Synergistic Effect and Mechanism of Nano-C-S-H Seed and Calcium Sulfoaluminate Cement on the Early Mechanical Properties of Portland Cement.

The combined utilization of mineral accelerators and nano-seeding materials is a novel method to promote the early strength of cement-based materials. In this paper, the effects of nano-C-S-H seed (NCS) on the early compressive strength of the Portland cement (PC)- calcium sulfoaluminate cement (CSA) binder were investigated. The results showed that NCS and CSA synergistically contributed to the early strength of PC. In detail, a 326.3% increase in the 10 h compressive strength of PC paste was obtained through the addition of NCS (2 wt%) and CSA (5%) in common. This was higher than the sum of the increases observed with the single additions of CSA (157.9%) or NCS (87.6%), with the same above dosage, in PC. Meanwhile, the early strength enhancement effects of NCS and CSA, when used together in PC, lasted longer than the effects of either used alone. Moreover, the synergetic effect mechanism was analyzed by isothermal calorimeter, QXRD, TGA, MIP, and SEM techniques. The calorimetry, XRD, and TGA results demonstrated that the synergistic mechanism was associated with the synergistic promotion effects of CSA and NCS on the hydrates. The fast hydration of CSA produced large amounts of ettringite and also consumed partial free water to promote the performance of the seeding effect of NCS which, simultaneously, further accelerated the precipitation of C-S-H gel and CH. The high alkie environment was also beneficial for the continuous generation of ettringite. In addition, the results of MIP and SEM measurements showed that the micro-filling effect of NCS significantly optimized the pore structure of a PC-CSA blend-hardened paste. Thus, the synergistic strength enhancement effects of CSA and NCS on PC were attributed to the matching of the promotion of hydration generation and the optimization of pore structures in hardening cement paste. The results of this article provide a new approach to achieving the rapid development of the early strength of cementitious materials, with potential applications in precast concrete and low-temperature construction.

Uniaxial tensile deformation and fracture process of structures forming by unsaturated intercalation of amine molecule into C-S-H gel.

The application of cement-based materials in engineering requires the understanding of their characteristics and subsequent deformation and fracture process of C-S-H gel in service. In this work, three types of amine molecules including tetraethylenepentamine (TEPA), polyacrylamide (PAM), and triethanolamine (TEA) were intercalated into C-S-H gel in an unsaturated status successfully. Systematical analysis was performed on the structures and properties for both C-S-H gel and corresponding amine molecules/C-S-H gel. It was found that the unsaturated intercalation of amine molecules into C-S-H gel plays a key role in the geometry and therein density of nanocomposites. Subsequently, radial distribution function (RDF), time-correlated function (TCF), and mean square displacement (MSD) were applied to characterize the structure and dynamic information of the as-generated nanocomposites, demonstrating the occurrence of interaction between amine molecules with Ca-Si layer and acceleration of water diffusion by unsaturated intercalation of amine molecules into the interlayer region in C-S-H gel. Finally, the deformation and fracture process of C-S-H gel and amine molecules/C-S-H gel under uniaxial tensile loads were given by molecular dynamics simulation. It was indicated that the tangent modulus of nanocomposites demonstrates a strain-softening nature, indicating a visco-elastic behavior. The breakage of Ca-O bonds and hydrogen bonds dominates the fracture of C-S-H gel. Weak interaction for TEPA/C-S-H gel or TEA/C-S-H gel leads to a decreased tensile strength. Local stress concentration in other interlayer region governs the deformation and fracture process in spite of the formation of strong interaction between double bonded polar oxygen atoms in PAM molecules and Ca atoms in C-S-H gel.

Research on the Hydration Properties of C4A3S-CSH2 Cement System at Different Temperatures.

