Calcium Sulfoaluminate-belite Cement Research Articles

Durability Assessment of Expansive and Nonexpansive Calcium Sulfoaluminate Belite Cement Concrete in Chloride-Rich Environments

Durability Assessment of Expansive and Nonexpansive Calcium Sulfoaluminate Belite Cement Concrete in Chloride-Rich Environments

Carbonation of calcium sulfoaluminate belite binder: mechanism and its implication on properties

Calcium sulfoaluminate belite (CSAB) binder is an alternative low CO2 binder. CSAB cement has ye’elimite as the primary clinker phase which hydrates to form monosulfate or ettringite as the main hydration product. The hydration and mechanical characteristics of CSAB cement and PC-CSAB blended cement have been reported in several studies; however, studies on the durability characteristics such as carbonation are limited. This study focuses on evaluating the microstructural and mechanical alterations due to carbonation in a CSAB binder system and understanding the underlying mechanism of development of the carbonation front. CSAB binder was found to carbonate rapidly compared to PC binder, accompanied by a reduction in compressive strength. This was attributed to the increase in the pore volume due to the carbonation of the ettringite-rich microstructure, facilitating further ingress of CO2 into the microstructure. The rate of carbonation in CSAB binders diverged significantly from the square root of time model used for PC binders.

Utilization of air granulated basic oxygen furnace slag as a binder in belite calcium sulfoaluminate cement: A sustainable alternative

Basic oxygen furnace (BOF) slag negatively impacts ordinary Portland cement performance when replacement levels exceed 5%. This necessitates the exploration of alternative applications for the slag. Simultaneously, a high-volume slag utilization is desired to benefit slag recycling as supplementary cementitious materials. Therefore, this study aims to optimize the air granulated BOF slag substitution potential in belite calcium sulfoaluminate cement by investigating the hydration products in standard mortar. The reactivity of the novel binder is correlated with workability, and mechanical performance by thermal, mineralogical, and microstructure analysis. Consequently, the 10–30% replacement delays the final setting time by inhibiting the ettringite formation leading to a decrease in mechanical performance till 28 days. At later ages (28–180 days), the 30–50% substitution exhibited the synergy in mechanical performance, which is attributed to the hydrogarnet, calcium silicate hydrate, and strätlingite formation. Moreover, all the mortar samples exhibited heavy metals’ leaching and drying shrinkage below the permissible limit.

Effect of additive compounding on early properties of belite sulfoaluminate cement

Belite sulfoaluminate cement is attracting increasing attention and being considered as a potential alternative to ordinary Portland cement owing to lower preparation temperature and carbon dioxide (CO2) emission. However, it still has disadvantages of poor workability and low early strength. This study investigated the effects of compound polycarboxylate superplasticiser (PCE) and early strength agent, calcium nitrite (Ca(NO2)2), on flowability and early strength of belite calcium sulfoaluminate (BCSA) cement. Variation patterns of setting time, fluidity, strength and polycarboxylate superplasticiser (PCE) adsorption of the paste were measured; the hydration products and microscopic morphology were also analysed by the heat of hydration, X-ray diffraction and scanning electron microscopy. Results showed that calcium nitrite effectively enhanced adsorption of PCE onto the surface of BCSA cement particles, leading to notable improvement in the fluidity of the paste (reaching up to 275 mm). In the initial hydration stage, 0.7% PCE compounded with 1.2% calcium nitrite inhibited the formation of ettringite (AFt), resulting in prolonged setting time. However, it deepened the degree of hydration of BCSA for 3 days and refined the hydration product, AFt crystals. Consequently, the compressive strength was increased to 95.75 MPa and 107.13 MPa for BCSA cement at 3 days and 28 days, respectively.

Investigations on the Suitability of Alternative and Supplementary Cementitious Materials to produce High Performance Aerogel Concrete

AbstractHigh Performance Aerogel Concretes (HPAC) are produced by embedding silica aerogels in high‐performance matrices, which were previously based on finely ground Portland cement CEM I 52.5 R and microsilica. On the one hand, this ensured a comparatively high compressive strength, but on the other hand it was associated with large shrinkage strains and a high global warming potential (GWP). In the following, approaches are presented on how these disadvantages can be counteracted by replacing Portland cement with alternative (ACM) or supplementary cementitious materials (SCM). In the experimental investigations, calcium aluminate cement, calcium sulfoaluminate cement, calcium sulfoaluminate belite cement and a CSH phase binder were used as ACMs; limestone powder, calcined clay, fly ash, glass powder, rice husk ash, brick dust and ground granulated blast‐furnace slag were used as SCMs. Based on extensive fresh and hardened concrete tests, it could be shown that with the (partial) replacement of Portland cement by ACMs or SCMs, High Performance Aerogel Concretes can be produced that have significantly reduced shrinkage deformations and a lower GWP with an unchanged good relationship between compressive strength and thermal conductivity.

