Study On The Mechanism Of Hydration Induction Period Of Portland Cement

Cement is one of the most important building materials because of its wide source of raw materials and low price.In the construction process,there are different requirements for the setting time of newly mixed cement-based materials.In order to control the setting time,it is necessary to understand the mechanism of the cement hydration process.The induction period is the crucial stage in the hydration process that determines the setting time of cement.At present,the major theories can not fully explain the hydration process in the induction period,and the mechanism of the hydration reaction in the induction period needs to be further clarified.In this paper,the effects of ion species(Ca²⁺,OH^-,SO₄^2-,Na⁺and K⁺)and their concentrations,material particle size（300μm,30μm,3μm,300nm quartz particles）,admixtures（calcium carbonate,magnesium calcium carbonate and strontium carbonate）and coagulants（glucose,cellulose,calcium formate and calcium chloride）on the induction period of hydration were studied.Firstly,the isothermal calorimeter was used to quantify their effects on the hydration of Portland cement.Then,the morphology of hydration products under the influence of different factors was observed by scanning electron microscopy.At the same time,the interaction of cement particles and admixtures with ions in solution was studied by zeta potential test.Furthermore,the main reasons affecting the hydration induction period of Portland cement are revealed by establishing the relationship between the adsorption of ions at the nano-scale,the formation of hydration products at the micro-scale,and the heat release of hydration at the macro-scale,so as to further clarify the mechanism of the hydration induction period of Portland cement.The results of this study may provide a theoretical basis and technical support for the regulation of cement setting time and the process of the hydration induction period.The results are as follows:（1）The increase of the main ion concentration in the cement pore solution will shorten the induction period,but its mechanism is different.The increase of Ca²⁺concentration shortens the saturation time of ion concentration,which promotes the nucleation and growth of hydration products,so the induction period is shortened.The increase of OH^-,Na⁺and K⁺concentration increases the p H value of the cement solution,it dissolves the gypsum,which promotes the dissolution of cement and shortens the hydration induction period.The increase of SO₄^2- concentration accelerates the dissolution of C₃A and promotes the hydration of C₃A,which lead to the rapid formation of ettringite and shortens the induction period.（2）The particle size of quartz will shorten the induction period.The heat release of cement paste with 300μm,30μm and 3μm quartz particles during the induction period is the same as that of cement paste,which has no effect on induction period.The 300nm quartz particles wrap a large amount of free water,which reduce the total amount of free water in the hydration reaction.This inhibits the early hydration and prolongs the induction period.（3）Carbonate admixture will shorten the induction period.Among them,strontium carbonate has a small particle size and a large specific surface area,it is easily attached to the surface of cement particles during hydration.It has a strong adsorption capacity for Ca²⁺,which promotes the nucleation of hydration products and shortens the induction period to the maximum.The adsorption capacity of calcium carbonate to Ca²⁺ is stronger than calcium magnesium carbonate,so the induction period is shortened more obviously.Calcium magnesium carbonate has the weakest adsorption capacity for Ca²⁺,so the induction period is shortened the least.（4）The coagulant will extend or shorten the induction period.Glucose and cellulose are attached to the cement surface,which hinders the nucleation of Ca²⁺ adsorption and prolongs the induction period.Calcium formate and calcium chloride increase the concentration of Ca²⁺ in the solution,it shortens the time for Ca²⁺ concentration to reach saturation,so the nucleation and growth of hydration products are promoted,thereby the induction period is shortened.