Effects Of NanoSiO₂on The Hydration And Hardening Properties Of Fly Ash Cementitious Materials

Application of fly ash in cementitious materials is one of the mostly used techniquesin modern concrete, and it is a vital method of eliminating the environmental problemscaused by fly ash, as well as making the concrete industry more green and sustainable.However, the slow property gain at early ages is one of the most critical factors thataffecting the development of fly ash cementitious materials and thus methods ofimproving the early age properties of this system are of great concern. It was reportedthat by adding a small dosage of nanosilica the properties of cement-based materials canbe greatly improved, especially the early age strength gain. Thus the application ofnanosilica in fly ash cementitious materials is becoming attractive. It has been reportedthat nanosilica has the advantage of increasing the early age property of fly ashcementitious materials, however, its effect on later age properties have been rarelyinvestigated and there are some controversial results regarding the later age propertygain. This is due to the lack of study of the effects of nanosilica on fly ash cementitiousmaterials, especially at later ages.To verify the advantages of nanosilica on the early age properties of fly ashcementitious materials, as well as clarifying its effects on later ages, the influences ofcolloidal nanosilica on fly ash cementitious materials at both the early and later ageswere experimently studied. Mechanisms governing the property evolution of nanosilica-added fly ash cementitious materials at early and later ages were investigated. Methodsof improving the adverse effect of nanosilica on this system were proposed with thehope of adding value to the study of nanomodification in fly ash cementitious materials.It reveals that nanosilica remarkably decreases the workability of cementitiousmaterials, but with the help of fly ash, the workability-decreasing tendency can beeliminated. With the addition of nanosilica, the slow strength gain of fly ashcementitious materials at early age can be eliminated as well. It shows that with5%nanosilica the setting and hardening properties of fly ash cementitious materials can begreatly improved and a higher fly ash replacement leads to a greater improving effect.The rate of strength gain of nanosilica-added fly ash cement mortar slows down at laterages and the compressive strength at three months is comparable to or lower thanmortar without nanosilica. The strength gain of nanosilica-added fly ash cementitiousmaterials is more greatly influenced by self-dissociation. The effects of nanosilica on the hydration and hardening properties of fly ashcementitious materials were investigated based on the study of its own hydrationproperties. It was found that the nucleation effect of nanosilica on cement hydration isinduced by the hydration gel of nanosilica, which is formed by following the first orderchemical reaction model. Based on investigations made by isothermal differentialscanning calorimetry, together with the ion analysis of hydration solution andmorphology of hydration products, it was found that the cement hydration accelerationeffect of nanosilica is caused by the acceleration of the dissolution of cement and therupture of semi-permeable membrane surrounding cement particles. It was revealed bymorphology, porosity and degree of hydration analyais that the dense structure ofhydration products of nanosilica can densify the cement structure. On the other hand,the dense hydration product, together with the quickly formed cement hydration product,forms a dense coating around unhydrated particles and the later hydration of theparticles can be hindered. Although the early age properties of fly ash cementitiousmaterials are greatly improved by nanosilica, grate consumption of Ca（OH）₂greatlyreduced the optimal dosage of fly ash in the system.To compensate the disadvantages of nanosilica on cementitious materials, severaltechniques were raised. It was found that by marching the size of raw materials theevolution of strength gain of fly ash cementitious materials can be altered and a coarsernanosilica and a finner cement are good for the porosity reduction and propertyimprovement. Experimentally-synthesized nanoCa（OH）₂induced an advantageouseffect on both the early and later ages of the fly ash cementitous materials.