• Journal of the Korean Recycled Construction Resources Institute
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• v.8 no.4
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• pp.475-482
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• 2020

In this study, three levels of solid capsules were prepared according to the composition ratio of core materials for the crystal growth type self-healing solid capsule, and a cement mortar was prepared with the crystal growth type self-healing solid capsule. The prepared solid capsule was mixed with 3% of the cement mass to evaluate the healing properties according to the crack induction age of the cement mortar. As a result of test, the crack healing properties according to the crack induction age of cement mortar mixed of solid capsules, it was confirmed that the self-healing performance of the cement mortar with the solid capsules was increased self-healing performance of 7 days than 28 days. This is because the unhydrated binder remains.

• Journal of the Korea Concrete Institute
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• v.17 no.3 s.87
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• pp.481-485
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• 2005

The present study examines the distribution of calcium hydroxide, unhydrated cement grain and porosity at the steel-concrete interface. The formation of calcium hydroxide has been confirmed by microscopic analysis using BSE images containing the ITZ between the steel and concrete. It was found that calcium hydroxide does not form a layer on the steel surface, different from the hypothesis that has been available in investigating the corrosion of steel in concrete, ranging from 5 to $10\%$ within the steel surface. Moreover, the high level of porosity at the ITZ was observed, accounting for $30\%$, which may reduce the buffering capacity of cement hydration products against a local fall in the pH. These findings may imply that the mole of ($Cl^-$) :($OH^-$) in pore solution as chloride threshold level lead to wrong judgement or to a wide range of values.

• Journal of the Korean Ceramic Society
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• v.17 no.4
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• pp.208-212
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• 1980

In order to clarify the mechanism of the hydration of supersulphated slag cement, the experiments of suspension hydration were performed in the mixtures of slag+CaSO4+water or NaOH solutions with a liquid/solid ratio of 10. The liquid and solid phase of the suspension was chemically analysed and discussed. In the slag-$CaSO_4$--NaOH system, $Al_2O_3$ was dissolved under high pH condition, and the ettringite was formed by a reaction of $CaSO_4$ and $Al_2O_3$ released in the solution. As hydration progressed, ettringite coated the surface of unhydrated slag grains and inhibited the hydration reaction of slag. $CaSO_4$ was not only an activator for slag hydration, but a reactant in the hydration.

• Journal of the Korean Ceramic Society
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• v.34 no.2
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• pp.163-170
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• 1997

The retarding effects of MgSiF6.6H2O on the hydration of portland cement were studied. The setting time, flow value and compressive strength of mortar were measured and the mechanism of retardation was also studied by ion concentration in solution, SEM, BET, and X-ray diffraction. The results are as follows ; 1. Setting time was delayed by the addition of MgSiF6.6H2O. 2. The flow value of mortar decreases depending upon the amount of MgSiF6.6H2O. 3. The compressive strength was almost same or some increase on 28 days hydration. 4. The main retardation mechanism of MgSiF6 on the hydration of portland cement may be explained by the following hypothesis. MgSiF6 depressing the Ca++ and K+ ion concentration of cement paste solution be-cause of the recrystalization of K2SiF6 and CaF2 phase. The new products of K2SiF6 and CaF2 deposit on the surface of unhydrated cement powder and harzard the mass transfer through these layer. The low con-centration of Ca++, K+ ion in solution was decreasing the hydration rate of portland cement.

• Proceedings of the Korean Institute of Building Construction Conference
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• 2013.05a
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• pp.31-32
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• 2013

ThThe purpose of this study is to investigate the compressive strength properties of fly ash using water eluted from recycled coarse aggregate. When fly ash come into contact with water, they have not a autonomously chemical reaction. But fly ash is a pozzolan reaction when fly ash come into contact with water and calcium hydroxide(Ca(OH)2) in alkaline environment. For that reason, if water eluted from recycled coarse aggregate use mixture water, fly ash is expected to reaction of pozzolan reaction property in early stage. According to the experimentation result, ICP-MS analysis showed water eluted from recycled coarse aggregate has a high alkali-ash value of pH of 12 and over. And mixing ratio 30% fly ash mortar using water eluted from recycled coarse aggregate showed a similar strength of plain mortar due to the pozzolan reaction. Also, poor strength in initial age, disadvantage of mortar using fly ash, can be improved as hydration in early age is expedited due to calcium hydroxide(Ca(OH)₂) and unhydrated cement component eluted from recycled aggregate mortar.

• Journal of the Korea Institute of Building Construction
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• v.12 no.2
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• pp.161-168
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• 2012

The purpose of this study was to investigate the influence of recycled aggregates powder (RP) contents on recycled aggregates (RA) using fly-ash (FA) mortar in a condition of zero cement targeting earth filling materials, and the results can be summarized as follows. First, there was a tendency that as RP contents increased, W/B and air contents increased also. In the case of compressive strength, the strength was hardly developed at the early age, which was prior to 14 days; however, with the starting point of 14 days of age, strength of around 1.5~2.0 MPa was developed when it arrived at 28 days. At a curing temperature of $20^{\circ}C$, the more RP contents increased, the more the compressive strength increased in comparison with FA 100% at all levels except RP 100 %, showing the highest compressive strength at RP 25 %. At a curing temperature of $35^{\circ}C$, the temperature-dependence appeared to be large, as the RP contents were decreased compared to the curing temperature of $20^{\circ}C$. In addition, based on SEM analysis, this study was able to confirm that a pozzolanic reaction formed by an alkali stimulus of RA with the lapse of certain days even in 100 percent FA, causing the densification of tissues, and with RP 25%, hydrate was created the most densely by the hydration of unhydrated cement particles and pozzolanic reaction of FA.

