The engineering properties and microstructure of sodium carbonate activated fly ash/ slag blended mortars with silica fume

Conventional cementitious materials as tunnel supporting materials are utilised in the construction of the final repository for spent nuclear fuel. However, the use of cementitious material releases alkaline ions from pH12 to pH13 plumed into groundwater. Such a high pH is detrimental to the performance of the bentonite functioning, which may possibly enhance the dissolution and alteration of the fracture buffer and filling materials. Instead, low-pH cementitious materials are being developed for use in geological repositories. This study is aimed at evaluating the usability of low-pH cementitious materials containing 40% silica fume or composites blended with 20% silica fume and 40% fly ash. Engineering properties were analysed and verified through experimental research using the flow, compressive strength, pH measurement and hydraulic conductivity. Test results show that the replacement level with 40% of silica fume or 20% of silica fume and 40% of fly ash was suitable for the mixture of low-pH cementitious. Compared to the compressive strength and water permeability of ordinary cementitious, those of low-pH cementitious enhanced better engineered performances at the age of 91 days. The information is contributed us to establish the long-term durability and environmental requirements of disposal repositories in Taiwan.

Download Full-text

Basic Properties of fly Ash/Slag -Concrete Slurry Waste Geopolymer Activated by Sodium Carbonate and Different Silicon Sources

Frontiers in Materials ◽

10.3389/fmats.2021.751585 ◽

2021 ◽

Vol 8 ◽

Author(s):

Yingli Gao ◽

Kairui Duan ◽

Shuncheng Xiang ◽

Wei Zeng

Keyword(s):

Fly Ash ◽

Silica Fume ◽

Sodium Carbonate ◽

Negative Impact ◽

Reference Group ◽

Setting Time ◽

Sodium Metasilicate ◽

Fly Ash Concrete ◽

Granulated Blast Furnace Slag ◽

Silicon Sources

As a kind of granular waste with complex composition and alkali corrosiveness, concrete slurry waste (CSW) has severe recycling limitations in the ordinary Portland cement (OPC). Considering this, a new type of geopolymer, prepared by granulated blast furnace slag/fly ash, concrete slurry waste, and powdered activators (sodium carbonate and different silicon sources including sodium metasilicate pentahydrate and silica fume), was adopted to conduct a comparative study with the OPC counterpart. In this study, the homogenized CSW was mixed in the OPC and geopolymer with a constant ratio of 50 wt%, respectively. Then the properties were studied in terms of the flowability, setting times, mechanical strengths, and microstructures. The results showed that better flowability (200 mm) could be achieved in the obtained geopolymer than in the OPC reference group (95 mm) by increasing the powdered activators. The setting time of the OPC was significantly shortened due to the addition of CSW. The strengths of geopolymer were supported by the produced C-A-S-H and carbonates, with less chemically bonded water than the hydration products in the reference group. The dominant size of pores in the hardened geopolymer was much smaller than that in the OPC group which was 80 nm. Silica fume could be the alternate of the sodium metasilicate pentahydrate and had an insignificant negative impact on the fresh and hardened properties and microstructures of the geopolymer when the incorporation rate was within 5%.

Download Full-text

The Effect of Incorporating Ultra-Fine Spherical Particles on Rheology and Engineering Properties of Commercial Ultra-High-Performance Grout

Crystals ◽

10.3390/cryst11091040 ◽

2021 ◽

Vol 11 (9) ◽

pp. 1040

Author(s):

Wei-Ting Lin ◽

Wen-Que Zhao ◽

Yi-Hua Chang ◽

Jiann-Shi Yang ◽

An Cheng

Keyword(s):

Compressive Strength ◽

Fly Ash ◽

Silica Fume ◽

High Performance ◽

Hunan Province ◽

Engineering Properties ◽

Spherical Particles ◽

Chloride Diffusion ◽

Chloride Diffusion Coefficient ◽

Volume Stability

In this study, ultra-fine spherical particles of silica fume and reactive ultra-fine fly ash were added to a mixture of commercial ultra-high-performance grout (UHPG) with the aim of enhancing the rheological properties, compressive strength, compactness, and permeability. This commercial UHPG study was conducted in collaboration with Triaxis Corporation (Changsha city, Hunan province, China). A water-to-binder ratio of 0.21 and a binder-to-fine aggregates ratio of 1.17 were used as fixed parameters, and the binders were a combination of type-II Portland cement, sulphoaluminate cement, silica fume, and reactive ultra-fine fly ash (RUFA). Polycarboxylate superplasticizer powder was used to control the rheology. The results revealed excellent compressive strength, volume stability, and resistance to chloride penetration. Mercury intrusion porosimetry and scanning electron microscopy tests revealed that the medium-sized RUFA particles with small silica fume particles completely filled the spaces between large cement particles to achieve optimal densification. This mixture also produced dense hydration and calcium-silicate-hydrates colloids, which filled the microstructures of the UHPG resulting in excellent engineering properties and durability. This commercially available UHPG mix responded to excellent compressive strengths approaching 120 MPa and exhibited good workability with a loss of slump-flow rate up to 33% after 60 min. It also exhibited very low abrasion resistance (0.5%), stable shrinkage and expansion rates (stabilization over 10 days), very low chloride diffusion coefficient (less than 0.1 × 10−14 m2/s) with a denser microstructure. This commercial UHPG (UHPG-120) has been developed to meet the needs of the market.

Download Full-text