Influence of iron content on processing conditions and early age hydration of ye’elimite

Ye’elimite is the primary reactive component in calcium sulfoaluminate-belite (CSAB) cement, and it has relatively lower raw material carbon footprint compared to Portland clinker phases. Ye’elimite produces ettringite when hydrated in the presence of calcium sulfate, and monosulfate in the absence of a sulfate source. The early age hydration properties in CSAB based binders are mainly controlled by the amount and reactivity of ye’elimite in the system. The reactivity of the ye’elimite can be affected by the presence of minor elements in raw mix during clinkering and distribution of other phases in the binder. This paper aims to study the formation and hydration o f ye’elimite incorporated with iron, which is commonly found in industrial by-products suitable for CSAB cement production. The influence of iron on the clinkering conditions and its incorporation in the ye’elimite phase are investigated using characterization techniques such as X-ray diffraction and thermogravimetric analysis. The iron-incorporated ye’elimite, synthesized using laboratory-grade chemicals, is examined for early-age hydration using isothermal calorimetry to study the effect of iron content on the hydration reactivity of ye’elimite. The iron content in the raw mix was found to affect clinkering reactions by accelerating the decomposition of calcite and calcium sulfate. The iron incorporation also resulted in changes in the crystal structure and early age hydration of ye’elimite.

Hydrate assemblage stability of calcium sulfoaluminate-belite cements with varying sulfate content

To assess the feasibility of employing calcium sulfoaluminate (CSA)-belite cements for radioactive waste encapsulation purposes, a series of 40 mixes were conducted on a 3-litre scale. This series of tests incorporated two commercially available CSA-belite clinkers, gypsum or anhydrite addition at 15, 25 or 35 wt%, a water to solids ratio of 0.5, 0.6 or 0.7, and a high or low mixing shear regime. CSA clinkers chosen contain predominantly ye’elimite, with some belite, and a small quantity of calcium sulfate as anhydrite. This paper outlines the effect of these parameters upon the phase composition and structure of these CSA samples tested at 7, 28, and 90 days. The development of the hydrate assemblage was monitored in order to assess the suitability of these grouts for use in encapsulation processes. The trends in phase development and evolution are consistent across both clinkers tested. The levels of ettringite formed as a hydration product are dependent upon the levels of calcium sulfate addition and the availability of water, with ettringite production evident, though at a decreasing rate, from between 7 and 90 days. Throughout the entire catalogue of mixes, no ettringite decomposition was detected. The majority of belite within both clinkers appears to remain unreacted, with a small increase in mixes producing less ettringite. Simulations also show similar phase assemblages to those encountered experimentally, with little phase development after 90 days.

Influence of expansive cement on rheological, strength performance and morphological characteristics of self-compacting concrete

This research is focused on the influence of type K expansive cement (calcium sulfoaluminate-belite cement) blended with self-compacting concrete (SCC) in the presence of alccofine and silica fume. The present work intends to evaluate the synergistic effect of expansive cement in various proportions from 0 to 15 % by cement weight by examining the rheological properties, mechanical strength and expansive characteristics for M30, M40 and M50 grades of SCC. Results revealed that the incorporation of expansive cement is constructive in improving fresh state properties, mechanical performance and reducing shrinkage properties effectively due to the formation of additional ettringite and calcium hydroxide. Results of fresh properties of expansive SCC show a good agreement with European standards. The test results illustrate a positive influence on strength properties of SCC at the early age with the inclusion of 10 % of expansive cement due to the formation of additional ettringite and calcium hydroxide. Incorporating 10 % of expansive cement enhanced the compressive strength by 12 %, 9% and 7 % for M30.M40 and M50 grades of SCC respectively relative to reference SCC at 28 days. However, a slight strength reduction in compressive strength is noticed for SCC with the inclusion of higher percentage (15 %) of expansive cement. The effect of expansive cement on the morphological characteristics of SCC was assessed by employing SEM, EDS, XRD and FTIR techniques to validate the experimental results. On the other hand, addition of expansive cement and supplementary cementitious materials in the production of SCC minimizes the use of Portland cement and significantly reduces carbon footprint.

