Hydration and micro-structure are important in the study of alkali-activated materials, especially used in building materials. To obtain information about hydration processes, we have investigated the hydration rate and micro-structural evolution. In this research, we evaluated the performances of nano-silica composite, which is produced by adding nano-silica into alkali activated slag. We also tested the impacts of nano-silica on hydration, micro-structure and porosity. Results of the experiments shows that the comprehensive strength is enhanced, the microscopic morphology and hydration characteristics are optimized. Also, hydration rate suggested that the impact of nano-silica on alkali activated slag early hydration is far greater than that of later hydration. Furthermore, the nano-silica action mechanisms on alkali-activated slag discussed in this paper mainly are that the nano-silica supplied siliceous precursor and served as nucleation seed and its accelerated early hydration effects.

Introduction

Alkali activated materials, which firstly invented and applied patent by Davidovits [1], were considered as alternative and supplementary materials to replace Ordinary Portland cement (OPC) with respect to reduce CO2 emissions. This kind of materials was famous for its lower energy consumption than Portland cement and wider range of raw materials resources, such as blast furnace slag [2], waste glass [3], fly ash [4], rice husk ash [5], red mud, steel slag [6] and other industrial waste residue [7,8], which were difficult to use in large scale in cement production industry. The truth is, alkali-activated materials has an advantage over other building materials such as ceramics and Portland cement, for it providing many important performance advantages, for example, high early compressive strength [9], high freeze-thaw resistance [10], strong acid-base corrosion resistance [11], excellent impermeability [12] and outstanding fire resistance [1]. By now, it is generally believed that alkali-activated materials’ hydration process has three stages [13]. The first stage is decomposition–coagulation reaction, which means that under strong alkaline condition, the alumino-silicate contained in raw materials depolymerized into ions. The second stage starts as the increase of precursor number and the reaction process accumulates. In the meantime, the third stage of condensation and crystallization happens, the hydrated calcium alumino-silicate gels and hydrated calcium silicate gels were produced in this stage [14].