Calcium Sulfoaluminate Cement Pastes Research Articles

Effects of EVA and SAE Emulsions on Hydration and Drying Shrinkage of Calcium Sulfoaluminate Cement Pastes

Effects of EVA and SAE Emulsions on Hydration and Drying Shrinkage of Calcium Sulfoaluminate Cement Pastes

A coupled chemo-transport-mechanical damage model for the mineral transformation in cementitious materials under a sulfate-rich environment

The present study proposes a coupled chemo-transport-mechanical damage model integrating thermodynamics, fluid-solid transformation, and mechanical damage modeling to predict the physicochemical characteristics of cementitious materials under a sulfate-rich environment. For verifying the proposed model, calcium sulfoaluminate (CSA) cements with various molar ratios of calcium sulfate over ye'elimite (m-values) were adopted for a case study. The thermodynamics model for the phase assemblage of CSA cements with various m-values under a sulfate-rich environment is compared with the experimental outcomes. The model parameters in the fluid-solid transformation and mechanical damage models are fitted through the available experimental data in the literature. Mineral compositions, crystallization pressure, effective elastic modulus, delayed ettringite formation, and microcracks of CSA cement pastes were modeled by a proposed coupled chemo-transport-mechanical damage model, with model parameters fitted to experimental data, thereby assessing the applicability of the proposed model. The predicted results show that an increase in m-value of the samples increases the rate of sulfate ion penetration and crystallization pressures. Moreover, DEF, microcrack evolution, and mechanical degradation can be attributed to the amount of ye'elimite and hydrated monosulfate of the samples.

Effects of calcined phosphogypsum replacement on hydration and properties of calcium sulfoaluminate cement at different curing temperatures

The present research aims to understand the cement hydration and microstructure formation mechanism of calcined phosphogypsum (CPG) supplemented calcium sulfoaluminate (CSA) pastes cured under hot-weather temperatures and to ensure the appropriate application and effective service of CSA cement-based materials, as well as to address the problem of overstocking of CPG. The effects of CPG dosage on the heat of hydration, electrical resistivity, hydration products, and bound water content of CSA cement pastes were investigated at curing temperatures of 20 °C, 35 °C, and 50 °C. Hydration kinetics was studied to elucidate the hydration mechanism, and thermodynamic modelling was performed to predict the phase assemblages. The results show that the incorporation of CPG facilitated the formation of ettringite and significantly reduced the transformation of ettringite to monosulfate, and these phenomena were enhanced with an increase in the CPG. The addition of CPG at elevated curing temperature (35 °C and 50 °C) significantly improved the compressive strength of CSA cement pastes. The growth rate and nucleation rate predicted by the BNG model for the CSA cement pastes cured at 50 °C were substantially higher compared to those at 20 °C.

Adhesion evaluation and interface characterization of 3D printed concrete for automatic repair

3D concrete printing can be used for automatic repair, aiming at increasing efficiency and reducing the use of manpower, as compared to conventional manual repair. Ensuring strong adhesion of the 3D printed materials and characterizing the interface is crucial for the success of this approach. To this end, we first evaluated the adhesion performance of 3D printed concrete based on a low-carbon binder, i.e., calcium sulfoaluminate (CSA) cement. CSA cement pastes with different re-dispersible polymer powder substitution rates were used as coating before 3D printing. We used two approaches, X-ray computed tomography (CT) and scanning electron microscopy (SEM), to characterize the interface region. Results indicate that the yield stress of the 3D printing material and the coating material plays an important role in the adhesion in both the fresh and hardened state. Moreover, the limited adhesion at the overlay interface can be due to the limited contact at the centimeter and micro scale, as reflected by CT and SEM, respectively. Applying a coating can fill the air voids at the interface, while the placement of a coating may not improve the bond strength and can even have a negative impact due to the limited liquid medium at the interface.