Influence of tartaric acid dosage on the early-age and long-term properties of calcium sulfoaluminate belite cement composites

Increasing the use of low carbon footprint calcium sulfoaluminate belite (CSAB) cement is a potential way to reduce carbon dioxide emissions related to the cement and concrete industry. However, rapid setting is one of the barriers limiting the large-scale applications of CSAB cement. To address this issue, experimental studies were carried out to investigate a sustainable retarder—tartaric acid—and its dosage on the hydration reaction, early-age properties, and long-term properties of CSAB cement composites from their fresh state to 2 years. The results confirmed that tartaric acid is a powerful retarder for CSAB cement; it showed a stronger retarding effect than citric acid, sodium gluconate, borax, and molasses. A proper tartaric acid dosage (≤ 0.75 %) extended the initial setting time of CSAB cement from 5 min to up to 154 min without compromising long-term strength. When the tartaric acid dosage was no more than 0.5 %, it hardly affected the strength from 2 h to 2 years. However, over dosage (≥ 1%) should be avoided since it significantly affected the strength development from 2 h to 2 years. Overall, this study provided a knowledge base for the mixture design of CSAB cement composites, which can benefit the popularization of low carbon footprint CSAB cement.

Performance of photocatalytic, carbonated calcium sulfoaluminate-belite cement

Carbonated and non-carbonated paste samples containing white Ordinary Portland Cement (OPC) or Calcium Sulfoaluminate-Belite (CSAB) cement with TiO2 were evaluated for NOx removal, optical properties, and CaCO3 polymorphs using chemiluminescence NOx analyzer, UV-Vis-NIR spectrophotometer, and X-Ray Diffraction, respectively. NOx removal of photocatalytic CSAB cement was 43% and 28% lower than white photocatalytic cement for non-carbonated and carbonated samples, respectively. Partially replacing the cements with fly ash decreased NOx removal in the non-carbonated state but reduced the negative impact of carbonation on photocatalytic efficiency. The decrease in NOx removal of carbonated samples were a combination of decrease in surface porosity, coverage of TiO2 sites by carbonation products, and decrease in activated TiO2 sites caused by a lower average diffuse reflectance in UV-A region despite an increase in photoconductance. All the TiO2-doped paste had a lower band edge energy than pure nano TiO2, and Urbach absorption tail, indicating presence of localized defects.

Ternesite-calcium sulfoaluminate cement: Preparation and hydration

Ternesite-calcium sulfoaluminate (TCSA) cement exhibits excellent properties. However, it is hard for sulfoaluminate (C4A3$) and ternesite (C5S2$) to coexist in the clinker. In this study, phosphogypsum (PG) was used for producing TCSA cement clinker. The joint effects of P2O5 and F in PG as well as the CaSO4 content in raw meals on the coexistence of C4A3$ and C5S2$ were investigated. The grindability and morphology of TCSA cement clinker were characterized. By the comparison with belite-calcium sulfoaluminate (BCSA) cement, the hydration and properties of TCSA cement were investigated. Results shows that the coexistence of C4A3$ and C5S2$ is achieved under the joint effects of P2O5, F and a sufficient amount of CaSO4. TCSA cement clinker could be prepared at 1125–1200 °C for 15–30 min and the clinker exhibits a good grindability. Ternesite in TCSA cement clinker presents a rodlike or granular shape Compared with BCSA cement, due to higher hydration rate of C4A3$, the setting time of TCSA cement is shorter and early compressive strength is higher. Due to the hydration of C5S2$, TCSA cement exhibits a higher late age compressive strength.

Mitigating early age cracking in 3D printed concrete using fibres, superabsorbent polymers, shrinkage reducing admixtures, B-CSA cement and curing measures

3D printed concrete is vulnerable to severe early age shrinkage-related cracking due to the absence of formwork and coarse aggregates. To address this, several potential mitigation measures are compared based on their ability to reduce free shrinkage and cracking severity at dosages which do not adversely affect the printability or buildability of a well-developed 3DPC mixture. The results show that a low dosage of short polypropylene microfibres eliminates early age cracking. The external application of a curing agent and multiple applications of misting water are less effective. Calcium sulfoaluminate–belite cement is used to accelerate structuration and proves beneficial in crack mitigation while enhancing buildability. Internal curing, with superabsorbent polymers, is less effective in mitigating plastic cracks but reduces autogenous shrinkage and increases the rate of stiffening. At the dosage used in this study, the shrinkage reducing admixture is relatively ineffective in crack mitigation, with higher dosages adversely affecting the buildability.

