Microstructure Of Blended Cement Pastes Research Articles

Utilization of coal gangue power generation industry by-product CFA in cement: Workability, rheological behavior and microstructure of blended cement paste

Circulating fluidized bed fly ash (CFA), by-product of coal gangue power generation industry based on circulating fluidized bed combustion technology, differs from ordinary fly ash (OFA) in its physical and chemical properties. This paper aims to investigate the influences of CFA with three particle sizes on the workability and rheological properties of cement paste. The results show that untreated raw CFA (RCFA) can significantly degrade many properties of fresh cement paste. Directly using CFA as a mineral admixture in cement-based materials is not suitable in terms of workability. However, CFA through grinding methods can significantly mitigate the detrimental effect of CFA on the flowability of cement paste. When 15% ultrafine CFA (UCFA) was added, the cement paste exhibited relatively favorable workability and rheological properties. The rheological behavior of cement paste containing CFA with three particle sizes follow the shear thinning, which is in line with the modified Bingham model. Furthermore, the reasons for the impact of CFA on the microscopic level of cement workability are analyzed and discussed. The addition of CFA reduces the amount of free solution in the cement paste, leading to the formation of a flocculated structure and an increase in the thixotropic loop area. Based on the analysis of TOC, BET and TEM, it was determined that numerous polycarboxylate superplasticizer (PCE) molecules are adsorbed onto the rough and porous surface structure of CFA particles, hence the destructive ability of PCE to the flocculation structure caused by cement hydration is weakened. Therefore, the mechanism that CFA affecting the working performance of cement paste can be determined as: the adsorption of free water and PCE molecules.

Effect of particle size and CO2 treatment of waste cement powder on properties of cement paste

This paper studies the role of CO2 treatment and the impact of particle size (<75 μm and 75–150 μm) of waste cement powder (WCP) with different cement replacement content (0%, 5%, 10%, 15%, 20%, and 30%) on the physical properties and microstructure of blended cement paste. The results show that carbonation of WCP can effectively increase the flowability of paste due to the formation of calcite and decrease the porosity of WCP microstructure, while the water demand to achieve the same workability decreases with increasing size of WCP particles. Cement paste containing decreased particles of carbonated waste cement powder possesses a higher 28 day compressive strength due to formation of a higher amount of calcite and hydration products, based on the thermogravimetric analysis.

Hydration and strength development in blended cement with ultrafine granulated copper slag.

This study aims at evaluating the effect of ultrafine granulated copper slag (UGCS) on hydration development of blended cement and mechanical properties of mortars. The UGCS with the median particle size of 4.78 μm and BET surface area of 1.31 m2/g was used as a cement replacement to prepare blended cements. Hydration heat emission of blended cement and mechanical performance of mortars were investigated by using isothermal calorimetry and strength tests, respectively. X-ray diffraction (XRD), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM) were applied to the analysis of pozzolanic reaction and hydration products. The results illustrate that UGCS has influence on the hydration heat evolution of blended cement due to its filler effect and pozzolanic reaction. The cumulative hydration heat of blended cement is reduced by partial cement replacement with UGCS. The test mortar prepared by using blended cements with 30 wt. % UGCS shows a retardation of strength development with a low value at early ages (7 days) and a rapid growth at later ages (28 days). The 90-day compressive strength of test mortar is 45.0 MPa close to that of the control mortar (49.5 MPa). The obtained results from XRD and TGA analysis exhibit an increase in calcium hydroxide (CH) consumption and calcium silicate hydrates (C–S–H) formation in blended cement pastes with curing time. The cement replacement with UGCS induces changes in microstructure of blended cement paste and chemical composition of hydration products.

Unveiling micro-chemo-mechanical properties of C–(A)–S–H and other phases in blended-cement pastes

Current demand for novel highly sustainable concrete urges improving the links between chemical and mechanical effects of supplementary cementitious materials (SCMs) on the microstructure of blended-cement pastes. In this work, coupled NanoIndentation and Quantitative Energy Dispersion Spectroscopy (NI-QEDS) was applied to disclose chemo-mechanical properties of microstructure phases in systems incorporating typical dosages of fly ash, slag, metakaolin, or glass powder. For a fixed water-to-binder ratio of 0.4, the C–(A)–S–H chemistry was significantly affected by the SCMs, while its average mechanical properties varied within the limited ranges M ≈ 25–27 GPa, H ≈ 0.7–0.8 GPa and C ≈ 180–230 GPa. The SCMs further changed the arrangement of anhydrous phases and minor hydrates embedded in the C–(A)–S–H matrix, which were also characterized (e.g., Portlandite, AFm such as carboaluminates, or an hydrotalcite-like phase). Finally, engineering properties of blended-cement matrices requires tailoring both the intermix of hydrates and the rigid anhydrous SCM inclusions.

The Effect of Palm Oil Fuel Ash as a Supplementary Cementitious Material on Chloride Penetration and Microstructure of Blended Cement Paste

This article presents the effect of palm oil fuel ash as a supplementary cementitious material on chloride penetration and microstructure of blended cement paste. Palm oil fuel ash (POFA) was ground to obtain two finenesses: one was the same size as the cement and the other was smaller than the cement. Type I ordinary Portland cement (OPC) was replaced by POFA at 0, 10, 20, 30, and 40% by weight of binder. All paste specimens were prepared using the same water to binder ratio as 0.35. The compressive strength, pore size distribution, total chloride content, free chloride content, and X-ray diffraction analysis of chloride penetration into blended cement pastes were investigated. The results indicated that, at 60 and 90 days, the blended cement pastes containing 30% of POFA with high fineness had 1.6 and 4.9% higher compressive strength than that of the OPC paste, respectively. POFA pastes had a lower chloride diffusion coefficient and shallower concentration profile of free chloride than that of the OPC paste. The specimens containing coarse fineness and small particle size POFA had lower chloride diffusion coefficient than OPC paste ranging between 13.2 and 61.0%. In addition, the chloride diffusion coefficient is linearly correlated with the critical pore diameters. Replacement of OPC by the fine-grained POFA resulted in the decrease in free chloride and in the chloride diffusion coefficient.

An Evaluation of Shrinkage Model Based upon Microstructure of Blended Cement Pastes

As shrinkage in concrete leading to the subsequent cracking and reducing durability, the study on shrinkage is quite important. Based on microstructure of pores obtained from Nitrogen absorption method and mechanical testing, a numerical shrinkage model was built. The standard shrinkage test results show consistent in the blended cement pastes with different water-to-cement ratioand admixture content to predict shrinkage values, which confirmes the shrinkage model based upon microstructure of blended cement pastes.

Effect of fly ash fineness on microstructure of blended cement paste

This research demonstrates the effect of fly ash fineness on pore size and microstructure of hardened blended cement pastes. Two sizes of fly ash, original fly ash and classified fly ash were used to replace Portland cement type I paste. Test results indicated that the pore sizes of hardened blended cement paste were significantly affected by the rate of replacement and the fineness of fly ash. The replacement of cement by original fly ash decreased the pore sizes of blended cement paste and the incorporation of classified fly ash resulted in a further decrease in the pore sizes of blended cement paste. The X-ray diffraction (XRD) results showed that the blended cement paste with classified fly ash was more effective at reducing the intensity of Ca(OH) 2 than that with the original fly ash. The scanning electron microscope (SEM) results revealed that the hardened blended cement paste containing finer fly ash produced a denser structure than the one containing coarser fly ash.