Appropriate 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.

Based 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 SO₄^2- to C₃A of 0.5 and 1.5, and one type of cement with mixed gypsum was prepared at a molar ratio of SO₄^2- 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.

For 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.

The 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.

In application, different types of concrete admixtures are usually mixed to solve different problems. The varying adsorption capacities of organic molecules or polymer additives on particle surface of different mineral phase result in “competitive” or “preferential” behaviors, leading to intricate interference or synergistic effects among the additives. Competitive adsorption occurs among retarders, viscosity modifying admixtures (VMA), and superplasticizers, especially when the surface coverage is relatively high. The investigation of competitive adsorption relies on the quantification of each type of admixture based on the special characteristic signal of each admixture (e.g. signal by light scattering or gel permeation chromatography due to the clear difference of solution size between VMA, polycarboxylate superplasticizer (PCE), and retarder molecules; signal of phosphorus in some P-containing retarders). Labeling of PCE molecule by chromophores is an effective strategy. However, the low signal intensity of the labeled PCE in the few former reports limited the universality. Herein the paper reported the synthesis of a labeled PCE by the grafting of naphthylamine onto backbone as chromophore. The mixture of naphthalene-ring-labeled PCE and conventional PCE could be quantified based on UV absorption spectra and total organic carbon (TOC) method. The effect of adding two PCEs with different side chain lengths on the fluidity of cement paste was studied.

Jiangsu Helin P·Ⅱ 52.5 Portland cement with density of 3.06 g/cm³ and BET specific surface area of 0.83 m²/g was used. The labeled PCE (PCE-A) was prepared by the simultaneously grafting naphthylamine (99%, Aladdin reagent) and polyetheramine (monofunctional, primary amine, with a terminal methyl group, from Zhejiang Lukean Chemical Co., Ltd) to polyacrylic acid (weight-averaged molecular weight M_w 2000) in the presence of 1% (in mass) H₂SO₄ under vacuum at 130 ℃. The molar ratio of chain unit as confirmed by ¹HNMR spectra was acrylic acid/naphthylamine/polyetheramine = 4.73/0.70/1.00. PCE-B (poly(methacrylic acid) grafted with poly(ethylene glycol) monomethyl ether (MPEG, M_w 1000), molar ratio of methacrylic acid to MPEG was 3/1) was supplied by Jiangsu Sobute New Materials Co., Ltd.

The UV absorption spectra was recorded between 200–400 nm. All the cement pastes were prepared with water to cement ratio of 0.18. The flow spread of cement paste (GB/T 8077—2012) with the addition of only PCE-A, only PCE-B, and both PCEs was measured, respectively. The total adsorption amount of PCE was measured by TOC method. The adsorption amount of PCE-B when both PCEs were added, was calculated based on UV absorption spectra. The standard curve (UV absorbance at 305 nm against TOC concentration) was obtained by the supernatant of cement paste when only PCE-B was added. The amount of ettringite (AFt) was calculated based on total heat absorbed between (40–105 ℃) in differential scanning calorimetry (DSC) as compared with synthetic AFt. The morphology of AFt was investigated by scanning electron microscope (SEM).

The main UV absorption band of PCE-A (naphthene ring) located between 250 nm and 320 nm, which enabled the differentiation with the background from the pore solution. The quantification by the UV method was confirmed by the supernatant of cement paste with only PCE-B. An excellent linear relationship (correlation coefficient of over 0.99) was found between absorbance at 295,305 nm and TOC concentration.

When only one type of PCE was added, the flow spread of cement paste achieved a maximum value at ~6 mg/g, then slight decrease could be observed at higher dosage. The maximum flow spread of cement paste with PCE-A (lower adsorption affinity, but higher saturated adsorption amount) is much larger than PCE-B, indicated a stronger steric hindrance, due to the longer side chain. When both PCE-A and PCE-B were added, at PCE-A of 2 mg/g and 4 mg/g, the flow spread of cement paste underwent a gradual increase and then decrease (especially after PCE-B of 4 mg/g) with the increase of dosage of PCE-B. At PCE-A of 6 mg/g, lower flow spread would be found at higher dosage of PCE-B. Regardless of PCE-A dosage, the adsorption amount of PCE-A would be lower at higher PCE-B dosage. The total adsorption amount always became lower at higher dosage of PCE-B, except for the condition of PCE-A 2 mg/g and PCE-B lower than 4 mg/g. The behind reason was, PCE-B could occupy the adsorption site for PCE-A. At fixed PCE-A dosage, the total surface coverage always increased with the addition of PCE-B. However, at high PCE-A dosage, the replacement of surface-adsorbed PCE-A would reduce the steric hindrance.

The increase of PCE dosage induced lower amount of AFt with finer morphology. The size of AFt particle with PCE-B is smaller than PCE-A, due to the even higher adsorption affinity for the inhibition of particle growth. When both PCEs were added, high amount of PCE-B would promote the transformation of AFt particles from rod-like to fine needle-like. The aspect ratio of AFt increased from ~10.6 (PCE-B 2 mg/g) to ~14.1 (PCE-B 6 mg/g). The increase of aspect ratio would result in worse particle packing and therefore lower flow spread.

A PCE labeled with naphthalene ring (UV chromophore) was prepared based on the grafting reaction of naphthylamine, which enabled the quantification of solution concentration in cement paste by absorbance at ~300 nm and therefore the investigation of competitive adsorption with another PCE. At extremely low water to cement ratio (0.18), as the dosage of short side chain PCE increases, the adsorption amount gradually increases, while the adsorption amount of long side chain PCE gradually decreases. At high dosage of PCE with long side chain, with the increase of dosage of short side chain PCE, part of the sites that could have been occupied by long side chain PCE are occupied by short side chain PCE. In addition, the morphology of ettringite (AFt) changes from rod-like to finer needle-shaped, showing a larger aspect ratio, which will deteriorate the packing behavior. The fluidity of the cement paste is therefore reduced.