Water-to-cement ratio of calcium sulfoaluminate belite cements: Hydration, setting time, and strength development

Little published data is available to guide engineers in designing calcium sulfoaluminate belite (CSAB) cement mixtures with adequate workability, strength, and durability. This lack of understanding of design factors, especially the effect of varying w/c, represents a significant barrier to widespread CSAB use. In this study hydration, setting time, and strength development of two CSAB cements with w/c from 0.3 – 0.6 were evaluated. CSAB reaction kinetics varied with increased w/c depending on CSAB composition, specifically calcium sulfate content – with higher w/c increasing retardation in higher anhydrite/ye'elimite content cement, but reduced retardation in lower anhydrite/ye'elimite cement. For both cements, greater w/c led to greater total hydration, increased setting times, and reduced compressive strengths in the pastes and mortar samples. Setting time was linked more closely to anhydrite content than w/c, with greater sulfate volumes shortening setting times.

Influence of calcium hydroxide and calcium sulfate on early-age properties of non-expansive calcium sulfoaluminate belite cement

Calcium sulfoaluminate belite (CSAB) cements are currently being promoted as alternative low CO2 cements. CSAB cements have ye'elimite as their primary phase. Ye'elimite hydrates rapidly to form ettringite which contributes towards high early-age strength. The hydration characteristics of ye'elimite are influenced by the presence of calcium sulfate and calcium hydroxide, which can also extend to the mechanical properties. Further, this change in phase composition can modify a non-expansive binder into an expansive binder. In this study, the influence of phase composition on hydration characteristics, expansion, pore structure, and mechanical properties was investigated using CSAB binders with varying gypsum and calcium hydroxide levels. The expansion was directly affected by the amount of calcium sulfate and calcium hydroxide. The differentiating factor between expansive and non-expansive systems was the critical pore size of CSAB binders. Expansive CSAB binders exhibited lower critical pore size than non-expansive binders, highlighting the role of supersaturation. The addition of calcium hydroxide accelerated the kinetics of ye'elimite hydration and increased the expansion. Moreover, the change from a non-expansive to expansive system did not lead to the deterioration of the CSAB binder matrix.

Effects of sodium gluconate on hydration reaction, setting, workability, and strength development of calcium sulfoaluminate belite cement mixtures

Fast setting is an important reason restricting the applications of eco-friendly calcium sulfoaluminate belite (CSAB) cement. To help spread the application of CSAB cement, this study investigated the influence of sodium gluconate (SG) as a setting retarder on the setting, workability, and strength development of CSAB cement mixtures. Quantitative X-ray diffraction (QXRD), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM) were performed to understand the effects of SG on the hydration and microstructure of CSAB cement mixtures. The results showed that SG effectively extended the setting time and improved the workability of CSAB cement pastes since it retarded ye’elimite hydration and impeded the loss of workability caused by ettringite formation. Low dosages (≤0.5%) of SG increased the unconfined compressive strength (UCS) of CSAB cement mortars at all ages (from 2 h to 90 days) due to the reduction in void content, fast diminishment of the retarding effect at early ages, and slight acceleration on belite hydration at later ages. However, high dosages (≥1%) significantly degraded the UCS of CSAB cement mortars at all ages since high dosages of SG strongly retarded ye’elimite hydration at early ages and caused a looser microstructure at later ages.

Early assessment of hydration and microstructure evolution of belite-calcium sulfoaluminate cement pastes by electrical impedance spectroscopy

In this paper, the interconnectedness among the electrical resistivity, the calorimetric heat and the chemical shrinkage of high Belite calcium sulfoaluminate (BCSA) cement are discussed. Microstructure analyses by using XRD, NMR, ICP-MS and SEM are applied to further demonstrate the relationship among the microstructure, the compositional variation and the electrical resistivity of the hydrated cement paste. Two distinctive hydration stages can be differentiated from the BCSA cementitious materials. First stage is mainly controlled by hydration process of both the ye’elimite and the gypsum, and the second stage is mostly controlled by the formation of AFm and amorphous phases as a secondary hydration process. According to the classic equation depicted the hydration degree of cementitious materials, a specific mathematical equation combining not only traditional parameters but also the electrical resistivity is proposed for the BCSA.

