Effect Of Mineral Admixtures On The Shrinkage Of Hardend Cement

Cement based materials is one of the largest building materials. In recent decades, the science andtechnology has been greatly developed, but cement based materials is still the leading building materials.With the development in science and technology of our country, the performance of cement based materialshas been greatly improved, which has played a significant role in promoting the development of buildingtechnology. However, in recent years, the cracking phenomenon of the construction site appears frequently,which arouse the attention of the majority of researchers and engineers,many studies have shown that themost important factors lead to the cracking of Portland cement based materials is the shrinkagedeformation.Especially for the mortar,the mortar is always attached to a surface of the substrate and workwith this substrate at the same time, there is the interaction inevitably between the substrate and the mortar.Besides the mortar is usually in the form of the thinner used in the project, the mortar is directly in contactwith the outside of the natural environment, it is easy to lose water and produce shrinkage deformation,because of the inconsistency shrinkage deformation between mortar layer and the grass-roots level,whichoften leading to mortar cracking. As a result it will affect the quality of construction engineering. In general,the shrinkage deformation of mortar and concrete are mainly from the drying shrinkage deformation ofhardened cement paste, the shrinkage deformation of the hardened cement paste is related to many factors.Mineral admixture is an indispensable component of modern concrete and mortar, especially theapplication of fly ash and slag have been quite common, this incorporation of component will affectshrinkage deformation of the hardened cement paste.In this paper, more systematic research work carried out, including the relationship between theproperties of the shrinkage deformation and dehydration of hardened cement paste, At the same time, theprocess of dried hardened cement which is put into water again water absorbing expansion is also studied.Namely the reversibility of the shrinkage deformation process and process of water loss of hardenedcement. The shrinkage deformation and the process of water loss of hardened cement under differentpre-cured age is studied. The influence of different contents of the mineral admixtures materials on the shrinkage and process of water loss of hardened cement is also studied.Moreover, investigate themechanism by means of the microscopic experiment the thermal analysis,X-ray diffraction analysis.Themain conclusions are as follows:a): The water loss rate of hardened cement paste increases and the shrinkage deformation decreaseswith the increasing of fly ash in a dry environment. Pre-curing period had no significant effect on theshrinkage deformation, Drying rate as the pre-curing period of the extension of significantly reducing andthere is no relationship pre-curing shrinkage deformation of the hardened cement paste with fly ash dopedvolume. When the dry hardened cement paste put into the water again, the specimen will be absorbent andproduce swelling,At the same time water absorption increase and the swelling decreases with the increaseof fly ash. The dry specimen put into the water again, the water loss rate of water absorption of thehardened cement paste can be more than dry, but the expansion rate is always less than the shrinkage rate ofcontraction. After a wet and dry cycle, the absorption of water of the hardened cement paste increases withthe increase of fly ash but which decreases with the pre-extension of the curing period. The fly ash did notshow a significant effect on the net contraction of the hardened cement paste, but the longer the pre-curingperiod, the smaller net contraction value seems.That is to say, the reversible of shrinkage deformation ofhardened cement paste is enhanced.b):The drying shrinkage of hardened cement paste increases and drying rate of the hardened cementpaste on a downward trend with increasing rate of the slag content in a dry environment,.The slag is similarto fly ash,the pre-curing had no significant effect on the shrinkage deformation ofhardened cement paste.Drying rate reduce significantly as the pre-curing period to extend. Moreover there is no relationshippre-curing shrinkage deformation of the hardened cement paste with slag doped volume. When the dryhardened cement paste put into the water again, we can see that no matter how long the pre-curing period,the hardened cement paste of the swelling has been reduced with the increasing of slag content, but theamount of water absorbed have no change basically. pre-curing period has significantly affect on expansionand water absorption, the longer the pre-curing period, the greater the expansion rate of the hardenedcement paste, the smaller water absorption, there is no relationship the swelling and water absorption ofthe difference under different pre-curing period with the slag content. There are large differences betweenprocess of water absorption and drying process, the water swelling of the hardened cement paste is always less than the drying shrinkage, but the water absorption rate of water loss is more than dry. After a wet anddry cycle, the water loss is fully recoverable, the water absorption can exceed the loss of water in the dryafter the dried specimen put into water again, but the shrinkage deformation of the drying process onlyrecover a small part. From the pre-curing time, the percentage of net water loss of hardened cement paste isreducing with the extension of pre-curing period, but no matter how long the pre-curing period, the netwater loss is increasing with the increase of slag content,the net contraction of the hardened cement pasteis increasing with increasing slag content.c):In general,though the test data regression analysis and variance analysis results show that,preventive maintenance one day, the behavior of hardened cement paste have no significant interactionunder difference of fly ash and slag content in a dry environment,the effect is independent. Preventivemaintenance for14days, both the shrinkage deformation or the rate of water loss, the fly ash has a certaininfluence on the role as the slag. However, the influence of slag content on the role of fly ash is onlymanifested in the shrinkage deformation, slag content is no affect on the role of fly ash in the water lossrate. Either fly ash or slag, the impact of the hardened cement paste behavior in a dry environment with itsdosage range, when the dosage range is small, the degree of influence is weak, or even almost no effect.When the dosage range, it is showed a more significant impact. Fly ash and slag on the hardened cementpaste shrinkage deformation behavior of the degree with pre-curing period, pre-curing period is shorter,their effect on the hardened cement paste is weaker. Pre-curing period is longer the degree of influence ofthe hardened cement paste has been enhanced, and may even show a certain degree of interaction.Another well-known, cement based materials is a brittle material, it has a high elastic modulus, butelastic deformation is small under usual conditions.It is exactly in virtue of this characteristic of thecement-based materials which makes the hardened cement paste extremely to crack. Because cracking ofthe hardened mortar is caused by the shrinkage deformation under the limit conditions, the hardened mortarcan’t crack under free conditions. Only under limited conditions, it is possible to make the cracking of thehardened mortar. Whether the hardened mortar cracking is the limit under the conditions of the shrinkagedeformation or not. Thus, in order to find such a relationship,in this paper, the force status of the hardenedcement under the limit shrinkage deformation is studied, The main conclusions are as follows:a):Study on influence of mineral admixtures on crack resistance of the hardened cement paste, it will help improve the crack resistance of the hardened cement paste when mixed with fly ash in cement paste;the small dosage of slag is negative for the crack resistance of hardened cement paste,It brought the matterto the crack resistance of the hardened cement paste when content40%,50%of the slag.When slag and flyash mixed with hardened cement paste, there is no significant interaction on crack resistance of thehardened cement paste under pre-curing period of1day as well as pre-curing of14days and high volumeof fly ash. only the large volume of slag and pre-conservation14days,it is showed a significant interactionon crack resistance of hardened cement paste.b):Comparing the different diameter steel of hardened cement paste.it is obtained that the shrinkagedeformation of hardened cement reduce with the increasing of bar diameter no matter how long thepre-curing period. but the small diameter reinforcement is conducive to improve the crack resistance ofhardened cement paste.while the opposite to large diameter reinforced.This study have a more profound understanding about the behavior of hardened cement paste in a dryenvironment and provide a scientific basis for a new assessment about the crack resistance indicator ofhardened cement paste.