Showing papers on "Ettringite published in 2021"

Effect of environmental pH values on phase composition and microstructure of Portland cement paste under sulfate attack

Abstract: The degradation of material induced by sulfate attack is major durability problem facing cementitious materials. In this paper, to illustrate the nanoscale chemical degradation, Portland cement samples were exposed to 5 wt% NaSO4 solution with different pH values (pH = 13, 8 and 5) as the function of corrosion ages (28 and 180 days). The effects of pH value on the hydrate phase, morphology and microstructure of cement hydration products under sulfate attack were characterized in combination with XRD, SEM-EDS, 1H, 27Al and 29Si NMR. The results demonstrate that reducing pH value of sulfate solution facilitates the formation and precipitation of columnar and platelet gypsum crystals in cement paste, whereas inhibiting the ettringite formation. A slight increase in the proportion of deleterious micropores larger than 1.6 μm as the sample exposed to sulfate with low pH value. The ettringite induced by sulfate attacking is transformed to AFm at early period and reversal transformation happens at later period. Reduction of the pH value of sulfate solution accelerates the transformation reaction of the hexa-coordinated aluminate phases. 29Si NMR test revealed that with pH reducing from 13 to 5, the hydration degree increase nearly 18%, the mean silicate chain length increase by 66.2% and Al[4]/Si ratio increase by nearly 30%. Also, the silicate polymerization enhancement is closely related with Al3+ ions extraction at early sulfate-attack period and re-entering at late sulfate-attack period. This research might provide nanoscale insights to guide more durable cement-based material in detrimental environment.

Utilization of ceramic tile demolition waste as supplementary cementitious material: An early-age investigation

Abstract: Construction and demolition (C&D) activities generate large amounts of waste, such as ceramic tile demolition waste (CTDW). However, the recycling ratio of C&D waste is generally of 3–10%. In this work, pastes with 0–30 wt% replacement of Portland cement by CTDW and limestone (reference filler) were produced. The particle packing, fresh-state properties (mini slump and rheometry), cement hydration (isothermal calorimetry and in situ X-ray diffraction – XRD) and compressive strength of the pastes were evaluated. The results showed that CTDW incorporation reduced the inter particle distance of the system while limestone increased it. Consequently, CTDW incorporation progressively increased the static yield stress and viscosity of paste while reducing the mini slump. CTDW enhanced the cement hydration kinetics compared with limestone, increasing the main heat flow peak, 24- and 168-h cumulative heat values by up to 8%, 5% and 6%, respectively, for the same incorporation level. Furthermore, in situ XRD and calorimetry indicated equivalent hydration for CTDW and limestone incorporation up to 8 h, from which the residue led to further alite consumption and hydrates formation (i.e. ettringite and portlandite). Compressive strength results at 1 and 7 days had good agreement with the cumulative heat values, indicating that CTDW led to strengths up to 5% higher than limestone did for the same replacement level and age. No significant pozzolanic reaction by CTDW was observed up to 7 days.

Carbonation of calcium sulfoaluminate cement blended with blast furnace slag

Abstract: The present study investigated the carbonation of calcium sulfoaluminate (CSA) cement blended with blast furnace slag. Paste samples were prepared by substituting CSA cement with slag at dosages of 0, 30, 50, and 70 wt%. The samples were carbonated for 28 days at a CO2 concentration of 3% after a sufficient pre-curing period. The test results indicated that slag incorporation did not affect the phase assemblage of the sample regardless of carbonation, while the incorporated slag promoted the formation of monosulfate and stratlingite upon carbonation in which this observation is closely related to the improved reaction degree of Si in the slag by carbonation. Meanwhile, monosulfate and ettringite were the first phases to be carbonated, while the C-A-S-H gel formed due to the carbonation of belite, thereby enhancing the compressive strength by refining the pore size in the gel pore region. On the other hand, the carbonation degree of the samples vastly increased as the slag content increased due to a small amount of ettringite formed along with the low reaction degree of slag which induced the development of macropores.