Influence of Nanosilica and Superplasticizer Incorporation on the Hydration, Strength, and Microstructure of Calcium Sulfoaluminate Cement Pastes

Influence of Nanosilica and Superplasticizer Incorporation on the Hydration, Strength, and Microstructure of Calcium Sulfoaluminate Cement Pastes

Effect of Class C and Class F Fly Ash on Early-Age and Mature-Age Properties of Calcium Sulfoaluminate Cement Paste

To promote the sustainable development of eco-efficient calcium sulfoaluminate (CSA) cements through the partial replacement of the CSA clinker with supplementary cementitious waste products, the effects of coal fly ashes on the early-age and mature-age properties of a calcium sulfoaluminate (CSA)-based cement paste were investigated. The impacts of both Class C and Class F fly ashes on the rheological properties, hydration kinetics, and compressive strength development of CSA cement paste were studied. Rheology-based workability parameters, representing the rate of loss of flowability, the rate of hardening, and the placement limit, were characterized for the pastes prepared with fixed water-to-cement (w/c) and fixed water-to-binder (w/b) ratios. The results indicate a slight improvement in the workability of the CSA paste by fly ash addition at a fixed w/b ratio. The isothermal calorimetry studies show a higher heat of hydration for the Class C fly ash-modified systems compared to the Class F-modified systems. The results show that fly ash accelerates the hydration of the calcium sulfoaluminate cement pastes, chiefly due to the filler effects, rather than the pozzolanic effects. In general, ettringite is stabilized more by the addition of Class F fly ash than Class C fly ash. Both fly ashes reduced the 1-day compressive strength, but increased the 28-day strength of the CSA cement paste; meanwhile, the Class C modified pastes show a higher strength than Class F, which is attributed to the higher degree of reaction and potentially more cohesive binding C-S-H-based gels formed in the Class C fly ash modified systems. The results provide insights that support that fly ash can be employed to improve the performance of calcium sulfoaluminate cement pastes, while also enhancing cost effectiveness and sustainability.

Nondestructive Evaluation for Hydration and Setting Time of Gypsum Modified Calcium Sulfoaluminate Cement Paste.

Calcium sulfoaluminate (CSA) cement is a promising solution for reducing CO2 emissions. While previous studies have attempted to investigate the usefulness of CSA cement via various approaches, early age nondestructive evaluations for the setting and hydration of CSA cement mixtures have not been reported. In this study, we measured the ultrasonic pulse velocity and electrical resistivity of early age CSA cement paste. Six types of samples were prepared according to different water-to-solid ratios and different amounts of gypsum. In addition, various microstructural analyses were performed to understand CSA cement hydration with the obtained nondestructive parameters. Consequently, the effect of added gypsum in CSA cement paste was discussed in terms of ye'elimite dissolution and the precipitation of ettringite, and different pore distributions produced by added gypsum were discussed in terms of compressive strength. The 5% addition of gypsum in CSA cement paste enhanced the hydration evolution, such as ettringite, and it can induce the faster setting time up to 6 h and strength development during 24 h.

Hydration characteristics of calcium sulfoaluminate-fly ash mixed with different alkalies

Due to its notable low carbon footprint, calcium sulfoaluminate (CSA) cement has become a research hotspot for cement chemists in recent years. Relevant studies have shown that supplementary cementitious materials (SCMs) can greatly improve the properties of CSA cement pastes, but the utility of SCMs in CSA cement has not yet been fully exploited due to their low alkalinity. To stimulate the activity of pozzolanic materials, the effect of different alkalies on the hydration of CSA-fly ash (CSA-FA) was systematically investigated, and the results indicate that after calcium hydroxide (Ca(OH)2) is incorporated into the CSA-FA paste, it first forms a saturated solution and then rapidly hydrates to form ettringite. The initial pH value of the suspension is approximately 12.65, which then slumps to a level lower than that without Ca(OH)2. The initial pH value of the CSA-FA suspension mixed with 1 M sodium hydroxide (NaOH) solution is 13.0, which is then rapidly decreased to a stable value of approximately 11.3. Ca(OH)2 can promote the hydration of CSA to form ettringite and improve the compressive strength of the paste, and the larger the Ca(OH)2 content, the more obvious this effect. NaOH inhibits the hydration of CSA. With increasing NaOH concentration, the compressive strength of the paste first increases and then decreases, and the effect of inhibiting the hydration of CSA becomes stronger. The CSA-FA paste blended with 1 % Ca(OH)2 and 1 M NaOH shows collapsing and cracking behavior, for which a large number of well-developed flaky aluminum hydroxide crystals are formed, and strong diffraction peaks for unhydrated CSA are observed.

Quantifying the Workability of Calcium Sulfoaluminate Cement Paste Using Time-Dependent Rheology.

