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 the inter facial zone between aggregate and paste of Portland cement as well as the orientation coefficient of portlandite ( Icn ) were preliminarily studied by XRD and SEM methods. The main products in the zone are C -S-H gel, portlandite, AFt and pores, micro-cracks as welt as unhydrated portland cement clinker particles. Potlandite not only grows well but exists in orientating forms, and near the interface it exists in a state parallel to the surface of aggregate by its (001) lattice plane. Icn rises with the increase of cement water ratio (w/c) and the development of the cement hydration. The higher the w/c, the more the pores and micro-cracks in the zone. w/c bears an exact relation to the decrease of bond strength. Icn, is decreased when 5. 0% silica fume is added to cement. Bond strengths at early ages are reduced by adding 0. 5% FDN (one kind of water-reducing admixture) due to the retarded hydration of cement, but they will increase 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 of principal minerals in Jiepai kaoline and demonstrates the black matter to be not organic matter but fine scaly mica-hematite aggregate in black vein mud, the nanometer mineral-portlandite Ca(OH)(2) in this kaoline is discovered.

Hydration and Mechanical Properties of Portland Cement Blended with Low-CaO Steel Slag

The hydration and mechanical properties of Portland cement blended with low-CaO steel slag were studied and reported. The steel slag was used to replace cement up to 30% and then blended cement powder, paste and mortar samples prepared for the experiment. The quantitative analysis of XRD shows that ettringite formation is greatly reduced by incorporation of steel slag but there was a relatively low reduction of portlandite. Thermal analysis by TG shows that slag injection reduced portlandite content in the cement by at least 50%. Generally, the slag cement pastes required less water to form a workable paste compared to the reference cement, reducing as the slag content was increased. However, the setting times were higher than the reference. The permeability of the blended cement samples were lower than the control. The incorporation of 5% slag could not have an effect on the compressive strength of the concrete. The results confirmed that whilst cements with up to 15% slag content satisfied the strength requirements of class 42.5 N and those containing 20%-30% produce Class 32.5R cement.

Hydration Properties of Calcined Clay Pozzolan and Limestone Mineral Admixtures in Binary and Ternary Cements

This paper investigates the properties of hydrated binary and ternary blended cements using limestone and calcined clay pozzolan as supplementary cementitious materials. The blended cements were hydrated and their phase compositions were evaluated by thermogravimetric and powder X-ray diffraction at 28 days. The morphology of the samples was also determined. The water demand, setting time, compressive and flexural strengths of mortar and concrete samples were determined up to 365 days. The study concluded that the portlandite [Ca（OH）2] content was considerably reduced whilst ettfingite formation were enhanced as a result of admixture reactions. The water demand and setting times of blended cements were lower than OPC with 5% admixture content but higher with increasing content. The mechanical test results also showed that Class 42.5N and 32.5R cements can be produced from the binary and ternary blends containing up to 10% and 20% admixtures, respectively.

Influence of Mineral Admixtures on Essential Properties of Ternary Cement Blends

Two CaCO3-based materials （limestone and clamshells） and steel slag were used as mineral admixtures in cement to produce ternary blends and their influences on hydration and portlandite formation were analyzed. Additionally, mechanical properties were determined. These properties were determined using X-ray diffraction and scanning electron microscopic/energy dispersive X-ray analytical techniques as well as applying methods specified by EN （European Standards） and ASTM （American Standards for Testing and Materials）. The portlandite （Ca（OH）E） content was considerably reduced from 36.9% of reference cement to between 13.79% and 15.5%. With the water demand and setting times of the cements containing up to 10%, admixtures did not change significantly. The mechanical tests results showed that ternary blends produced 2-day strengths higher than that specified by EN 197-1 and that blends containing up to 20% admixtures can be used to produce both Class 32.5N and 42.5N cements.