IntroductionRetarders are necessary in construction projects such as long-distance transportation, high-temperature construction in summer and large-volume concrete construction. However, in practical applications, abnormal setting phenomena such as rapid or non-setting of concrete often occur due to the possibly incorrect or excessive mixing, which can affect the progress of the project and cause economic losses. It is thus of great significance for accurately regulating the setting and hardening properties of cement-based materials to clarify the cement hydration process and its mechanism under the action of retarders. Citric acid (CA) and its salts (mainly sodium citrate (SC) and potassium citrate (PC)) are commonly used as retarders. However, there are still some controversies about their effect and mechanism in the existing studies. In this paper, CA, SC and PC with different dosages and addition methods were used to clarify the effect and mechanism of these retarders on cement hydration and solve the problem of abnormal setting during the use of such retarders. Their effects on the setting time of cement paste, hydration heat, and adsorption capacity under the action of different sulfate carriers (anhydrite (AH) and hemihydrate (HH)) were investigated. In addition, the possible mechanism was also analyzed.MethodsA fixed water-to-cement mass ratio of 0.4 was used. The dosage of citric acid and its salt was calculated based on a mass ratio of citrate to cement, and the dosage of citrate ion was 0.045%, 0.090%, 0.180% and 0.360% (mass fraction), respectively. Two addition methods were used, i.e., W addition method and S addition method. For W addition method, the retarders were first dissolved in water and then mixed with cement. For S addition method, the retarders were first mixed with cement as a solid powder, and then the mixture was added to water. Two kinds of cement with different types of sulfate carriers were prepared, i.e., AH-contained cement prepared with anhydrite and HH-contained cement prepared with hemihydrate.The setting time was measured according to the Chinese standards GB/T 1346—2011. The hydration of cement paste at (25.00±0.02) ℃ was measured by a model TAM Air 8-channel isothermal calorimeter (Newcastle, DE, USA). The adsorption capacity of different retarders after hydrating for 5 min was measured by a model TOC-L total organic carbon analyzer (Shimadzu Co., Japan). The change in conductivity when AH and HH dissolved in CA, SC and PC solutions with a citrate concentration of 0.047 6 mol/L was determined by a model FE38 conductivity meter (Mettler Toledo Co., Switzerland). In addition, the ionic concentrations of calcium and sulfate during the dissolving process were measured by a model 5800 inductively coupled plasma emission spectrometer (Agilent Co., USA).Results and discussionThe effect of retarders on the cement hydration process can be affected by different addition methods, dosages, cations of retarders and different sulfate types in cement. For cement with different sulfate carriers, the setting time of HH-contained cement is longer than that of AH-contained cement regardless of the dosage of retarder and the addition method used. The abnormal rapid setting phenomenon of retarder can occur in the AH-contained cement rather than in HH-contained cement. Regardless of the type of retarders used, the initial dissolution capacity of HH is greater than that of AH. For different addition methods, the retarder with S addition method shows a greater retarding ability. The effect of citric acid and citrates indicates that the retarding ability of PC is greater than that of SC.The results of hydration heat show that a phenomenon of abnormal rapid setting in AH-contained cement can be caused due to the rapid hydration of the aluminate-containing phase. The results of conductivity and ionic concentration indicate that HH quickly dissolves and releases sufficient sulfate ions, thus inhibiting a rapid reaction of C₃A and avoiding the occurrence of rapid setting. Moreover, the ability of PC to promote the dissolution of sulfate carrier is greater than that of SC possibly due to its greater complexing ability with calcium ions. Therefore, the complexation can play an important role in determining the effect of retarders.ConclusionsThe type of sulfate carrier in cement could change the effect of citric acid/citrate. For AH-contained cement, there existed a phenomenon of first retarding and then rapid setting with the increase of retarder dosage. Such a phenomenon was avoided in HH-contained cement. The addition method of citric acid/citrate greatly changed its effect. The S addition method of retarders enhanced its retarding ability. The adsorption and complexation of citric acid/citrate with calcium ions both played an important role in determining the cement hydration process.