Showing papers on "Ettringite published in 2022"

Hydration, reinforcing mechanism, and macro performance of multi-layer graphene-modified cement composites

Abstract: Multi-layer graphene (MLG) has excellent mechanical properties and a unique stacked structure. In this paper, sulphoaluminate cement replaced ordinary portland cement to prepare low-carbon ecological ultra-high-performance-concrete (UHPC); the effects of MLG on the hydration, microstructure, and mechanical properties of UHPC were investigated, revealing the hydration mechanism and reinforcing mechanism of MLG on UHPC. Results show that adding MLG can significantly enhance the macro performance of UHPC. When the MLG content is 0.08%, UHPC has better macro performance. Compared with the M0 group, the flexural strength and compressive strength of the M3 group increased by 31.6%, 35.3%, 43.3%, 50.9%, and 9.5%, 13.5%, 22.2%, 21.7% after curing for 1 d, 7 d, 28 d, and 56 d, respectively. We quantitatively characterized the content of hydration product changes in UHPC. Various characterization analyses showed that the adsorption effect and nucleation effect of MLG promoted the hydrolysis and ions exchange of Ca2+ and Al3+, which provided sites for the growth of hydration products and accelerated the hydration process of cement, forming more hydration products, including AFt (ettringite) and AH3 (gibbsite). AFt, AH3, and MLG are closely connected, filling the pores and reducing the porosity of the matrix, optimizing the pore structure, and the medium and large pores transformed into micro-nano pores. Revealing the multi-level reinforcing mechanism, namely hydration products (AFt, AH3) and MLG filling the pores; MLG prevented the extension of micro-cracks and changed micro-cracks development path through filling effect, deflection effect, pulling out, and bridging effect.

A comprehensive review on effects of mineral admixtures and fibers on engineering properties of ultra-high-performance concrete

Abstract: The unparalleled attributes of ultra-high-performance concrete (UHPC) reported over the last three decades sum up the development of this material in two phases: UHPC and ultra-high-performance fiber-reinforced concrete (UHP-FRC)/ultra-high-performance hybrid fiber-reinforced concrete (UHP-HFRC). The integration of scientific knowledge pertaining to material interactions and technical observations of developed UHPC has laid the foundation for further research and development. Microstructural research has revealed the contribution of finer sizes mineral admixtures along with lower water/binder ratios in the range of 0.16–0.24 to the development of high-density calcium silicate hydrate with a very low possibility of ettringite formation and alkali silica reactions in UHPC matrix. The addition of fibers is challenging when seeking to maintain the rheology of UHPC; therefore, technically sound concrete experts are required for the field applications. The challenges preventing the widespread use of UHPC are the higher initial cost, the requirement of special skilled labor for execution, and the lack of open discussions about various UHPC standards available worldwide to reach an agreement on minimum strength achievements and test standards. The hurdles hindering the use of UHPC in recent emerging 3D printing technology are also discussed in the present study. The vast literature compilation also reveals that less research has concentrated on UHP-HFRC as compared to UHPC and UHP-FRC. Therefore, a comprehensive review regarding the effects of mineral admixtures and fibers on the microstructural characteristics , fresh properties, and mechanical properties overall to address the upcoming challenges preventing the widespread use of UHPC and UHP-FRC/UHP-HFRC is essential.

A comprehensive review on effects of mineral admixtures and fibers on engineering properties of ultra-high-performance concrete

