IntroductionAppropriate workability is a key to ensuring the smooth pouring, building of concrete, and quality of the project. However, in practical applications, the workability of concrete often undergoes unexpected drastic changes in the early mixing stage, such as rapid loss of flowability, false set and so on. The change of flowability of cement paste overtime in the early stage is mainly related to the early hydration of cement and the adsorption behavior of superplasticizer. Cement pastes with different mineral compositions can exhibit different change rules overtime. In the early stage of hydration, especially in the first hour, the hydration reaction of tricalcium aluminate (C₃A) mainly occurs. Since the reaction of C₃A with water is extremely rapid, a large amount of calcium sulfate is incorporated into cement, and gypsum dihydrate (G, CaSO₄·2H₂O) is usually the most important source of calcium sulfate. Although the existing studies focus on the influence of gypsum types on the hydration of C₃A, the fundamental reasons are still need to be explored.In this paper, the effect of gypsum types was considered to elucidate the fundamental reasons behind the drastic changes in early flowability of fresh concrete. Five cements were prepared using clinker combined with different types of gypsum (i.e., G, gypsum dihydrate and H, hemihydrate gypsum). The study focused on the time-dependent changes in the flowability of pastes within the first hour, and examined the processes such as adsorption of superplasticizer, dissolution of mineral phase and precipitation of hydration products, thus analyzing the intrinsic mechanisms behind the time-dependent changes in flowability.MethodsBased on the content of C₃A in the clinker, four types of cements with a single gypsum type were prepared via adding G and H, respectively, at two molar ratios of SO₄2⁻ to C₃A of 0.5 and 1.5, and one type of cement with mixed gypsum was prepared at a molar ratio of SO42⁻ to C₃A of 1.5 and a molar ratio of G to H of 2. In the test, the water binder ratio was fixed at 0.3, and the amount of PC was adjusted to achieve the similar initial flowability (i.e., (250 ± 5) mm at 2 min). At 2, 5, 10, 20, 30 and 60 min after adding water, the paste was taken out for various tests (i.e., flow spread, analysis of interstitial liquid, adsorption and hydrated sample preparation).The flow spread of paste was tested according to the Chinese standard GB/T 8077—2012. The adsorption behavior of PC and the concentrations of [Ca] and [S] in the interstitial liquid were obtained by TOC and inductively coupled plasma mass spectrometry (ICP-MS) on the interstitial liquid obtained by centrifugation. The hydration was stopped with cold isopropyl alcohol (5 ℃) to perform the microscopic analysis on the hydrated sample, including differential scanning calorimetry (DSC) and surface area measurement by BET based on nitrogen adsorption.Results and discussionFor five groups of cement pastes with the similar initial flowability, the addition of G leads to a rapid loss of flowability in the first 10 min, and increasing the amount of G can reduce the loss of flowability. The addition of H also leads to a rapid loss of flowability in the first 10 min, but then the flowability slowly increases. Moreover, the higher the amount of H added is, the more significant loss of flowability in the first 10 min and the greater the subsequent increase of flowability will be.For cement with only G added, the precipitation of ettringite mainly occurs in the first hour, and AFt rapidly forms in the first 5 min and then increases slowly. However, for cement with H, two processes occur simultaneously, i.e., ettringite precipitation and H dissolution -G precipitation. H promotes the formation of ettringite, and the ettringite content in the paste is higher than that with only G added.For cement with only G added, the precipitation of ettringite leads to a large increase in the specific surface area of the system, which in turn causes the loss of paste flowability over time. For cement with H added, the precipitation of ettringite and large-sized G leads to a rapid loss of the early flowability of the paste, but the gradually decreasing sulfate ion concentration in the interstitial liquid is conducive to the adsorption of PC on the surface of the mineral phases, which in turn leads to a reverse increase in the subsequent flowability.ConclusionsThe dissolution of mineral phases and the precipitation/growth of hydration products were key factors causing the drastic changes in early fluidity of the paste. For cement pastes containing dihydrate gypsum, the formation of ettringite led to a significant increase in the specific surface area of the samples, causing a time-dependent loss in fluidity. In contrast, for cement paste containing hemihydrate gypsum, in addition to ettringite, the precipitation of large-sized dihydrate gypsum accelerated the early loss of flowability. However, the gradually decreasing concentration of SO₄^2- was conducive to the adsorption of superplasticizer, thus leading to an increase in flowability of paste.