The Thermodynamics and Kinetics of Early Cement Hydration Based on Pore Solution Analysis and Improved BNG Model

Author:Chen Heng

Supervisor:sun wei jiang jin yang paulo jmmonteiro


Degree Year:2018





Cement hydration is the origin of all the performance of cement-based materials.The early behavior of cement hydration determines the effectively available time of transportation and casing of fresh concrete.In addition,the late behavior of cement hydration determines the basic properties of hardened concrete such as the mechanical performance and durability.Therefore,the investigate on the mechanism of cement hydration of significance in the point of views of engineering practice and theory science.By now,the investigation of cement hydration mechanism is focused on the induction period,acceleration period and deceleration period,and no consensus is reached on their mechanisms.Cement hydration is mainly composed of the dissolution of clinker(mainly C3S)and precipitation of hydration products(mainly C-S-H).Pore solution is the chemical environment of the two processes,therefore,it will contribute to reveal the mechanism of all periods in cement hydration by investigating the influence of pore solution on the thermodynamics and kinetics of cement hydration.Therefore,this study first statically analyzed the relation between the induction period and acceleration period,based on which discussed the roles of C3S dissolution and C-S-H precipitation in the two periods.Then,the influence of pore solution on cement hydration from the point of view of thermodynamics was investigated by exploring the effect of the calcium and pH of pore solution on the rate of cement hydration and chemical driving forces of all processes during cement hydration(C3S dissolution and the precipitation of C-S-H,CH,and gypsum).Finally,based on the improved BNG model,investigated the influence of pore solution composition on kinetics of cement hydration.First,this study investigated the relation between the induction period and acceleration period by statically analyzing the heat flow curves of different cement systems such as systems with chloride salts(sodium chloride,magnesium chloride and calcium chloride),small organic molecules,organic polymers and composite mixture of mineral admixture and chemical admixture.The results showed that there is linear relationship between the length of induction period and the silicate peak time in all the systems.This means that the change of induction period length(35)Li is related with the change of acceleration period length(35)La by the equation:(35)La=(s-1)(9)(35)Li.In addition,the slope of systems with organic admixtures is around 1((35)La≈0),indicating the organic admixtures have little effect on acceleration period.For the system with sodium chloride,the slope of the fitting line is around 2((35)La≈(35)Li),suggesting that both the induction period and the acceleration period are controlled by a same process.For the systems with calcium chloride and magnesium chloride,the slopes of their fitting lines are around 5((35)La≈4(35)Li),suggesting that these chloride salts have higher influence on acceleration period than on induction period.The above phenomenon can be reasonably interpreted by the assumption that the induction and acceleration are controlled by the nucleation and growth of C-S-H,respectively.Second,by the addition of calcium chloride(calcium concentration:0.06 mol/L and0.12 mol/L),hydrochloric acid(hydrogen concentration:0.09 mol/L and 0.18 mol/L),and potassium hydroxide(hydroxyl ion concentration:0.28 mol/L and 0.56 mol/L)into mixing water,this study investigated the influence of calcium concentration,pH value,undersaturation degree of C3S,and supersaturation degree of C-S-H on the rate of cement hydration.The results showed the undersaturation degree of C3S kept increasing whereas the supersaturation degree of C-S-H kept decreasing,meaning that the chemical driving force of C3S dissolution kept increasing and that of C-S-H kept decreasing.This indicates that the rate of C-S-H precipitation is higher than that of C3S dissolution,which can be reached according to constantly reduction of silicate concentration with time.Since C3S hydration is a multistep reaction composed of C3S dissolution and C-S-H precipitation,the slower step of C3S dissolution would be the direct factor(limiting factor)of C3S hydration.This conclusion is further verified by the apparent positive correlation of undersaturation of C3S with the length of induction period and the silicate peak intensity.Since higher C-S-H precipitation rate is the reason for constantly increasing undersaturation degree of C3S,the precipitation of C-S-H should be the deciding factor(controlling factor)of undersaturation degree of C3S and the overall rate of C3S hydration.Considering the positive correlation of calcium concentration with the length of induction period and the silicate peak intensity,it can be inferred that the increase of calcium concentration(decrease of pH)would promote the precipitation of C-S-H,increase the undersaturation degree of C3S,and finally the rate of C3S dissolution and cement hydration.Third,by the addition of oxalic acid and potassium oxalate with different molar ratio,this study investigated the coupling effect of calcium concentration and pH value on cement hydration.The results showed that in the systems with the same amount of oxalate ions(total amount of potassium oxalate and oxalic acid)but with different oxalic acid(it will remove calcium and hydroxyl ions)and potassium oxalate(it will remove calcium ions but increase hydroxyl ions)ratio,with the increase of oxalic acid in the mixing water,the length of induction period increases linearly and the intensity of silicate peak decreases linearly.Furthermore,the addition of nitric acid with the same amount of hydrogen ions with oxalic acid in above systems and the addition of potassium oxalate with the same amount of potassium ions with potassium oxalate in above systems would have little influence on cement hydration.It indicates that the hydrogen ions of oxalic acid and the potassium ions of potassium oxalate are not the key factors and the amount of oxalic acid in mixing water are the key factor.That is to say,simultaneous removal of calcium ions and hydroxyl ions is the main reason for inhibiting cement hydration instead of the only removal of calcium ions or hydroxyl ions.Finally,by the fitting on both the heat flow and cumulative heat(the sulfate depletion peak removed)of different systems with calcium chloride,hydrochloric acid and potassium hydroxide with improved BNG model,this study analyzed the correlation between pore solution composition,thermodynamics and kinetics of cement hydration.Results showed that with the increase of the linearly growth rate of C-S-H,the length of induction reduces linearly and the intensity of silicate peak increases linearly,suggesting that linearly growth rate of C-S-H is the key factor of cement hydration.With the increase of calcium concentration,the linearly growth rate of C-S-H increases linearly and gradually level off at around 30 mM,suggesting that calcium concentration is main factor of the linearly growth rate of C-S-H.