•The effects of alkali sulfate on cement hydration and hardening were studied.•The adverse effect of alkali sulfate originated from clinker is significantly.•The Na2Oeq/SO3 molar ratio in cement is ...better <0.5 for good properties.•Ca-langbeinite could play a good role as gypsum in mortar.
Alkali sulfates originate from cement clinker, occurring as arcanite (K2SO4), thenardite (Na2SO4), aphthitalite (K3NS¯4) and Ca-langbeinite (KC2S¯3). The impacts of alkali sulfates, prepared through high temperature heating in the laboratory, on cement hydration and hardening properties were studied. Results show that the initial setting time would not significantly shorten or extend with the increase of alkali sulfates/SO3 molar ratio. Alkali sulfates cannot promote 1 d compressive strength, and result in significantly linear decrease of 28 d compressive strength and increase of drying shrinkage with increasing of the molar ratio of Na2Oeq/SO3. The adverse effect of alkali sulfate on cement hydration and hardening properties is significant, and the content is better less than 0.5 of Na2Oeq/SO3 molar ratio. Ca-langbeinite in cement slightly decreases compressive strength and increases drying shrinkage, but can replace gypsum to play a good role in cement. Besides, it is found that alkali sulfates would promote the growth of C-S-H and portlandite during cement hydration.
•Ettringite seeds remarkably accelerated the hydration of pastes in the presence of PCE.•3-h compressive strength increased by 290% when containing 4% ettringite seeds.•The compressive strengths ...increased with seed content less than 1% at all curing ages.•Ettringite seeds improved the morphology and porosity of CSA pastes.
Polycarboxylate superplasticizers have been applied in calcium sulphoaluminate (CSA) cement-based materials to obtain a good workability, while the concomitant retardation in hydration results in a slow early strength development after setting. Ettringite seed crystals synthesized in this research are expected to work as seed nucleus and accelerate the hydration of CSA at early ages in the presence of polycarboxylate-ether (PCE) superplasticizer. The effect of ettringite seed on setting time, rheological properties, compressive strength, volume changes, phase development and microstructure of CSA pastes was experimentally investigated. Results showed that the addition of ettringite seed crystals indeed promoted the hydration and significantly improved the compressive strength and microstructure of CSA pastes at early ages, while reduced the setting time, the yield stress and plastic viscosity. Compared with the control paste, 3-h compressive strength of CSA paste containing 4% ettringite seed increased by 290%. The compressive strengths increased with increasing ettringite seed content less than 1% at all curing ages, with an adequate setting time and rheological properties.
To understand the effects of colloidal nanoSiO2 (CNS) on cement hydration and gel properties in the early and later age, hydration heat, calcium morphology, hydroxide content, non-evaporable water ...(NEW) content and nanoscale mechanical properties were measured. Some comparison studies were conducted on silica fume (SF) paste, as well. Results revealed that the accelerating effect of CNS on hydration in the early age is achieved by the acceleration of cement dissolution and hydrate nucleation on reacted nanoSiO2 particles. Although cement hydration can be greatly accelerated by CNS in the early age, its later age hydration is hindered. The NEW content of CNS-added paste experiences a higher rate of increase initially, but gradually becomes smaller than that of the control paste due to changes in the gel structure, making NEW content an unsuitable method for monitoring the hydration of CNS-added paste. However, nanoindentation results revealed that CNS modifies the gel structure to increase the high-stiffness C–S–H gel content.
Knowing and promoting the strength development of concrete at an earlier age is essential for accelerating formwork circulation of the on-site construction and precast product manufacture. The ...strength development rate at earlier ages of less than the initial 24 h was investigated. The effect of measures of adding silica fume, calcium sulfoaluminate cement, and early strength agent on the strength development of earlier concrete at ambient temperatures of 10, 15, 20, 25, and 30 °C was studied. The microstructure and long-term properties were further tested. It is shown that the strength increases exponentially first and then logarithmically, different from what is commonly recognized. Increasing cement content exhibited a certain effect only above 25 °C. When the cement content increased from 420 to 460 kg/m
, the strength only increased from 6.2 to 6.7 MPa after 12 h at 25 °C. The early strength agent could increase the strength significantly, the strength could be increased from 6.4 to 10.8 MPa after 20 h at 10 °C and from 7.2 to 20.6 MPa after 14 h at 20 °C. All measures for promoting earlier strength did not have an evident negative effect. The results could be potentially referred for the formwork removal at a suitable moment.
► We investigated to make use of CFBC fly ash and slag containing about 10% SO3. ► Up to 97% of CFBC fly ash and slag can be used to make an autoclaved brick. ► The compressive strength of the ...autoclaved brick reached 14.3MPa. ► The autoclaved brick had no destructive expansion even cured in water for 270days. ► There was no AFt and dihydrate gypsum in the autoclaved brick.
Circulating fluidized bed combustion (CFBC) fly ash and slag are wastes from CFBC power plant, and have such a high sulfur content (usually more than 5% SO3 by weight) that would result in a great risk of destructive expansion when used in building materials, which are produced in normal temperature. This work investigated the preparation conditions of autoclaved brick with CFBC fly ash and slag, and compared the long-term volume stability and the hydration products between the autoclaved brick and adobe one. It is shown that the autoclaved brick could be made up of 77% CFBC fly ash, 20% CFBC slag and 3% cement by weight, and exhibited good long-term volume stability, and achieved the compressive strength of up to 14.3MPa, and that there was no dihydrate gypsum and ettringite formation in the autoclaved brick so that the destructive expansion could be avoided. This work has carried out practical production and application of the autoclaved brick. It is concluded that CFBC fly ash and slag can be used to make high-quality brick as long as it is autoclaved.