Abstract: The unparalleled attributes of ultra-high-performance concrete (UHPC) reported over the last three decades sum up the development of this material in two phases: UHPC and ultra-high-performance fiber-reinforced concrete (UHP-FRC)/ultra-high-performance hybrid fiber-reinforced concrete (UHP-HFRC). The integration of scientific knowledge pertaining to material interactions and technical observations of developed UHPC has laid the foundation for further research and development. Microstructural research has revealed the contribution of finer sizes mineral admixtures along with lower water/binder ratios in the range of 0.16–0.24 to the development of high-density calcium silicate hydrate with a very low possibility of ettringite formation and alkali silica reactions in UHPC matrix. The addition of fibers is challenging when seeking to maintain the rheology of UHPC; therefore, technically sound concrete experts are required for the field applications. The challenges preventing the widespread use of UHPC are the higher initial cost, the requirement of special skilled labor for execution, and the lack of open discussions about various UHPC standards available worldwide to reach an agreement on minimum strength achievements and test standards. The hurdles hindering the use of UHPC in recent emerging 3D printing technology are also discussed in the present study. The vast literature compilation also reveals that less research has concentrated on UHP-HFRC as compared to UHPC and UHP-FRC. Therefore, a comprehensive review regarding the effects of mineral admixtures and fibers on the microstructural characteristics, fresh properties, and mechanical properties overall to address the upcoming challenges preventing the widespread use of UHPC and UHP-FRC/UHP-HFRC is essential.

Use of recycled gypsum in the cement-based stabilization of very soft clays and its micro-mechanism

Abstract: This paper presents an experimental study and micro-mechanism discussion on gypsum role in the mechanical improvements of cement-based stabilized clay (CBSC). A soft marine clay at two initial water contents (i.e. 50% and 70%) was treated by reconstituted cementitious binders with varying gypsum to clinker (G/C) ratios and added metakaolin to facilitate the formation of ettringite, followed by the measurements of final water contents, dry densities and strengths in accordance with ASTM standards as well as microstructure by mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM). Results reveal that the gypsum fraction has a significant influence on the index and mechanical properties of the CBSC, and there exists a threshold of the G/C ratio, which is 10% and 15% for clays with 50% and 70% initial water contents, respectively. Beyond which adding excessive gypsum cannot improve the strength further, eliminating the beneficial role. At these thresholds of the G/C ratio, the unconfined compressive strength (UCS) values for clays with 50% and 70% initial water contents are 1.74 MPa and 1.53 MPa at 60 d of curing, respectively. Microstructure characterization shows that, besides the common cementation-induced strengthening, newly formed ettringite also acts as significant pore infills, and the associated remarkable volumetric expansion is responsible, and may be the primary factor, for the beneficial strength gain due to the added gypsum. Moreover, pore-filling ettringite also leads to the conversion of relatively large inter-aggregate to smaller intra-aggregate pores, thereby causing a more homogeneous matrix or solid skeleton with higher strength. Overall, added gypsum plays a vital beneficial role in the strength development of the CBSC, especially for very soft clays. • Gypsum fraction has a significant influence on the index and mechanical properties of the CBSC. • Optimal G/C ratio determined in CBSC was obviously greater than that of mortar or concrete. • Ettringite-induced filling and cementation are responsible for the improved mechanical performance of the CBSC. • The abnormal beneficial role of gypsum is closely associated with the increase in the density. • Design guideline for cementitious binders that are only applicable to soft clays was re-established.

Shrinkage, hydration, and strength development of limestone calcined clay cement (LC3) with different sulfation levels

Abstract: This study investigated autogenous shrinkage of limestone calcined clay cement (LC3)-based mixes with various sulfation levels of hemihydrate. Physicochemical evolution and strength development were also monitored. Superior mechanical performance of the mixes was achieved at an optimal sulfation level, although the shrinkage was reduced with the sulfate content. Calorimetry data showed the hemihydrate delayed and broadened the second aluminate peak. Both gypsum and ettringite formed early during hydration, and carboaluminates were observed after 3 days of curing. Ettringite in the LC3 mixes was of smaller sizes in comparison to that in the ordinary Portland cement (OPC) mixes, attributable to the pore refinement in the LC3 matrices. The content of portlandite (Ca(OH)2) was reduced with the curing time due to pozzolanic reactions. The findings demonstrated a coupling effect of both the chemical and microstructural change on the observed autogenous shrinkage and the consequent performance of LC3-based concrete composites.

