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Showing 10 results for Allahverdi
EFFECTS OF SILICA MODULUS AND ALKALI CONCENTRATION ON ACTIVATION OF BLAST-FURNACE SLAG
A. Allahverdi, E. Najafi Kani, S. Esmaeilpoor,
Volume 5, Issue 2 (spring 2008 2008)
Abstract
Abstract: The use of alkali-activated cementitious materials especially over the past decades has significantly been increased. The goal of this research is to investigate the effects of silica modulus and alkali concentration on alkali-activation of blast-furnace slag. In this research, the most important physical characteristics of cementitious systems, i.e. the 28-day compressive strength and final setting time, were studied by changing influencing parameters such as silica modulus, i.e. SiO2/Na2O, (0.44, 0.52, 0.60, and 0.68) and Na2O concentration (4, 6, 8 and 10% by weight of dry binder) at a constant water-to-dry binder ratio of 0.25. Final setting time of the studied systems varies in the range between 55-386 minutes. The obtained results show that systems cured at an atmosphere of more than 95% relative humidity at room temperature exhibit relatively high 28-day compressive strengths up to 107 MPa.
APPLICATION OF FTIR-SPECTROSCOPY TECHNIQUE IN CHARACTERIZING MOLECULAR STRUCTURE OF NATURAL POZZOLAN-BASED GEOPOLYMER CEMENT
A. Allahverdi, E. Najafi Kani,
Volume 6, Issue 2 (Spring 2009 2009)
Abstract

geopolymer cement (inorganic polymeric binder) based on natural pozzolan are investigated. SiO

of activator and total molar ratios of Nacomposition exhibiting the highest 28-day compressive strength. Mixtures exhibiting the highest compressive strength were studied for their molecular structure using laboratory technique of Fourier transform infrared spectroscopy.Results obtained confirm that changes in chemical composition and curing condition can result in variations in degree of silicon substitution by aluminum in the second coordination sphere. Hydrothermal curing affects the molecular structure so that by increasing the hydrothermal curing temperatures, a lower degree of silicon substitution by aluminum in the second coordination sphere is observed. The molecular structure of the studied inorganic polymeric binde is composed of Si-O-Si chains bonded to Al-O and Si-O units creating two and three dimensional networks.

In this paper, the effects of chemical composition and curing conditions on molecular structure of2/Na2O molar ratio2O/Al2O3, and H2O/Al2O3 were changed to determine the optimum chemical
EFFECT OF POLYVINYL ALCOHOL ON FLEXURAL STRENGTH AND SOME IMPORTANT PHYSICALPROPERTIES OFPORTLAND CEMENT PASTE
A. Allahverdi,, K. Kianpur, M. R. Moghbeli,
Volume 7, Issue 1 (winter 2010 2010)
Abstract

Abstract:

This work was designed to investigate the simultaneous effect of both water-to-cement (W/C) and polymer-to-cement

(P/C) ratios on flexural strength of type V Portland cement pastes containing polyvinyl alcohol. The obtained results

confirmed the strong effect of W/C-ratio on the effectiveness of the added polymer so that there exist optimum values

for both W/C and P/C-ratios. Pastes prepared at optimum W/C and P/C-ratios of respectively 0.30 and 0.016 exhibited

the highest flexural strength which was more than two times the strength of unmodified pastes. Further measurements

on hardened paste specimens prepared at W/C=0.30 show that increasing the P/C-ratio increases dry bulk specific

gravity of the specimens and result in significant reductions in both total permeable pore volume and water absorption

of the specimens confirming the effective packing caused by polyvinyl alcohol acetate.