Poor workability is a common feature of calcium sulfoaluminate (CSA) cement paste. Multiple chemical admixtures, such as set retarders and dispersants, are frequently employed to improve the workability and delay the setting of CSA cement paste. A quantitative assessment of the compatibility, efficiency, and the effects of the admixtures on cement paste workability is critical for the design of an appropriate paste formulation and admixture proportioning. Very limited studies are available on the quantitative rheology-based method for evaluating the workability of calcium sulfoaluminate cement pastes. This study presents a novel and robust time-dependent rheological method for quantifying the workability of CSA cement pastes modified with the incorporation of citric acid as a set retarder and a polycarboxylate ether (PCE)-based superplasticizer as a dispersant. The yield stress is measured as a function of time, and the resulting curve is applied to quantify three specific workability parameters: (i) the rate at which the paste loses flowability, (ii) the time limit for paste placement or pumping, marking the onset of acceleration to initial setting, and (iii) the rate at which the paste accelerates to final setting. The results of the tested CSA systems show that the rate of the loss of flowability and the rate of hardening decrease monotonously, while the time limit for casting decreases linearly with the increase in citric acid concentration. The dosage rate of PCE has a relatively small effect on the quantified workability parameters, partly due to the competitive adsorption of citrate ions. The method demonstrated here can characterize the interaction or co-influence of multiple admixtures on early-age properties of the cement paste, thus providing a quantitative rheological protocol for determining the workability and a novel approach to material selection and mixture design.

Effects of sodium aluminate and quicklime on the properties of CSA grouting materials

With the advantages of rapid setting, high strength and durability properties, calcium sulfoaluminate (CSA) cement is usually used as rapid repairing material and grouting material in building and other engineering projects. In order to improve the physical properties of CSA grouting materials, the effect of adding sodium aluminate and quicklime was investigated through hydration heat, DTA-TG, XRD and SEM. Their effects on the kinetics of ettringite formation in CSA grouting materials were analyzed. The results show that with the increase of sodium aluminate in Slurry A (CSA cement paste) from 0% to 1.5% (by weight of CSA cement), the setting time of the grouting materials gradually shortened, the compressive strength at early ages (2 h and 3 days) rapidly increased, but the compressive strength at 28 days decreased continuously. The greatest compressive strength was achieved when 20% quicklime and 80% anhydrite were used in Slurry B (quicklime + anhydrite paste). The results from hydration heat, DTA-TG and XRD show that the formation of ettringite (AFt) was accelerated at early ages by sodium aluminate and quicklime, and quicklime could accelerate the formation of AFt at all ages. It was found that sodium aluminate only accelerated the early formation of ettringite but did not affect the overall amount of hydration products, and the initial accelerated hydration could provide a proper structural layout for the later strength development.

Influence of metakaolin and calcined montmorillonite on the hydration of calcium sulphoaluminate cement

Calcium sulfoaluminate (CSA) cement-calcined clay blends have shown great potential as new lower CO2 footprint cementitious materials. The influences of metakaolin and calcined montmorillonite on the hydration of CSA cement were studied from aspects of setting time, physical strength and drying shrinkage, while fly ash was chosen for comparison. Meanwhile, hydration heat evolution and the microstructure of hydrates of modified CSA cement paste at early age were determined by means of calorimetry, XRD, SEM and DTA-TG. Results show that both metakaolin and calcined montmorillonite do not contribute to the formation of new kinds of hydrates. They shorten the setting time and reduce the strength, while effectively reduce the cumulative hydration heat within 24 h and inhibit the drying shrinkage of CSA cement mortars. By contrast, calcined montmorillonite has a better shrinkage compensation than metakaolin, owing to that it contributes to the formation of ettringite. It is notable that low dosage of calcined montmorillonite can effectively decline the drying shrinkage of CSA cement mortar, whereas higher content is detrimental to the strength development. It lies in that calcined montmorillonite dramatically accelerates the initial heat evolution rate of CSA cement paste in 0.4 h and leads to a rapid setting. On the contrary, increasing amount of metakaolin prominently reduces the drying shrinkage of CSA cement mortar, with a slight shortening on the setting time.

Experimental Study on the Rheology and Setting Behavior of Calcium Sulfoaluminate Cement Paste Modified with Styrene-Butadiene Copolymer Dispersion

Experimental Study on the Rheology and Setting Behavior of Calcium Sulfoaluminate Cement Paste Modified with Styrene-Butadiene Copolymer Dispersion

Effect of the nanosilica source on the rheology and early-age hydration of calcium sulfoaluminate cement pastes

This work investigates the role of different nanosilica (NS) sources on the rheology and early-age hydration of calcium sulfoaluminate (CSA) cement paste. Specifically, pastes were produced with three NS (two powder and one colloidal suspension) in 1 wt% replacement of cement, with and without superplasticizer. Rotational rheometry, isothermal calorimetry, and in-situ X-ray diffraction (XRD) tests were conducted. The results showed that incorporation NS increased the yield while decreased the viscosity and delayed the shear thickening onset point of CSA pastes, regardless of the NS source. Calorimetry and in-situ XRD revealed that NS incorporation enhanced the CSA cement hydration kinetics while superplasticizer delayed it. Overall, colloidal NS led to the best overall early-age performance, leading to the lowest viscosity and the highest hydration degree over time among the CSA cement pastes investigated.