Quantitative in Situ X-ray Diffraction Analysis of Early Hydration of Belite-Calcium Sulfoaluminate Cement at Various Defined Temperatures

The influence of temperature on the early hydration of belite-calcium sulfoaluminate cements with two different calcium sulfate to calcium sulfoaluminate molar ratios was investigated. The phase composition and phase assemblage development of cements prepared using molar ratios of 1 and 2.5 were studied at 25, 40 and 60 °C by in situ X-ray powder diffraction. The Rietveld refinement method was used for quantification. The degree of hydration after 24 h was highest at ambient temperatures, but early hydration was significantly accelerated at elevated temperatures. These differences were more noticeable when we increased the temperature from 25 °C to 40 °C, than it was increased from 40 °C to 60 °C. The amount of calcium sulfate added controls the amount of the precipitated ettringite, namely, the amount of ettringite increased in the cement with a higher molar ratio. The results showed that temperature also affects full width at half maximum of ettringite peaks, which indicates a decrease in crystallite size of ettringite at elevated temperatures due to faster precipitation of ettringite. When using a calcium sulfate to calcium sulfoaluminate molar ratio of 1, higher d-values of ettringite peaks were observed at elevated temperatures, suggesting that more ions were released from the cement clinker at elevated temperatures, allowing a higher ion uptake in the ettringite structure. At a molar ratio of 2.5, less clinker is available in the cement, therefore these differences were not observed.

Influence of exposure conditions on expansion characteristics of lime-rich calcium sulfoaluminate-belite blended cement

Expansive characteristics of calcium sulfoaluminate-belite (CSAB) cement can be harnessed for making shrinkage-compensating concrete. The level of expansion is dependent on several factors, such as the amounts of ye'elimite and calcium sulfate available in CSAB cement. Moreover, calcium sulfate from an external source can also influence the expansion characteristics by increasing the supersaturation with respect to ettringite. In this study, the influence of externally available calcium sulfate was investigated by curing a blend of Portland cement (PC) and CSAB cement to different concentrations (1 g/l and 5 g/l) of gypsum solutions. Expansion of the PC-CSAB blended cement was found to be directly affected by the ye'elimite content (CSAB dosage) and the gypsum concentration in exposure solution. The presence of external calcium sulfate during the hydration of ye'elimite was found to affect the hydrate phase assemblage, the pore structure, and the morphology of ettringite. Furthermore, the level of expansion depended on the age at which the specimens were exposed to the gypsum solution, indicating the influence of matrix stiffness on resultant expansion.

Phase development and hydration kinetics of belite-calcium sulfoaluminate cements at different curing temperatures

The influence of different curing temperatures on the hydration of belite-calcium sulfoaluminate cement was investigated at 20, 40 and 60 °C. The hydration kinetics and the hydrated phase assemblages were studied by isothermal calorimetry, X-ray powder diffraction, differential thermal analysis and thermogravimetric analysis, as well as field emission scanning electron microscopy. The compressive strength development of the cement pastes was also determined. Results showed that, at early ages, hydration was faster and early compressive strength was higher at elevated temperatures than at ambient temperature. On the other hand, at late ages in cement pastes cured at 60 °C, the amount of ettringite decreased, leading to lower compressive strength, indicating that the degree of hydration was lower at higher temperatures. Moreover, at elevated temperatures prismatic ettringite crystals became smaller due to faster hydration. Other hydration products present were aluminium hydroxide, which is formed together with ettringite from the hydration of calcium sulfoaluminate and gypsum, and C–S–H which precipitates as a main hydration product of belite. Belite hydrated in a lesser amount, especially at 60 °C, when the lowest amount of C–S–H was observed.

Immobilization of heavy metals, selenate, and sulfate from a hazardous industrial side stream by using calcium sulfoaluminate-belite cement

Release of heavy metals from different industries and industrial waste is a major global threat for as well humans as ecosystems. In this study, immobilization of an industrial filter sludge (FS) with an extremely high content of several heavy metals (24.6 wt% Pb, 21.7 wt% Hg, and 9.00 wt% Se) and sulfate via calcium sulfoaluminate-belite (CSAB) cement was tested. The ratios of 25%, 50% and 75% of CSAB addition were tested, and the target was to achieve immobilization of the hazardous components. The leaching of Pb, Hg, SeO4, SO4, Ni, Cd, Cu, and As was monitored, and the structure of the immobilized materials was examined via X-ray powder diffraction (XRD) and field emission scanning electron microscopy-energy dispersive spectroscopy (FESEM-EDS) analysis. It was observed that Hg, Cu, As, Cd, and Ni were immobilized completely and leaching of Pb was reduced by 69% from the theoretical release. On the other hand, the leaching of SeO4 and SO4 experienced major increase when CSAB was added. XRD indicated significant ettringite formation as the amount of added CSAB increased, and the formation of gypsum as the amount was decreased. FESEM-EDS indicated that the immobilization was largely based on encapsulation into the CSAB binder, but chemical immobilization into the ettringite binder was also observed. It was concluded that the increased release of SO4 and SeO4 might have resulted from an excess amount of sulfates (added gypsum) during hydration.