• Journal of the Korean Recycled Construction Resources Institute
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• v.6 no.1
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• pp.103-110
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• 2011

This study is to comparatively analyze the characteristics of pH decrease in recycled fine aggregates for embankment and landfill produced from waste concrete by using natural process and artificial process. The result was as follows In case of recycled fine aggregates left outdoor, it was found that pH level was decreased if the thickness of embankment becomes thinner, or the materials left outdoors owing to high concentration of $CO_2$ in atmosphere caused by respirations of people. When the air was permeated, pH level was decreased more effectively. It was analyzed that this phenomenon was caused by efficient supply of $CO_2$ in the recycled fine aggregates owing to high-pressure ventilators. In case of water spraying treatment, sprayed water facilitated hydration of unhydrated cement to dissolve calcium hydroxides which neutralized $CO_2$ in the atmosphere during desiccation process and decrease pH level by a considerable margin. In case of Immersed treatment, decrease of pH was not sufficient. When facilitating the supply of $CO_2$, pH level of the recycled fine aggregates was decreased by the largest margin. It was analyzed that this phenomenon was caused by efficient supply of $CO_2$. From the above results, it was analyzed that the most effective method of reducing pH level of the recycled fine aggregates from the aspects of pH reduction performance, economic efficiency and workability was repeated wet-dry cycles of spraying water to the aggregates in the proportion of 1:0.5 by weight and then treating by forcefully blowing $CO_2$ gas into the aggregates.

• Journal of the Korean Recycled Construction Resources Institute
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• v.4 no.4
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• pp.463-469
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• 2016

In this paper, compressive strength and water permeability performance for two types of crack self-healing materials such as SH-PO-0 composed of mineral admixtures(expansive agent, swelling material and crystal growth agent) and SH-PO-(5, 15, 30) blended with SH-PO-0 and phosphate additive(PO) dissolving easily calcium ion, were evaluated. The test results show that the water flow of SH-PO-0 decreased steeply at the early age although compressive strength decreased about 9% at 28 days compared with OPC. The higher PO replacement ratio is, the lower compressive strength and more improved water permeability performance is, and thus, based on such results, adequate PO replacement ratio is 15%. It is also found that the self-healing performance of SH-PO-15 was quite improved at the early ages and however, the performance of SH-PO-15 is similar to one of SH-PO-0 at long-term ages, and 28 days compressive strength of SH-PO-15 decreased about 8% compared with SH-PO-0. In addition, it is confirmed from the analysis of SEM-EDS that calcium ions of SH-PO-15 were crystallized more than those of SH-PO-0.

• Journal of the Korea Concrete Institute
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• v.29 no.1
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• pp.11-21
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• 2017

The paper presents experimental results of self-healing effects of OPC-GGBFS paste with cracked and uncracked specimens. The self-healing of cracked specimens is processes of crack closing with re-hydration of unhydrated in crack surface. The OPC paste with GGBFS replacement ratios of 0%, 10%, 20% and 30% were prepared having a constant water-binder ratios of 0.5. The OPC-GGBFS paste specimens immersed in tap-water and sea-water. The temperature of tap and sea-water was $5^{\circ}C$, $15^{\circ}C$ and $25^{\circ}C$. The cracked specimens after deterioration were immersed for 60 days. The relationship between self-healing effect and age-effect was calculate based upon the experimental results. The self-healing effect was measured in ultrasonic pulse velocity (UPV) before and after loading. When the relative change rate of UPV is increases with the increase in GGBFS replacement ratios. Moreover, the self-healing effect is increased with the temperature of tap-water is increase. But the cracked specimens immersed in sea-water was unclear effects of different temperature. Furemore, most of the healing for OPC-GGBFS specimens immersed in tap-water and sea-water occurred during the first 30 days. The self-healing effect until 30 days is higher than that the age-effect. After 30 days, self-effect and age-effect was largely decreases. SEM/EDS analysis of crack on the surface of the specimens in tap-water were covered with aragonite, and sea-water were covered with brucite.

• Journal of the Korea Concrete Institute
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• v.27 no.2
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• pp.95-102
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• 2015

In this paper, the effects of sodium hydroxide (NaOH) and aluminum potassium sulfate ($AlK(SO_4)_2{\cdot}12H_2O$) dosage on strength properties were investigated. For evaluating the property related to the dosage of alkali activator, sodium hydroxide (NaOH) of 4% (N1 series) and 8% (N2 series) was added to 1~5% (K1~K5) dosage of aluminum potassium sulfate ($AlK(SO_4)_2{\cdot}12H_2O$) and 1% (C1) and 2% (C2) dosage of calcium oxide (CaO). W/B ratio was 0.5 and binder/ fine aggregate ratio was 0.5, respectively. Test result clearly showed that the compressive strength development of alkali-activated slag cement (AASC) mortars were significantly dependent on the dosage of NaOH and $AlK(SO_4)_2{\cdot}12H_2O$. The result of XRD analysis indicated that the main hydration product of $NaOH+AlK (SO_4)_2{\cdot}12H_2O$ activated slag was ettringite and CSH. But at early ages, ettringite and sulfate coated the surface of unhydrated slag grains and inhibited the hydration reaction of slag in high dosage of $NaOH+AlK(SO_4)_2{\cdot}12H_2O$. The $SO_4{^{-2}}$ ions from $AlK(SO_4)_2{\cdot}12H_2O$ reacts with CaO in blast furnace slag or added CaO to form gypsum ($CaSO_4{\cdot}2H_2O$), which reacts with CaO and $Al_2O_3$ to from ettringite in $NaOH+AlK(SO_4)_2{\cdot}12H_2O$ activated slag cement system. Therefore, blast furnace slag can be activated by $NaOH+AlK(SO_4)_2{\cdot}12H_2O$.