The potential use of drinking water sludge ash as supplementary cementitious material in the manufacture of concrete blocks

Abstract: The effect of drinking water sludge ash (DWSA) on the properties of DWSA-cement pastes is investigated in this study using x-ray diffraction, scanning electron microscopy coupled with energy-dispersive spectrometry, and thermogravimetry techniques. The pozzolanic reactivity of DWSA is confirmed; the filler effect of DWSA can significantly and rapidly accelerate the hydration reaction at an early curing age. The presence of DWSA in blended pastes promotes the incorporation of aluminum into the C-(A)-S-H gel; the original ‘Al-minor’ C-(A)-S-H gel in pure cement paste is converted to ‘Al-rich’ C-(A)-S-H gel. The added DWSA leads to the formation of aluminum-bearing hydrates, such as ettringite and calcium aluminate hydrates (C-A-H). The compressive strength of sludge-derived concrete blocks is studied; blocks containing 10% DWSA exhibited higher compressive strength at a curing age of 90 d than the reference samples. Such benefits are not observed in block samples with 20 and 30% DWSA, which is attributed mainly to the physical effect of unreacted DWSA particles.

Hydration properties and kinetic characteristics of blended cement containing lithium slag powder

Abstract: Lithium slag (LS) powder is a potential supplementary cementitious material due to its large content of active aluminosilicate minerals. To clarify the properties of LS powder as a supplementary cementitious material, the ionic dissolution properties of LS powder and the hydration properties and kinetic process of blended cement containing LS powder were studied in this paper. ICP was used to analyze the dissolution of Si 4+, Al3+ and Ca2+ ions in LS powder in the simulated cement alkali environment, and the mineral composition of the dissolved LS powder was analyzed by XRD . The results showed that SiO 2 and Al2O3 were the main active sources of LS powder and that an increase in the alkaline solution temperature promoted the dissolution of silicate in the LS powder and breaking of the Si–O and Al–O bonds. The hydration characteristics of blended cement were studied by isothermal calorimetry combined with the K-D model. The results showed that LS powder reduced the structural compactness of the hardened cement paste to varying degrees in the early stage of hydration, but in the later stage of hydration, the high content of S in the LS powder promoted the formation of rod-shaped ettringite in the hydration product . The proper amount (10%) of LS powder made the paste structure more compact. As determined by the hydration kinetic analysis, the hydration process of blended cement can be divided into crystal nucleation and growth (NG), phase boundary reaction (I) and diffusion (D) processes, and the rate of crystal nucleation is far greater than the rates of the phase boundary reaction and diffusion. According to the SEM results, the structure of blended cement is more compact when 10% LS powder was mixed in the cementitious system .

Hydration characteristics of calcium sulfoaluminate (CSA) cement/portland cement blended pastes

Abstract: The hydration characteristics of calcium sulfoaluminate cement (CSA) and ordinary Portland cement (OPC) blended pastes incorporating calcium sulfate (C S ‾ ) are investigated. The experimental characterization techniques were employed and complemented with thermodynamic calculations to provide in-depth understanding of the hydration of CSA/OPC blended system. The results showed that the hydration of C3S in the CSA/OPC blended pastes was significantly delayed in comparison with the sole OPC paste. The delayed hydration of C3S negatively affected the strength development of the CSA/OPC pastes. As the amount of C S ‾ increases, the induction period occurring between the initial peak and the intermediate peak was extended, thus increasing the initial setting time. The addition of C S ‾ to the CSA/OPC blends led to the two phenomena in the hydration of C4A3 S ‾ , (1) retardation of C4A3 S ‾ hydration, and (2) formation of more ettringite after a few hours of hydration. The thermodynamic calculations indicated that the setting of the CSA/OPC blended system is mainly facilitated by hydration of C4A3 S ‾ and by the formation of ettringite, while the major strength-giving phase is C–S–H that originates from the hydration of C3S.

Supersulfated cements based on pumice with quicklime, anhydrite and hemihydrate: Characterization and environmental impact

Abstract: This investigation reports on the mechanisms of reaction, microstructure, hydration products, compressive strength and environmental impact of supersulfated cements based on pumice (SSC-PM), using 10–20%wt of quicklime (CaO) as the alkaline activator and 5–15%wt of sulfatic activators of anhydrite or hemihydrate; three different initial curing temperature (ICT) regimes were used for 24 h, of 20, 40 and 60 °C. The main hydration products were C–S–H and ettringite; minor phases included monosulfate, gypsum, portlandite and calcite. The ICT of 60 °C accelerated the reactions of formation of ettringite and C–S–H, improving the early strength, whereas; the higher contents CaO (20%) and An or HH (15%) slowed the hydration reactions and consequently the early strength development. The activation with CaO-Anhydrite densified the microstructure leading to 360-day strengths of up to 34 MPa. These SSC-PM represent a good sustainable alternative as these showed emissions of 159–281 kgCO2-eq/t, which are at least 67% lower than the manufacture of clinker of Portland cement.

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.