Some water-soluble polymers have proved potentials for improvement in properties of Portland cements.
INVESTIGATING SHRINKAGE CHANGES OF NATURAL POZZOLAN BASED GEOPOLYMER CEMENTPASTE
E. Najafi Kani, A. Allahverdi,
Volume 8, Issue 3 (september 2011 2011)
Abstract
Shrinkage behavior of a geopolymer cement paste prepared from pumice-type natural pozzolan was studied
by changing parameters of chemical composition including SiO2/Na2O molar ratio of activator and total molar ratios
of Na2O/Al2O3, and H2O/Al2O3. For investigating the effect of curing conditions on shrinkage, hydrothermal curing
was also applied. The obtained results clearly revealed the governing effect of chemical composition on shrinkage.
Mixes with different Na2O/Al2O3 molar ratios exhibited different shrinkage behavior due to variations made in
SiO2/Na2O molar ratio. Application of hydrothermal curing after a 7-day period of precuring in humid atmosphere
also showed strong effect on shrinkage reduction.
INVESTIGATING THE LINEAR EXPANSION, SET, AND STRENGTH BEHAVIORS OF THE BINARY MIXTURE PORTLAND CEMENT CLINKER-GYPSUM
A. Allahverdi, E. Najafi Kani, M. Fazlinejhad,
Volume 8, Issue 4 (december 2011)
Abstract
Abstract: The linear expansion, early-age compressive strength and setting times of the binary mixtures of gypsum and Portland cement clinkers of relatively low C3A-contents were investigated. For this reason, type 1, 2, and 5 of Portland cement-clinkers were selected and a number of binary mixtures were designed. At relatively lower percentages of gypsum (about 5%), the early strength behavior is improved. Results obtained for compressive strength of mixtures with 5% gypsum confirm the possibility of achieving 28- and 90-day compressive strengths up to values higher than 100 MPa and 130 MPa, respectively. At relatively higher percentages of gypsum (more than 25%), excessive expansion caused by ettringite formation results in the formation of micro-cracks effectively weakening the strength behavior. The work suggests that type S expansive cements could be produced from Portland cement clinkers of relatively low C3Acontents.
INVESTIGATING THE RELATIONSHIP BETWEEN HEAT OF HYDRATION AND PHYSICO-MECHANICAL PROPERTIES OF CALCIUM SULFATE HEMIHYDRATE IN THE PRESENCE OF ADDITIVES
E. Najafi Kani, M. Nejan, A. Allahverdi,
Volume 13, Issue 4 (December 2016)
Abstract

This article addresses the interplay between heat of hydration and physico-mechanical properties of calcium sulfate hemi-hydrate in the presence of retarding additives such as citric and malic acids and sodium citrate. The heat of hydration was measured using a semi-isothermal calorimeter. Results proved that citric and malic acids had superior impact on hydration and mechanical properties. While the concentration of additives was increasing, the maximum heat of hydration was decreasing from 56.15 cal/g.min for blank sample to 33 cal/g.min for high concentrations of citric and malic acids. Consequently, the measured time to this maximum heat of hydration and thus the induction period were increased significantly from 5 to 105 min. Mechanical results indicated that the increase in the amounts of additive led to the reduction of the compressive strength from 16.25 MPa in the blank sample up to 74% for the highest concentration of malic acid


Physico-Mechanical Properties of Organo-Modified Bentonite Reinforced Cement Mortars
A. Allahverdi, Z. Padar, M. Mahinroosta,
Volume 16, Issue 2 (June 2019)
Abstract
It is demonstrated that the addition of organo-modified Na-bentonite (OMB) particles to Portland cement mortar can promote its physical and mechanical properties. A series of experimental works on some important physico-mechanical properties of Portland cement mortars mixed with various dosages of hydrophobic OMB were performed. The obtained results confirm that the OMB provides a dense packing effect. An optimum replacement level of around 3.5% (by weight) at an increased water-to-cement ratio of 0.53 results in an almost 11.43% increase in 28-day compressive strength along with about 20.78 and 16.20% reductions in total volume of permeable pore space and water absorption, respectively. Also, at the optimum replacement level, an increase of about 2.72% is taken place in dry bulk specific gravity.