Thermo-chemo-mechanical characterization, modeling, and analysis of hydration of calcium-sulfoaluminate cement paste

The reaction kinetic of a commercial calcium sulfoaluminate (CSA) cement was characterized by heat flow calorimetry. Distinct behavior is observed whether tartaric acid is added as retarding agent or not. Employing the so-obtained calorimetric data, heat of hydration and chemical affinity of the investigated binder mixture was determined. Furthermore, continuous ultrasonic propagation velocity measurement gave access to the stiffness gain of paste samples. However, the ultrasonic test setup is characterized by a non-constant temperature history (temperature rise and fall). Modeling the temperature history and the history of the degree of hydration with a thermo-chemical analysis scheme, the intrinsic material function of the stiffness evolution of the investigated hydrating CSA paste was obtained. Furthermore, the robustness of the thermo-chemical analysis scheme was demonstrated by the comparison of the measured with the simulated temperature evolution. The versatility of the approach may contribute to a systematic assessment and optimization of the heat/mechanical evolution of CSA binder systems under various geometric, temporal, and thermal boundary condition.

State of water in calcium sulfoaluminate cement paste modified by hydroxyethyl methyl cellulose ether

To obtain satisfying water retention and improved viscosity, cellulose ether (CE) has been widely used in fields such as tile adhesives, repair mortars and self-levelling mortars. But the impacts of CE on the hydration and microstructure of calcium sulfoaluminate (CSA) cements, especially on the hydration and water evolution are still missing some basic insights. This paper mainly focusses on the influence of two hydroxyethyl methyl cellulose ether (HEMC) on the water evolution and hydration properties of commercial CSA cements with variable composition. The properties at fresh state such as setting times, air void content and water retention were recorded, while the interaction between HEMC and CSA cements was investigated by the combination of calorimetry, 1H NMR and DSC-TG. Results indicate that regardless of the composition of CSA cements, HEMC greatly increases the air void content and water retention. Setting time and hydration heat evolution within 1 d as well as the formation of cement hydrates (ettringite, monosulfate and alumina gel) are only slightly or unaffected by the addition of HEMC. As shown in the T2 distribution of 1H NMR pattern, it is interesting to notice that the addition of HEMC leads to the appearance of a new peak around 100 ms–1000 ms within 1 h and it disappears after 150 min. This finding is assigned to the evolution of a special water state in cement pastes containing HEMC. Additionally, variations in the polymer structure of HEMC have comparatively weaker impact than CSA cement type on the properties of prepared cementitious materials.

Effect of calcium sulfoaluminate cements composition on their durability

Calcium Sulfoaluminate Cements (CSA) are considered as an alternative to Ordinary Portland Cement (OPC) for some applications. There is still a lack of knowledge on their durability and degradation mechanisms. The goal of this study is to investigate the influence of CSA cement composition on CSA cement paste durability. In addition to a commercial cement, two CSA cements with different amounts of ye’elimite (50 and 75 w.t. %) were synthetized for this study Three cement pastes were prepared and leached for one year in pure water at a temperature of 20 °C. The leachates were analyzed by ICP/AES and the cement pastes were characterized by X-Ray Diffraction (XRD), thermogravimetric analysis (TGA/DTG), Mercury Intrusion Porosimetry (MIP), and Scanning Electron Microscope (SEM). The results show that the leaching mechanism is first dissolutive than diffusive independently of the composition of calcium sulfoaluminate cement. Before leaching, the paste generated by the cement containing 50 w.t. % of ye’elimite had the highest porosity and was the only one to contain Stratlingite. During leaching, this material exhibited the lowest decalcification rate and showed the smallest degraded zone after one year of degradation. This study reveals that the presence of stratlingite has an important role in improving the durability of calcium sulfoaluminate cement paste.