Sulphate resistance of one-part geopolymer synthesized by calcium carbide residue-sodium carbonate-activation of slag

Abstract: One-part geopolymers attract wide interests due to their much lower CO 2 emissions and similar practice as ordinary Portland cements (OPC), that is “just add water”, instead of mixing alkaline solution with precursors. To apply this type of new cementitious material in sulphate-rich environments, such as wastewater system and sewer network, we investigated the chemical and microstructure properties of one-part geopolymers synthesized by calcium carbide residue (CCR)-sodium carbonate-activated slag when they were exposed to sodium sulphate and magnesium sulphate solutions . We highlighted the main synthetic parameter of CCR dosage (from 2.5% to 10%) and its impact on kinetics of degradation, and meanwhile proposed the conceptual degradation mechanisms to clarify the influences of cation type (Na + and Mg 2+ ). We found that the relatively higher CCR dosages (7.5% and 10%) significantly accelerated the kinetics of degradation in Na 2 SO 4 solution. The formation of ettringite depended mainly on the portlandite availability in the binders. In comparison with the Na 2 SO 4 attack, all the mixtures were more susceptible to the MgSO 4 attack, which could be explained by two mechanisms: the formation of gypsum via the reaction between SO 4 2− with Ca 2+ , and the transformation of main binding gel phase C-A-S-H into non-cementitious and fibrous M-A-S-H. • One-part geopolymers were synthesized by calcium carbide reside-sodium carbonate-activation of slag. • Conceptual degradation mechanisms in Na 2 SO 4 and MgSO 4 solutions were proposed. • Ettringite and gypsum were main degradation products. • Binders were more susceptible to MgSO 4 degradation. • Relatively higher CCR dosages significantly accelerated kinetics of degradation.

Synergy effect of synthetic ettringite modified by citric acid on the properties of ultrafine sulfoaluminat cement-based materials

Abstract: In our previous study, ultrafine ettringite was prepared in the presence of citric acid by using a nucleation and aging separation method. Due to the seed effect, ultrafine ettringite enhanced the hydration rate and compressive strength of sulfoaluminat cement-based materials at all ages. However, when the particle size of sulfoaluminat cement-based materials became smaller, ultrafine ettringite retarded the hydration rate of ultrafine sulfoaluminat cement-based materials (USCM). In this paper, ettringite was prepared under different citric acid concentrations and characterized by XRD, FT-IR, TG-DSC, PSD and TOC measurements. The effect of ettringite on the properties of USCM was experimentally studied. Results showed that citric acid can absorb on the surface of ettringite. The average particle size of the citric acid-ettringite composites（CECs）were in the range of 100–300 nm, and increasing the concentration of citric acid in composites lead a smaller size of CECs. When present in the alkaline solution of pH 13, the higher citric acid concentration used in the preparation of CECs, the more amount of tartaric acid released from CECs. CECs retarded the cement hydration, modified the pore structure and enhanced the compressive strength of USCM pastes at all ages due to the synergistic effect of nano crystal nucleus and citric acid released from CECs.

Utilization of Calcium Carbide Residue as Solid Alkali for Preparing Fly Ash-Based Geopolymers: Dependence of Compressive Strength and Microstructure on Calcium Carbide Residue, Water Content and Curing Temperature

Abstract: Calcium carbide residue (CCR) is a solid waste resulting from acetylene gas production. In this study, CCR was used as an alkali activator to prepare fly ash (FA)-based geopolymers without any alkali supplementation. We studied the factors (FA/CCR ratio, curing temperature, and water/binder ratio) influencing the mechanical property of FA/CCR-based geopolymers. The compressive strength results showed that, by optimizing these three factors, the FA/CCR mixture has great potential for use as a cementitious material and geopolymer with a dense microstructure having a maximal compressive strength of 17.5 MPa. The geopolymers’ chemical structure, microstructure, and chemical composition were characterized and determined by a combination of techniques. All these results revealed that amorphous C-(A)-S-H (calcium (aluminate) silicate hydrate) gels mainly formed after geopolymerization resulting from the reaction of FA and CCR. In addition, some crystallines, such as ettringite and monosulfate, were also formed. Further, geopolymers prepared with a suitable FA/CCR ratio (1:1 or 1:2) possessed a compact microstructure because of their sufficient reactive SiO2 and Al2O3 and high-enough alkalinity, responsible for higher content of C-(A)-S-H formation and better mechanical property. Too high curing temperature or water content induced the formation of a loosely bound geopolymer matrix that strongly weakens its mechanical property.