Resistance of Alkali-activated Slag Cement Against Sodium Sulfate
A. Allahverdi, H. Hashemi, M. Mahinroosta,
Volume 17, Issue 1 (March 2020)
Abstract
This work evaluates the resistance of alkali-activated slag (AAS) mortar against sodium sulfate attack. The effects of immersion in 5% sodium sulfate solution under room temperature and wetting-drying cycles on the compressive strength of mortar specimens were considered for evaluating the extent of degradation. Mortar specimens prepared from type II and V Portland cements (PC2 and PC5) in accordance with ASTM standard were also used as reference. To characterize the chemical products of the degradation process due to sodium sulfate attack, the specimens were also studied by X-ray diffractometry, scanning electron microscopy, and the elemental analysis by energy dispersive X-ray spectroscopy. After 360 days of exposure to the sodium sulfate solution, PC2, PC5 and AAS cements showed 71, 52 and 45% reduction in compressive strength, respectively. According to the obtained results, AAS cement exhibits a higher sulfate resistance compared to PC2 and PC5.

Mortar Properties Improvement by Using Fine Portland Cement Clinker as Reactive Aggregate
Jafar Shafaghat, Ali Allahverdi,
Volume 18, Issue 1 (March 2021)
Abstract

Microscopic studies has shown that adjacent to the interface between cement paste and aggregate, there exists an area with high porosity and low binding compounds that is referred to as interfacial transition zone (ITZ). ITZ in concrete and mortar imposes a number of negative effects, including flexural and compressive strengths reduction and permeability enhancement. That’s why many research attempts have been devoted to limit ITZ and its negative effects. The present study investigates the possibility of utilizing fine Portland cement (PC) clinker as a reactive aggregate in mortar for the same purpose. For this, natural quartz sand in normal mortar (NM) was totally replaced with PC clinker of the same particle size distribution and the most important engineering properties of the new mortar referred to as Reactive Aggregate Mortar (RAM) were measured and compared with NM as control. The results of compressive strengths measurements represented 65% and 21% increases at curing ages of 7 and 90 days, respectively, for RAM compared to NM. Chloride penetration depth in RAM displayed reductions by about 33% and 26% after 14 and 28 days of exposure, respectively. The effect of PC clinker reactivity on the microstructure and size of ITZ was studied by using scanning electron microscopy.


Hydration and Engineering Properties of Mechanically-Activated Composite Cement from Processed Steel Slag and Limestone
Mojtaba Hosseini, Ali Allahverdi, Mohammad Jaafar Soltanian Fard,
Volume 19, Issue 1 (March 2022)
Abstract
The aim of the present research work is to evaluate the feasibility of processing and utilizing steel slag
in binary and ternary cement blends with limestone. The physical and microstructural properties of binary and
ternary composite cements produced by inter-grinding mixtures of ordinary Portland cement clinker, processed
steel slag and limestone in a laboratory ball mill with replacement levels varying from 0 wt.% to 30 wt.% were
studied. The effects of processed steel slag and limestone incorporation on density of dry cement mixes and water
consistency, setting time and volume stability of fresh and hardened cement pastes were investigated. Also,
density, water absorption, total open pore volume (%) and compressive strength of cement mortars were measured.
The mix with 15 wt.% limestone and 15 wt.% processed steel slag was selected as a typical ternary cement mix
for complementary studies including X-ray diffractometry, thermal gravimetry, Fourier-transform infrared
spectroscopy, and scanning electron microscopy analyses. The results show that removal of relatively high
metallic content of steel slag increases its grindability for mechanical activation and improves its hydraulic
properties effectively and makes it suitable for being recycled in cement industry. The results show that
mechanical activation of the cement mixes enhances the poor hydraulic activity of the processed steel slag and
compensates the strength loss to some extent. The physical and chemical properties of all studied composite
cement mixes comply with ASTM standard specifications, except the compressive strength of the cement mixes
at 28-days containing 20 wt.% or higher amounts of limestone ground to the relatively low Blaine specific surface
area of about 3000 cm2/g.

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