Water retention mechanism of cellulose ethers in calcium sulfoaluminate cement-based materials

To explore the mechanism to water retention of cellulose ether (CE) in calcium sulfoaluminate (CSA) cement-based materials, three cellulose ethers (CEs) with similar viscosity but different substitution groups were selected in this paper, namely hydroxyethyl cellulose (HEC) and hydroxyethyl methyl cellulose (HEMC) with high and low degree of substitution (DS). Water retention of CE modified CSA cement mortar with different CE dosages was characterized by filter paper method. The water state and content in CE modified CSA cement paste were determined by low field nuclear magnetic resonance (1H NMR). The adsorption and dispersion characteristics of CE in CSA cement paste were analyzed by measuring the organic carbon content in the pore solution. Combined with the measurement of dynamic viscosity and surface tension of pore solution, the mechanism to water retention of CEs in CSA cement-based materials was comprehensively analyzed. The results show that the addition of CE can significantly improve the water retention of CSA cement mortar. At the same dosage, the water retention effect of HEMC is better than HEC, and DS has little effect on the water retention capacity of HEMC. The mechanism to water retention of HEC and HEMC added less than 0.1% dosage in system is governed by water sorption effect of CE. While more than 0.1% dosage, HEC and HEMC in CSA cement-based materials achieve water retention respectively through the adsorption of CE on cement and the formation of three-dimensional polymer network in pore solution.

Effect of Steel Slag on Shrinkage Characteristics of Calcium Sulfoaluminate Cement

The effects of steel slag with 0, 10%, 20 % and 40% content on the chemical shrinkage, autogenous shrinkage, internal relative humidity, and drying shrinkage of calcium sulfoaluminate cement paste were studied. The results show that the compressive strength of calcium sulfoaluminate cement paste at an early stage decreases gradually when the content of steel slag increases. When the steel slag content is 0 and 10%, the compressive strength of hardened cement pastes gradually decreases at 90 and 180 days, but the samples with steel slag content of 20% and 40% maintain the compressive strength growth within 180 d. With the extension of curing period, the gap of compressive strength is gradually narrowed. The autogenous shrinkage decreases with the increase of steel slag content and has a good linear relationship with the relative humidity inside the paste. The proportion of autogenous shrinkage to chemical shrinkage is deficient, and most chemical shrinkage occurs in the form of the pore volume. Although the trends of drying shrinkage and autogenous are consistent, the former is more severe than the latter.

Mechanical and Microstructural Characteristics of Calcium Sulfoaluminate Cement Exposed to Early-Age Carbonation Curing.

The early-age carbonation curing technique is an effective way to improve the performance of cement-based materials and reduce their carbon footprint. This work investigates the early mechanical properties and microstructure of calcium sulfoaluminate (CSA) cement specimens under early-age carbonation curing, considering five factors: briquetting pressure, water–binder (w/b) ratio, starting point of carbonation curing, carbonation curing time, and carbonation curing pressure. The carbonization process and performance enhancement mechanism of CSA cement are analyzed by mercury intrusion porosimetry (MIP), thermogravimetry and derivative thermogravimetry (TG-DTG) analysis, X-ray diffraction (XRD), and scanning electron microscope (SEM). The results show that early-age carbonation curing can accelerate the hardening speed of CSA cement paste, reduce the cumulative porosity of the cement paste, refine the pore diameter distribution, and make the pore diameter distribution more uniform, thus greatly improving the early compressive strength of the paste. The most favorable w/b ratio for the carbonization reaction of CSA cement paste is between 0.15 and 0.2; the most suitable carbonation curing starting time point is 4 h after initial hydration; the carbonation curing pressure should be between 3 and 4 bar; and the most appropriate time for carbonation curing is between 6 and 12 h.

Pore characteristics of calcium sulfoaluminate cement paste with impact of supplementary cementitious materials and water to binder ratio

The main hydration products of calcium sulfoaluminate (CSA) cement were analyzed by X-ray diffraction and thermogravimetry. The pore characteristics of CSA cement paste with the incorporation of different supplementary cementitious materials (SCMs) were studied by a mercury intrusion porosimeter. Water-to-binder ratios between 0.24 and 0.73 were used in the mixtures. The microstructure of the CSA cement concrete was observed by backscattering electron microscopy. The test results indicate that compared with ordinary Portland cement (OPC) paste, CSA cement paste has a coarser pore volume. Silica fume can substantially reduce the median pore diameter (MPD) of CSA cement paste. With the action of SCMs and the adjustment of the water to binder ratio, the pore structure of the CSA cement paste is significantly refined, harmful pores and air pores are greatly reduced. SCMs can play a substantial filling role in CSA cement concrete, which can optimize the pore structure and improve the related properties.