Effect of Limestone Powder Mixing Methods on the Performance of Mass Concrete.

Using limestone powder (LP), the by-product of manufactured sand, to replace part of fly ash (FA) or manufactured sand could not only turn waste into treasure and decrease the price of concrete, but could also enhance the performance of concrete and reduce environmental pollution. However, the impact of various LP incorporation methods on the performance of mass concrete was inconsistent. In this paper, the effects of LP on the workability, compressive strength, constrained expansion rate, hydration temperature and impermeability of mass concrete were studied by replacing FA or manufactured sand alone and replacing FA and manufactured sand simultaneously. The results showed that the impact of LP on the performance of mass concrete was equal when it replaced FA alone and FA and manufactured sand at the same time. When the replacement amount was 20%, the workability, expansibility and early strength of concrete were improved, but the later strength and impermeability were slightly reduced. The workability, compressive strength, expansibility and impermeability of mass concrete were improved when manufactured sand was replaced alone, and the optimal dosage was 10%. The LP, moreover, reduced the hydration temperature peak of concrete in three kinds of mixing methods, but the temperature peak appeared earlier. At lower dosages, LP optimized pore structure and promoted the early hydration of cement through filler effects and nucleation effects. When LP replaced manufactured sand, the microstructure of concrete was more dense, so the replacement of manufactured sand had a better effect on the improvement of concrete properties. A reference value for the use of LP in mass concrete is provided in this study.

Effect of Sodium Gluconate on Properties and Microstructure of Ultra-High-Performance Concrete (UHPC).

The properties of concrete can be significantly affected by sodium gluconate (SG) at very small dosages. In this paper, the effects of SG on the fluidity, setting time, heat of hydration, and strength of ultra-high-performance concrete (UHPC) were studied. The results show that (1) in the plastic stage, SG inhibited the formation of early ettringite (AFt) and delayed the hydration of tricalcium silicate (C3S) and dicalcium silicate (C2S). SG increased the initial fluidity of UHPC without decreasing within 1 h. When the SG dosage was ≥0.06%, the slumps at 30 min and 60 min increased slightly. (2) In the setting hardening stage, the addition of SG inhibited the formation of calcium hydroxide (CH), which significantly extended the setting time of UHPC. When the dosage of SG was 0.15%, the initial and final setting times were 5.0 times and 4.5 times that of the blank group, respectively. SG had no obvious effect on the hydration rate of cement in the accelerated period, but the peak hydration temperature of UHPC was increased when the SG dosage was 0.03~0.12%. (3) In the strength development stage, the 1 d and 3 d strength of UHPC decreased significantly with the increase in the SG dosage. However, SG could promote the formation of AFt at the pores and aggregate interface in the later stage, reduce the porosity of cementite, and improve the compressive strength of UHPC in 28 d, 60 d, and 90 d. When the SG dosage was 0.12%, the 90d strength increased by 13%.

Effect of Moisture Condition of Brick-Concrete Recycled Coarse Aggregate on the Properties of Concrete.

The application of brick-concrete recycled aggregates can alleviate the problem of increasing construction waste and increasing scarcity of natural aggregates. The different moisture condition of coarse aggregates can significantly affect the performance of brick-concrete recycled aggregate concrete. In this paper, the additional water quantity of dry and air-dried brick-concrete recycled coarse aggregate concrete was determined. Additionally, the fluidity, rheological parameters, autogenous shrinkage, strength and chloride ion penetration resistance were tested, and compared with saturated surface dry recycled brick-concrete coarse aggregate concrete and natural aggregate concrete. The results showed that the slump of concrete was increased, whereas the plastic viscosity, static and dynamic yield stress were decreased by adding additional water or using saturated surface dry coarse aggregate. Compared with the dry and saturated surface dry state, the air-dried recycled coarse aggregate concrete has the smallest 28 days autogenous shrinkage value, higher compressive strength and splitting tensile strength, and less adverse effects on chloride permeability. It is most beneficial to the performance and economy of concrete to adopt the air-dried state when the brick-concrete recycled coarse aggregate is applied in engineering.

Effect of Naphthalene-Based Superplasticizer and Polycarboxylic Acid Superplasticizer on the Properties of Sulfoaluminate Cement.

In order to study what the effect of superplasticizers on the setting time, fluidity and compressive strength of calcium sulfoaluminate cement (CSA) a naphthalene-based superplasticizer (BNS) and a polycarboxylic acid superplasticizer (PC) were selected to interact with CSA pastes and ye'elimite, respectively. X-ray diffraction (XRD), thermogravimetric (TG) analysis and scanning electron microscopy (SEM) analytical methods were used to investigate the class, amount and microstructure of the CSA pastes and ye'elimite pastes hydration products under the effect of the superplasticizers. The results showed that the addition of BNS can promote ettringite generation and thus improve the early compressive strength. As the addition of BNS increased from 0.8 wt% to 2.0 wt%, the initial setting time was prolonged 10 min, the final setting time was prolonged 7 min, the 5 min fluidity was improved from no fluidity to 220 mm. However, as the addition of PC increased from 0.08 wt% to 0.20 wt%, the setting time of the PC just changed within 3 min; the 5 min fluidity increased from 110 mm to 195 mm and no 15 min fluidity at all was observed. AS seen by SEM, it can be stated that generated ettringite under the addition of PC was layered and lacking bonding, and its morphology changed from rod-like to flake-like, leading to a decrease in early compressive strength.