the surface of C-S-H.?5?The addition of NaCl in C₃S shortens the induction period,accelerates the acceleration period and shortens the main peak time.The hydration products are C-S-H and CH,which have a little effect on the morphology of C-S-H,and promote the formation of CH.Na⁺promotes the crystallization of CH and forms a cationic adsorption layer on the surface of C-S-H,which affects the adsorption of Cl^-on C-S-H.When MgCl₂ is added to C₃S,the induction period is shortened,the acceleration period is accelerated,and the main peak time is shortened;and with the increase of MgCl₂ concentration,this effect is more obvious;hydration products are C-S-H,CH and MH,but the content of MH is very small,which obviously changes the morphology of C-S-H and tends to foil-like growth with more porosity;Mg²⁺will affect the growth of CH and partly replace Ca²⁺in C-S-H as substituted solid;Mg²⁺adsorbs on the surface of C-S-H to form a cationic adsorption layer,which affect the adsorption of Cl^-to C-S-H.Compared with Na⁺,Mg²⁺has more obvious effect on hydration and Cl^-has more obvious effect.?6?When NaCl is added into C₃A+gypsum system,the first stage is almost unchanged,the strength of main peak increases and the width of peak narrows;the hydration products are C₃AH₆,AFm,and related amorphous phases,Friedel salt and Na₂SO₄.The surface of hydration products is more granular and compact,and there are smaller crystals attached to the sheets.Cl^-acts to form Friedel salts,while Na⁺acts to form sodium sulfate crystals in porous solution,providing more nucleation sites for the second stage.When C₃A+gypsum system is added with MgCl₂,the first stage time is shortened and the main peak strength is increased;hydration products are C₃AH₆,AFm,and related amorphous phases,Friedel salt and MH;hydration products are mainly composed of non-directional growth of sheet-like substances and surface granular distribution deposits,and the structure is compact,the surface of sheet-like substances is rough,the concentration is further increased,and the edges and corners are weakened;Cl^-acts as Friedel salt,and Mg²⁺competes with Ca²⁺to reduce the surface adsorption of Ca²⁺or CaSO₄ on C₃A.Mg²⁺binds with OH^-to form MH,which provides more nucleation sites.