Sulphoaluminate cement has the advantage of low-temperature application performance, but its hydration mechanism at low temperatures is not yet clear. Anhydrous calcium sulfoaluminate (C4A3S) is the main mineral in the composition of sulfoaluminate cement clinkers. In this paper, C4A3S mixed with gypsum (CaSO4∙2H2O) to form a C4A3S-CSH2 cement system; X-ray diffraction (XRD), thermogravimetric analysis (TG-DTG), scanning electron microscopy (SEM) and mercury intrusion analysis (MIP) to clarify the effect of temperatures on the hydration properties of C4A3S-CSH2 cement system. The results showed that hydration of the C4A3S-CSH2 cement system could carry on at low temperatures, even at -15 °C. The main hydration product was ettringite. Low temperatures did not change the types of the hydration products, but the low temperature of 0 °C was more favorable for the formation of ettringite. The early hydration of the C4A3S-CSH2 cement system was inhibited by the decrease in temperature. However, hydration of the cement at 0 °C continued at a high rate after one day. Morphologies of the ettringite for the C4A3S-CSH2 cement system at -15 °C were needle-like structures, while they were of columnar structure at 0 °C. The compressive strength of samples at 0 °C reached 82 MPa, which is significantly higher than that at 20 °C.

Effect of Portland Cement versus Sulphoaluminate Cement on the Properties of Blended Lime-Based Mortars Prepared by Carbide Slag.

In order to improve the properties of lime-based mortars and promote the green development of the construction industry, blended lime-based mortars were prepared by using carbide slag instead of hydrated lime, and the additions of Portland cement and sulphoaluminate cement were studied in our work. The paper focused on mechanical properties, porosity, capillary water absorption and drying shrinkage of both types of blended mortars. The chemical composition and microstructure of hydration products were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results show that sulphoaluminate cement provided more contributions to mechanical properties, capillary water absorption and early shrinkage compared to Portland cement.

Performances and working mechanism of a novel polycarboxylate superplasticizer synthesized through changing molecular topological structure.

A novel star-shaped polycarboxylate superplasticizer (SPCE) was synthesized through a simple two-step method. 1H Nuclear Magnetic Resonance (1H NMR) and Infrared Spectroscopy (IR) measurements were used for structural characterization. SPCE and comb-shaped polycarboxylate superplasticizer (CPCE) with same molecular weights were designed and synthesized. The cement paste containing SPCE exhibited better fluidity, fluidity retention, water reduction, 25% lower saturated dosage of PCE, 10% longer setting time, lower hydration heat, more delayed hydration heat evolution and lower amount of hydration products at early ages. Furthermore, the adsorption behavior of SPCE and CPCE in cement pastes and the zeta potential were investigated, and then the working mechanism of SPCE was theoretically explained. It is interesting that changing topological structure from comb-shape to star-shape can achieve the optimization of dispersion effect, and further improve the working effectiveness. The aims of this study are to provide a new avenue to synthesize superplasticizer with novel structure achieving the chemical diversity of superplasticizer structure, and to verify the contribution of optimizing molecular shape. This new type of superplasticizer can be used as a rheology modifying agent in fresh cement-based materials.

Preparation, Characterization and Performances of Powdered Polycarboxylate Superplasticizer with Bulk Polymerization.

A polycarboxylate superplasticizer (PCE) was synthesized in a non-solvent system with bulk polymerization and then was pulverized into powdered form to achieve a rapid transportation and convenient preparation. PCE synthesized by using isopentenyl polyethylene glycol (TPEG) or isobutenyl polyethylene glycol (IPEG) as a macromonomer exhibited the best fluidities and retaining properties at 80 °C and 75 °C, respectively. Besides, azobisisobutyronitrile (AIBN) was suitable as an initiator, and the fumaric acid was suitable as the third monomer. The test results of ¹H nuclear magnetic resonance (¹H NMR) confirmed the occurrences of polymerization, and the measurement results of molecular weight and distribution showed that PCE molecular weight characteristics were in accordance with their fluidity properties in cement paste. The application performances in cement showed that PCEs with the best paste fluidity retentions had the longest final setting time and the shortest setting time interval, and the PCEs with good fluidity properties can obviously delay the hydration process and lower the hydration heat. Accordingly, this is a novel, energy-saving and economical method to prepare powdered PCE in the field of concrete admixtures.