The Study Of Alkali-silica Reaction And Carbonation In Alkali-activated Slag Mortar

Alkali activated cement is a new type of building material, which is increasingly known because of its excellent performance. At present, the durability of alkali activated cementitious materials has become a hot topic, but there is some controversy in two aspects:(i) whether alkali-silica reaction will occur in alkali activated cement motar and concete;(ii) whether alkali activated cementitious materials is susceptible to carbonation. These are the important factors that hinder the promotion of application of alkali activated cementitious materials.Slag, fly ash and metakaolin usually used as the cementitious materials for alkali activated cement. To get a better activation, they must be mixed with a certain amount of alkalis. The high alkali concentration in the cement may cause expansion destruction as a result of alkali-silica reaction. Therefore, the high alkali content of alkali-activated cement has been questioned. A systematic investigation was performed on alkali-silica reaction of alkali-activated slag mortars activated by waterglass with different dosage of alkali and waterglass modulus. The accelerated mortar bar method was used to measure the expansion of the specimen caused by alkali-silica reaction. The pore alkalinity, mass change, porosity and pore size distribution and compressive strength were also investigated. The scanning electron microscope and energy spectrum was applied to analyze the morp Hology and chemical composition of the reaction products. A comprehensive exploration of the mechanism of alkali-silica reaction in alkali-activated slag mortar bar was analyzed.At the end of the study, an improved understanding of mechanism of alkali-silica reaction in activated slag mortar with fly ash and metakaolin was also made using the same testing procedure.Some researchers reported that alkali activated cementitious materials is more susceptible to carbonation than Portland cement-based materials. However, other researchers have pointed out that carbonation of alkali activated cementitious materials is largely affected by carbon dioxide concentration. The carbonation process of alkali activated cementitious materials is much different when concrete is cured at high concentrations of carbon dioxide in laboratory or in natural conditions. The impact of alkali dosage and water glass modulus on carbonation of alkali-activated slag were investigated under an accelerated condition. For comparasion, samples under natural conditions were also investigated. The main tests includeddetermination of carbonation rate, characterization of pore structures before and after carbonation, change in compressive strength. The changes in phases were also analyzed with IR and differential thermal analysis before and after carbonationBased on the present study, the results indicated that:According to the ASTM 1260 standard, the alkali-silica reaction for waterglass-activated slag mortars containing sand from Xiangjiang rive as aggregate has the potential to occur with a certain amount of alkali content and different waterglass modulus. The specimen containing high dosage of alkali has a high alkali concentration in the pore solution, but the expansion of the mortar bar is not necessarily high. The effect of alkali content on alkali silica reaction in alkali activated slag mortar is more complicated, which further needs to be studied.Therefore, high alkali content may not lead to damage by alkali silica reaction for alkali activated slag cement mortar. With the same alkali content, the alkali-silica reaction expansion first increased then decreased with increasing waterglass modulus,In other words, it has the most unfavorable modulus regarding alkali-silica reaction.Fly ash and metakaolin can effectively inhibit the alkali silica reaction in alkali activated slag mortar, where metakaolin is more effective than fly ash. For both water glass activated slag-fly ash and water glass activated slag-metakaolin mortars, the mass growth is consistent with the expansion of mortar bars. The extent of mass growth of mortar bar can reflect the amount of reaction products formed by alkali silica reaction. Fly ash can effectively reduce the alkali concentration in the pore solution of water glass activated slag, and the extent of the reduction depends on the relative amount of fly ash. When 30% of fly ash was added, the alkali concentration in the pore solution was the lowest. Metakaolin can significantly reduce the pore alkalinity of the water glass activated slag mortar bar. and the more metakalin added,the smaller the alkalinity.Under the same carbonation condition, the alkali activated slag mortar specimens is easier to be carbonated as compared to the Portland cement mortar. The carbonation rate of the alkali activated slag mortar specimens increased with increasing the age of exposure, while the Portland cement mortar show differently. Under the same water glass modulus, the carbonation rate of mortar decreased with the increasing alkali content. The carbonation rate of the Na OH activated slag samples follows the aforementioned rules. Under the condition of the same alkali content, the carbonation rate of the activated slag mortar becomes smaller with the increasing modulus. When the mortar was carbonated, the porosity and average pore size of the matrix incrased,the compressive strength decreased. However, matrix density and the compressive strength of the sodium hydroxide activated slag mortar specimens and Portland cement specimens increased.