Melhoramento da resistência à carbonatação do cimento álcali ativado por meio de adições minerais sob condições naturais e aceleradas de ensaio

Abstract

Alkali activated cement (AAC) is composed by an aluminosilicate mineral and an alkalineactivator solution which raises the pH allowing the minerals to dissolve and hydrate. ForAAC to be widely used, issues related to its durability remain. Carbonation reaction thatoccurs due to the penetration of carbon dioxide (CO2) into the structure is one of them.CO2in contact with water forms carbonic acid, attacks hydration products and reducesthe pH of the medium, causing the risk of depassivation of steel in reinforced concrete. Itis consensus in literature that AAC’s perform less than Portland cement when subjectedto carbonation. Among other factors, that is due to the absence of calcium hydroxide(Ca(OH)2) in their hydration process, which favors the decalcification of C-S-H andconsequent disintegration of the matrix. However, few studies focused on measures toimprove the resistance of these materials to this reaction. Therefore, the present studyaims to improve the resistance of AAC to carbonation by using mineral admixtures likecalcium hydroxide (AAC-C), calcium sulfate (AAC-F) and magnesium oxide (AAC-M).The evaluation was performed in both natural and accelerated tests. The influence of theadmixtures on non-carbonated AAC pastes and concretes were investigated as regardsto hydration heat, compressive strength, voids index and microstructure. Sampleswere exposed to natural carbonation for 4, 12, 24 and 48 weeks and accelerated carbonation for 4, 8, 12 and 16 weeks. Carbonation depth, compressive strength andmicrostructure analysis by X-ray diffraction (XRD) were performed over time of exposure.Best performance was obtained with magnesium oxide which provided less carbonationdepth and strength loss under natural test. As regards with accelerated carbonationAAC-M kept higher compressive stregth than all others despite of total carbonationof all samples. It was observed in microstructure that magnesium oxide was beingconsumed. This might indicate its reaction with CO2and consequent mitigation ofC-S-H decalcification. