Calcium Sulfoaluminate Cement Pastes Research Articles

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.

Influence of Different Lithium Compounds on Hydration and Mechanical Properties of Calcium Sulfoaluminate Cement.

This work investigated the influence of three different lithium compounds, lithium carbonate (Li2CO3), lithium sulfate (Li2SO4) and lithium chloride (LiCl), on the hydration and mechanical properties of calcium sulfoaluminate (CSA) cement mixtures. Five concentrations of Li+, 0, 0.05, 0.11, 0.16 and 0.22 mmol/g of cement, were chosen, and then the proportions (by mass) of three lithium compounds were determined. Compressive strengths at 8 h, 24 h and 28 days were tested. Meanwhile, an early hydration heat test, thermogravimetric (TG) analysis, X-ray diffraction (XRD) and scanning electron microscope (SEM) techniques were performed to study the influences of different lithium compounds on properties of CSA cement mixtures. The experimental results show that three lithium compounds can all accelerate the early hydration process of CSA cement. There is not a remarkable difference on the properties of CSA cement pastes with a different content of Li+. The anion of lithium compounds can also affect the properties of CSA cement pastes, the accelerating effects of LiCl and Li2SO4 are more significant than that of Li2CO3, but there is not a distinct difference between LiCl and Li2SO4.

Effect of steel slag on the hydration and strength development of calcium sulfoaluminate cement

In this study, effects of steel slag on the hydration and strength development of calcium sulfoaluminate (CSA) cement were investigated. Pastes with 0%, 10%, 20%, and 40% (by mass) of steel slag replacements for CSA cement were tested for setting time, chemical shrinkage, electrical resistivity, and compressive strength. The hydration products, bound water content, pore structure, and pH value of the pore solutions of these pastes were also analyzed. The results indicate that at early ages (from 2 h to 28 d), steel slag worked like a filler and had a dilution effect on CSA cement hydration. Therefore, setting time increased and chemical shrinkage, electrical resistivity, chemically bound water content, and compressive strength of the pastes reduced with increase of steel slag replacements. But, at late ages (≥90 days), these properties in pastes with steel slag replacements surpassed those of the plain CSA cement paste due to hydration of steel slag, which increased pH values of pore solutions and subsequently stabilized ettringite in the pastes. Steel slag replacement for CSA cement also accelerated strätlingite formation. Both ettringite stabilization and strätlingite formation appeared to be the primary mechanisms for strength compensation of CSA cement paste at late ages.

Influence of citric acid and sodium gluconate on hydration of calcium sulfoaluminate cement at various temperatures

To reveal the effect of set retarders on calcium sulfoaluminate (CSA) cement at different temperatures, plain paste and retarded ones with citric acid and sodium gluconate were prepared. Curing temperatures of 20, 30 and 40 °C were adopted. Initial and final setting times of fresh paste were measured. Compressive strength of samples at 1 day, and 3, 7, and 28 days were tested. XRD, DSC-TG and SEM were used to reveal the impact of retarders on hydration and microstructure of CSA cement. Results reveal that, retarders extend the setting of CSA cement paste, however accelerate the hydration of it at later age. Finer and more AFt is observed in retarded paste. Nevertheless, rise of temperature diminishes the retarding effect of citric acid and sodium gluconate and meanwhile inhibits the hydration of CSA cement with and without retarders. On most occasions, compressive strength of CSA cement paste increases when retarders are employed with the content up to 1.5%. At the content of 0.5 or 0.25%, retarded pastes achieve the highest compressive strength gains relative to their plain counterpart. This varies with the type of retarders and curing temperature. For citric acid and sodium gluconate, the contents are 0.25 and 0.5% at 20 °C, respectively. With the rise of temperature to 30 and 40 °C, the contents are 0.5 and 0.25%, respectively. Rise of temperature from 20 to 30 °C enhances the gain of compressive strength. However, similar gains are observed at temperatures of 30 and 40 °C.

Influence of water removal techniques on the composition and microstructure of hardened calcium sulfoaluminate cement pastes

Influence of different drying techniques, namely, vacuum drying, oven drying (40 °C) and D-drying on the composition and microstructure of the calcium sulfoaluminate (CSA) cement was investigated in this study. The vacuum drying and oven drying were performed after the samples were immersed in acetone or ethanol. The results show that D-drying decomposes ettringite (AFt) greatly. Following solvent replacement, oven drying slightly decomposes AFt when compared with the vacuum dried sample, possibly due to the carbonation of AFt in the oven drying process. The decomposition of AFt due to D-drying and oven drying affects both the compositional and microstructural analysis of CSA hydrates. In addition, the soaking time of the samples in the solvents has no effect on the thermogravic (TG) analysis of the hydration products of CSA cement, which indicates that the CSA cement hydrates have no interaction with the solvents.

Yield stress and thixotropy control of 3D-printed calcium sulfoaluminate cement composites with metakaolin related to structural build-up

The three-dimensional (3D) structural build-up with the precision has proposed new requirements related to yield stress and thixotropy in the 3D-printed cementitious materials. In this study, the static and dynamic yielding behaviours of 3D-printed calcium sulfoaluminate (CSA) cement composites with metakaolin were studied to understand the relationship between thixotropy and structural build-up through the powder-based extrusion system. Experimental results show that the static yield stress implemented by the constant shear-rate-controlled protocol in the 3D-printed CSA cement paste increased significantly with rest time. Dynamic rheological behaviours were investigated by the Bingham and Herschel-Bulkley model. Given the change of flocculation structure and shear-thinning behaviour during the shear period, Herschel-Bulkley model is more suitable for studying the dynamic rheological properties of 3D-printed CSA cement paste. Moreover, the thixotropy can be well improved by the addition of metakaolin, and the correlation between the structure deformation and thixotropy of 3D-printed CSA cement paste further proved that the improved thixotropy is beneficial to 3D-printed structural build-up. In conclusion, developing a 3D-printed CSA cement composites with the controllable yield stress and thixotropy is significant for structural build-up in the 3D-printed construction materials.

Utilization and performance evaluation of molasses as a retarder and plasticizer for calcium sulfoaluminate cement-based mortar

The objectives of this study are to utilize and investigate the performance of molasses as a retarder and plasticizer for calcium sulfoaluminate (CSA) cement and examine the effects of molasses on the hydration reaction, microstructure, and mechanical properties of CSA cement mortar. In this study, experiments were conducted to measure the setting time, hydration heat flow, and workability of the fresh CSA cement pastes and mortars with different molasses dosages (i.e., 0%, 0.25%, 0.5%, 0.75%, and 1%). The density, water absorption, voids contents, and pore size distribution were determined at 28 days. Further, thermogravimetric analysis (TGA) and unconfined compressive strength (UCS) tests were conducted at various ages. The results show that molasses can retard the hydration reaction and improve the workability without compromising the mechanical properties of CSA cement mortar. This study concludes that molasses is a suitable retarder and plasticizer for CSA cement.

Rheological behavior of calcium sulfoaluminate cement paste with supplementary cementitious materials

Calcium sulfoaluminate (CSA) cement has the potential to become the ‘future cement’ given its excellent characteristics, such as low carbon, energy saving, and low shrinkage. However, compared with ordinary Portland cement (OPC) paste, with the same water–to–cement (w/c) ratio and similar fluidity, the plastic viscosity of the CSA cement paste increases remarkably and exhibits significant shear thickening behavior, which is unconducive to the formulation and application of CSA cement concrete. Rheological behavior of CSA cement paste with supplementary cementitious materials (SCMs) is systematically studied. Results show that silica fume (SF) can significantly reduce the maximum shear stress and plastic viscosity of CSA cement paste, but the yield stress increases evidently. The incorporation of fly ash and blast-furnace slag can substantially eliminate the negative effect of the increase in the yield stress of the paste caused by the SF. As the w/c ratio decreases, the plastic viscosity of the CSA cement paste increases significantly. Under the action of the SCMs, the plastic viscosity of CSA cement paste with low water–to–binder (w/b) ratio is reduced, and the plastic viscosity of the high w/b CSA cement paste is increased, all CSA cement pastes exhibit shear thinning behavior.

Effects of Retarding Admixture, Superplasticizer and Supplementary Cementitious Material on the Rheology and Mechanical Properties of High Strength Calcium Sulfoaluminate Cement Paste

In this paper, the effects of retarding admixture, polycarboxylate (PCE) superplasticizer and supplementary cementitious material (SCM) on the rheology of high strength calcium sulfoaluminate (CSA) cement paste are investigated. At the water to binder ratio of 0.3, one PCE, two retarding admixtures, and three SCMs were used in the test. The test results show that much more PCE is needed for paste with citric acid than that with sodium borate to achieve a similar initial flowability. The reference paste and pastes mixed with sodium borate or fly ash exhibit shear thickening; pastes mixed with citric acid or silica fume exhibit shear thinning. The addition of citric acid increases the maximum shear stress of pastes, while the maximum shear stress can be significantly reduced by silica fume. Both the retarding admixtures and SCMs will strongly reduce the early-age compressive strength of the cement paste. At 28 days of age, CSA cement paste incorporating retarding admixtures has comparable strength to the reference paste. Silica fume has less negative effect on the compressive strength of the CSA cement paste due to the significant filling effect.

Recycling of Reverse Osmosis (RO) Reject Water as a Mixing Water of Calcium Sulfoaluminate (CSA) Cement for Brick Production

This study explored the possibility of using reverse osmosis (RO) reject water as a mixing water for producing cementitious bricks using calcium sulfoaluminate (CSA) cement along with gypsum, and it investigated the changes in the properties of CSA cement pastes when RO reject water was used. The results were compared with those obtained using purified water and seawater. Overall, the use of RO reject water improved the cement paste’s strength. Given that the use of RO reject water very slightly affected ettringite formation but more significantly influenced the Al2O3-Fe2O3-mono (AFm) phases (i.e., monosulfate, kuzelite, and Friedel’s salt) and amorphous aluminum hydroxide (AH3), the strength improvement was likely mainly due to the formation of Friedel’s salt rather than ettringite formation. This study also demonstrated that the use of RO reject water for brick production satisfied the Korean Standards (KS) F 4004 and toxicity characteristic leaching procedure (TCLP); thus, it is recommended to use RO reject water as a mixing water to produce CSA cement bricks for use in construction.

Early hydration of CSA cement modified with styrene–butadiene copolymer dispersion

The effect of styrene–butadiene copolymer (SB) dispersion on the early hydration of calcium sulfoaluminate (CSA) cement was investigated. Pastes with five different ratios (0, 5, 10, 15 and 20%) of SB to CSA cement at a constant water to cement ratio of 0·4 were investigated by ultrasonic measurements, isothermal calorimetry, X-ray diffraction, thermogravimetry and scanning electron microscopy. It was found that SB retarded the initial hydration of the CSA cement paste within 3 h, during which time the formation of ettringite was strongly delayed. The hydration degree of the CSA cement paste was caught up by that of the SB-modified CSA cement paste at 3 h based on ultrasonic and calorimetry analysis. In addition, an increase in the addition of SB led to the generation of more ettringite and aluminium hydroxide. The most ettringite was formed and the size of ettringite crystals was increased in the pastes with 20% SB. Meanwhile, the addition of SB tended to reduce the amount of hemicarboaluminate in CSA cement pastes.

Effect of curing temperature and relative humidity on the hydrates and porosity of calcium sulfoaluminate cement

Abstract The main objective of this study was to investigate the effect of curing temperature and relatively humidity (RH) on the hydrates and porosity of calcium sulfoaluminate (CSA) cement pastes. A series of experiments including X-ray diffraction (XRD), thermogravimetric (TG) and mercury intrusion porosimetry (MIP) were performed. The results indicate that the formation of hydration products and pore characteristics of CSA cement are dependent on the curing conditions. The formation of ettringite is favor of curing with low temperature and high RH. The rate of formation of aluminum hydroxide (AH3) and monosulfoaluminate (Ms) is well promoted when curing temperature and RH are rising. The total porosity of CSA cement paste is gradually decreased by curing with low temperature and high RH.

Continuous monitoring for leaching of calcium sulfoaluminate cement pastes incorporated with ZnCl2 under the attacks of chloride and sulfate

Ionic zinc is considered as an environmental pollutant. This work systematically investigated leaching mechanisms of calcium sulfoaluminate cement (CSA) pastes incorporated with/without ZnCl2 under the attacks of chloride and/or sulfate. The leaching behaviors of CSA pastes in the leaching solution are in-situ and continuously monitored by innovative non-contact electrical impedance measurement (NCEIM) and pH meter. The dissolution and diffusion during the leaching process are experimentally identified. Other techniques are also performed to verify the finding of NCEIM: the ion chromatograph and inductively coupled plasma optical emission spectrometer reveal the leaching or decomposition sequence of CSA pastes during the leaching process. Besides, results from XRD and SEM techniques demonstrate that main solid products in CSA pastes are ettringite and calcium monosulfoaluminate hydrates. The incorporation of Zn in the pastes has great impact on the decomposition of CSA pastes in the temperature elevation. External chloride and/or sulfate attacks significantly alter the pore structure of CSA pastes during the leaching process.

Porosimetric features of calcium sulfoaluminate and Portland cement pastes: testing protocols and data analysis

Investigating pore structure of calcium sulfoaluminate cement paste is significantly important to understand the hydration process of cement, to develop the models that describe the cement paste microstructure, and eventually to improve the properties of the cement paste such as shrinkage compensation and self-stressing. The history, manufacture, and hydration of calcium sulfoaluminate cement are investigated. Two most common experimental protocols, nitrogen sorption and mercury intrusion porosimetry, are systematically summarized with required theories for analysis of porosimetric features of the cement pastes. Both calcium sulfoaluminate and portland cement pastes were tested with a water-to-cement ratio of 0.4, 0.5, and 0.6 using the two test methods. Since the measurement range of pore diameter by the nitrogen sorption porosimetry is relatively narrow (about 0.3–300 nm), the microporous properties of cement paste can be well determined. The mercury intrusion porosimetry is useful to evaluate the porosimetric features of cement pastes including bulk durability due to the wide measurement range of pore diameter (approximately 3 nm–200 μm).

Volume and surface fractal dimensions of pore structure by NAD and LT-DSC in calcium sulfoaluminate cement pastes

This paper presents a preliminary work to evaluate influences of water to cement ratio, hydration age and content of superplasticizer on porosity, volume and surface fractal dimensions of pore structure in 56 groups of calcium sulfoaluminate cement pastes by nitrogen adsorption/desorption (NAD) tests and low temperature differential scanning calorimetry (LT-DSC). Additionally, a fractal sponge model is developed to analyze intrinsic relations among porosity, volume and surface fractal dimensions by changing the removed number of cells, iterate steps and number of fractal cells.

The role of Ca(OH)2 in sulfate salt weathering of ordinary concrete

In the present paper, a micro-analysis of sulfate salt weathering of calcium sulfoaluminate (CSA) cement paste and concrete was conducted to offer reference values and to elucidate the sulfate salt weathering of ordinary concrete. The test results showed that Na2SO4 crystals could be clearly identified in CSA cement paste and concrete, causing the failure of the paste and concrete in the evaporation zone. Based on the surface physicochemical theory of crystallization in a pore, a solution film between the pore wall and the salt crystal causes crystal growth and can be formed due to the inert relationship between sulfates and CSA cement hydration products without the effect of Ca(OH)2. However, after the inevitable chemical reaction between the sulfates and hydration products of Portland cement due to the existence of Ca(OH)2, the solution film is not formed, and Na2SO4 crystallization cannot occur in the Portland cement concrete.

Physico-chemical interaction between mineral admixtures and OPC–calcium sulfoaluminate (CSA) cements and its influence on early-age expansion

The present study aims at examining the physico-chemical factors influencing the expansion characteristics of OPC–CSA blend in the presence of mineral admixtures. Three different admixtures: Class ‘F’ fly ash (‘F’FA), Class ‘C’ fly ash (‘C’FA) and silica fume (SF) were used as 15%, 15% and 5% replacement of total cementitious binder. Longitudinal expansion of cement pastes prepared at w/cm – 0.44 showed that the Class ‘F’FA increased the expansion whereas the Class ‘C’FA and SF reduced the expansion. The pore solution of the OPC–CSA cement pastes was extracted at different ages to monitor the concentration of various ionic species. The saturation level of ettringite was determined using a geochemical modeling program (GEMS). Furthermore, an upper bound of crystallization stress was estimated. The expansion behavior in the presence of Class ‘F’FA and SF was found to be influenced by the changes in the stiffness, whereas the expansion of the Class ‘C’FA-based mixture was governed by faster hydration of ye'elimite.

The damage of calcium sulfoaluminate cement paste partially immersed in MgSO4 solution

One of controversies on the sulfate salt weathering of ordinary concrete is that MgSO4 does not show physical crystallization distress on concrete as the same as on masonry. In this paper, the Calcium sulfoaluminate (CSA) cement paste was partially immersed in 10 % MgSO4 solution to study if MgSO4 crystal can be identified in the damaged paste because the hydration products of CSA cement paste do not contain Ca (OH)2 that playing a vital role in chemical sulfate attack on concrete. The results showed that the low parts in the solution showed more severe damage observation than the upper part in contact with air after 5 months exposure. MgSO4 could not be identified in the damaged paste of upper part. The damage of paste was still a typical chemical sulfate attack characterized by amounts of CaSO4·2H2O crystals formation due to the decomposition of ettringite by Mg2+, even without the effect of Ca (OH)2. The so-called physical sulfate attack still cannot be confirmed on MgSO4 attack on CSA cement paste.

Hydration stage identification and phase transformation of calcium sulfoaluminate cement at early age

This paper studies hydration stages and phase transformation mechanism of fresh calcium sulfoaluminate (CSA) cement. Features of different hydration stages of CSA cement paste are identified by non-contact impedance measurement (NCIM), X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transforms infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), heat evolution test, ion chromatograph and inductively coupled plasma. Ettringite (AFt) and monosulfate (AFm) are found as metastable phases and have poor crystallization degree at the early-age hydration. The unique transformation between AFt and AFm is observed. The influence of different w/c ratios is further studied by NCIM, setting time and compressive strength test.

Hydration studies of calcium sulfoaluminate cements blended with fly ash

The main objective of this work is to study the hydration and properties of calcium sulfoaluminate cement pastes blended with fly ash (FA) and the corresponding mortars at different hydration ages. Laboratory X-ray powder diffraction, rheological studies, thermal analysis, porosimetry and compressive strength measurements were performed. The analysis of the diffraction data by Rietveld method allowed quantifying crystalline phases and overall amorphous contents. The studied parameters were: i) FA content, 0, 15 and 30wt.%; and ii) water addition, water-to-CSA mass ratio (w/CSA=0.50 and 0.65), and water-to-binder mass ratio (w/b=0.50). Finally, compressive strengths after 6months of 0 and 15wt.% FA [w/CSA=0.50] mortars were similar: 73±2 and 72±3MPa, respectively. This is justified by the filler effect of the FA as no strong evidences of reactivity of FA with CSA were observed. These results support the partial substitution of CSA cements with FA with the economic and environmental benefits.