Effect of calcination temperature on the pozzolanic activity of maize straw stem ash treated with portlandite solution

The effect of calcination temperature on the pozzolanic activity of maize straw stem ash(MSSA)was evaluated.The MSSA samples calcined at temperature values of 500,700,and 850℃ were dissolved in portlandite solution for 6 h,thereby obtaining residual samples.The MSSA and MSSA residual samples were analyzed using Fourier transform infrared spectroscopy,X-ray powder diffraction scanning electron microscopy,and X-ray photoelectron spectroscopy to determine vibration bonds,minerals,microstructures,and Si 2p transformation behavior.The conductivity,pH value,and loss of conductivity with dissolving time of the MSSA-portlandite mixed solution were also determined.The main oxide composition of MSSA was silica and potassium oxide.The dissolution of the Si^(4+) content of MSSA at 500℃ was higher than those of the other calcination temperatures.The conductivity and loss of conductivity of MSSA at 700℃ were higher than those of the other calcination temperatures at a particular dissolving time due to the higher KCl content in MSSA at 700℃.C-S-H was easily identified in MSSA samples using X-ray powder diffraction,and small cubic and nearly spherical particles of C-S-H were found in the MSSA residual samples.In conclusion,the optimum calcination temperature of MSSA having the best pozzolanic activity is 500℃,but excessive agglomeration must be prevented.

The pH of Cement-based Materials:A Review

Cement-based materials (CBMs),such as paste,mortar and concrete,are highly alkaline with an initial high pH of approximately 12.0 to 13.8.CBMs have a high pH due to the existing oxide mineral portlandite and alkali metal contents in Portland cement.The high pH of concrete provides excellent protection and reinforces the steel bars against corrosion.The pH of concrete does not remain constant due to ageing and other defect-causing factors,such as chloride ingress,alkali leaching,carbonation,corrosion,acid attack,moisture and biodegradation process.Reducing the concrete pH has negative impact on the strength,durability and service life of concrete buildings.However,the high pH of concrete may also cause concrete structure deterioration,such as alkali silica reaction,porosity and moisture related damages in concrete structures.The pH of CBMs can be influenced by high temperatures.For instance,the extremely high volume (85%-100%) of slag-blended cement pastes shows considerable pH reduction from 12.80 to 11.34 at 800 ℃.As many concrete structure deterioration are related to concrete pH,using an accurate and reliable method to measure pH and analyse the durability of reinforced concrete structure based on pH values is extremely important.This study is a comprehensive review of the pH of CBM in terms of measurement,limitations and varying values for different CBM types.

Effect of High Temperatures on the Microstructure of Cement Paste

The microstructural and compositional changes within the cement paste exposed to high temperatures were monitored by XRD, FTIR, TGA/DTA and SEM techniques to understand the nature of decomposition of C-S-H gel and the associated physicomechanical properties of thermally damaged cement paste. OPC paste (w/c ratio 0.27) was hydrated for 28 days then fired up to 750°C for 2 hours (heating rate 10°C/min). The relative mass percent of calcium hydrates and portlandite was estimated by calculations derived from TGA results. Beyond 450°C, the percentage of portlandite sharply diminishes and C-S-H progressively decomposes into C2S and C3S until complete loss of calcium hydrates content occurs at 750°C. An increase of the total porosity, severe loss in mechanical strength and propagation of harmful cracks occurs. The thermal shock as a result of cooling of the heated cement paste and the rehydration of lime enhance the propagation of harmful cracks.

INVESTIGATIONS ON THE INTERFACIAL ZONE BETWEEN AGGREGATE AND PASTE OF PORTLAND CEMENT

The composition and structure of theinterfacial zone between aggregate and paste ofportland cement as well as the orientationcoefficient of portlandite(Icn) werepreliminarily studied by XRD and SEMmethods.The main products in the zone are C—S—H gel,portlandite,AFt and pores,micro-cracks as well as unhydrated portlandcement clinker particles.Potlandite not onlygrows well but exists in orientating forms,andnear the interface it exists in a state parallel tothe surface of aggregate by its(001)latticeplane.Icn rises with the increase of cementwater ratio(w/c)and the development of thecement hydration.The higher the w/c,themore the pores and micro-cracks in thezone.w/c bears an exact relation to the decreaseof bond strength.Icn is decreased when 5.0%silica fume is added to cement.Bond strengthsat early ages are reduced by adding 0.5% FDN(one kind of water-reducing admixture)due tothe retarded hydration of cement,but they willincrease continually and become greater than those of other samples after 28 days.

THE STUDY OF MINERAL COMPONENTS IN KAOLINE FROM HUNAN JIEPAI

This paper finds out distributive pattern ofprincipal minerals in Jiepai kaoline and demonstrates theblack matter to be not onganic matter but fine scaly mica-hematite aggregate in black vein mud,the nanometer mineral-portlandite Ca（OH）2 in this kaoline is discovered.