Creep deformation is an important aspect of magnesium phosphate cement (MPC) used as a structural material. In this study, the shrinkage and creep deformation behaviors of three different MPC ...concretes were observed for 550 days. The mechanical properties, phase composition, pore structure, and microstructure of MPC concretes after shrinkage and creep tests were investigated. The results showed that the shrinkage and creep strains of MPC concretes stabilized in the ranges of -140 to -170 με and -200 to -240 με, respectively. The low water-to-binder ratio and the formation of crystalline struvite were responsible for such low deformation. The creep strain had almost no effect on the phase composition; however, it increased the crystal size of struvite and reduced the porosity, especially the volume of pores with diameters <20 nm and >200 nm. The modification of struvite and densification of microstructure led to an improvement in both compressive strength and splitting tensile strength.
Magnesium phosphate cement (MPC) is a potential inorganic binder for steel coating due to setting and hardening rapidly, and bonding tightly with steel. NH4H2PO4-based MPC as a fire-retardant coating ...for steel was investigated in this work. MPC coatings were prepared from MPC paste and MPC mortar with expanded vermiculite (EV). The physical-mechanical properties and fireproof performance of MPC coatings were investigated in detail. An infrared thermal imager was employed to collect the temperature distribution and temperature rise with time on the coating samples automatically. The X-ray diffraction (XRD) and Scanning Electron Microscope (SEM) analyses were carried out on the MPC coating after the fireproof test. Re-fire test and corrosion resistance were performed preliminarily on the MPC coating. The results showed that the fireproof performance of MPC coating met the fire protection requirement for steel as long as the thickness of the MPC paste coating was up to 10 mm, while the thickness of MPC mortar coating decreased to 4 mm when adding 40% EV (by mass). Dehydration and decomposition of reacted products in the hardened MPC coating were, to some extent, contributed to the excellent fireproof performance during the fire test. The slight ceramic formation and integration of MPC coating during the fire test would compensate for the decreasing of strength due to the dehydration and decomposition, so that the MPC coating would keep certain fireproof performance when undergoing fire again. MPC is suitable for a fire-retardant coating, while higher tensile bonding strength with steel and potential corrosion resistance on steel, as well as rapid surface drying and hardening can be achieved.
Experiments were performed to determine the effect of calcined phosphogypsum on the strength of fly ash–lime binders. Significant strength increases compared to binders without calcined phosphogypsum ...were observed due to the activation. But lowering the lime to calcined phosphogypsum ratio of blends with the same fly ash content yielded a relatively lower compressive development at late ages. Strengths of samples cured first at 45
°C in over 90% R.H. for 12
h and then at room temperature were better than those cured at room temperature all the time. X-ray analysis suggests that the activation of calcined phosphogypsum to the systems was due to the formation of ettringite and dihydrate calcium sulfate during the hydration process.
Magnesium phosphate cement (MPC) is a promising alternative cement. However, the rheological property of this new binder is still to be explored. In this study, Response Surface Methodology (RSM) was ...adopted with Central Composite Design (CCD) to establish mathematical models describing the rheological characteristics of MPC in terms of initial mini slump (Y1), mini-slump loss (Y2), yield stress (Y3) and plastic viscosity (Y4), as a function of three independent variables, namely, water-to-solid ratio (W/S ratio, X1), MgO to MKP ratio (M/P ratio, X2) and borax dosage (X3). The results show that the M/P ratio and borax dosage could significantly affect the yield stress and mini-slump loss of MPC, while the W/S ratio was the significant coefficient influencing plastic viscosity and initial mini slump. The numerical optimised values of X1, X2 and X3 were 0.280, 7.528 and 0.170, respectively, and an MPC paste with desirable rheological characteristics (Y1 161.858 mm, Y2 11.282, Y3 0.680 Pa, Y4 0.263 Pa·s) with the highest desirability of 0.867 can be obtained.
Ternesite–ye’elimite (TCSA) cement is a new type of environmentally advantageous binder prepared by introducing ternesite, a reactive phase, into belite calcium sulfoaluminate cement clinker. This ...paper reports the laboratory production of TCSA cement by the addition of minor elements to achieve the coexistence of ternesite and ye’elimite. The influence of dopants on the mineralogical composition of clinkers and the clinkering conditions for the preparation of TCSA cement clinkers were investigated by X-ray powder diffraction and scanning electron microscopy. The mechanical properties and hydration products of the cement pastes were also studied. The results indicated that the addition of CaF2, P2O5 and Na2O can promote the coexistence of ternesite and ye’elimite, and that Na2O is the most effective candidate. TCSA cement clinkers could be successfully prepared at 1150 °C for 30 min by doping 0.3% Na2O. The TCSA cement clinkers exhibited shorter setting times than the BCSA cement clinkers. The later strength of TCSA cement showed a significant increase compared with BCSA cement. The effect of Na2O was different on the strength development for TCSA and BCSA cement. The dissolution of ternesite could promote the formation of ettringite. The reactivity of belite was higher in TCSA cement due to the formation of strätlingite.
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