Synergy effect of synthetic ettringite modified by citric acid on the properties of ultrafine sulfoaluminat cement-based materials

Abstract: In our previous study, ultrafine ettringite was prepared in the presence of citric acid by using a nucleation and aging separation method. Due to the seed effect, ultrafine ettringite enhanced the hydration rate and compressive strength of sulfoaluminat cement-based materials at all ages. However, when the particle size of sulfoaluminat cement-based materials became smaller, ultrafine ettringite retarded the hydration rate of ultrafine sulfoaluminat cement-based materials (USCM). In this paper, ettringite was prepared under different citric acid concentrations and characterized by XRD, FT-IR, TG-DSC, PSD and TOC measurements. The effect of ettringite on the properties of USCM was experimentally studied. Results showed that citric acid can absorb on the surface of ettringite. The average particle size of the citric acid-ettringite composites（CECs）were in the range of 100–300 nm, and increasing the concentration of citric acid in composites lead a smaller size of CECs. When present in the alkaline solution of pH 13, the higher citric acid concentration used in the preparation of CECs, the more amount of tartaric acid released from CECs. CECs retarded the cement hydration, modified the pore structure and enhanced the compressive strength of USCM pastes at all ages due to the synergistic effect of nano crystal nucleus and citric acid released from CECs.

The effects of biomineralization on the localised phase and microstructure evolutions of bacteria-based self-healing cementitious composites

Abstract: Microbially induced calcite precipitation (MICP) is one of the most effective mechanisms to achieving self-healing abilities in cementitious composites. However, there has only been limited understanding of the effect of the MICP process on the mineralogy and microstructure of the cementitious matrix closely mixed with the healing products. This study systematically assessed the effect of biomineralization on the localised cementitious binders at micro and atomic level combining different characterisation techniques (i.e. XRD, FTIR and μCT). The results show that, in addition to the formation of CaCO3 polymorphs that close the crack space, the MICP process will also modify the phase assemblages near the healed cracks. For the first time we observed that when the most common source of calcium for the MICP process (calcium hydroxide) is limited, ettringite and C–S–H can also act as the providers of the calcium for the biomineralization process to take place. The detailed microstructure characterisations support that, apart from the dense thin layer (around 0.5 mm) of healing products formed on the surface of the cracks, loose particle-like calcium carbonate crystals can also form in pores and voids, suggesting that healing can also be generated in deeper sections of the crack. The outcomes of this study advance the fundamental understanding of the MICP process in Portland cement binders, and will also assist the further evaluation of the durability performances of these self-healed cementitious composites.

Formulation of an alkali-free accelerator and its effects on hydration and mechanical properties of Portland cement

Abstract: An alkali-free accelerator (AMD) mainly composed of aluminum sulfate, magnesium sulfate and diethanolamine (DEA) was formulated and its effects on hydration and mechanical properties of Portland cement were investigated. Results indicate that AMD accelerator can efficiently shorten the setting time and enhance the early strength of Portland cement. The rapid setting and hardening properties were achieved by promoting the formation of ettringite (AFt), Mg(OH)2 and MgAl-hydrotalcite (MgAl-LDHs). Although AMD accelerator retarded early hydration of alite due to the accelerated hydration of C3A and caused higher porosity, it had no negative effect on or even slightly increased long-term strength development. Except for environmentally-friendly advantage, this type of alkali-free accelerator is more beneficial to strength development compared to accelerators with fluoride salts.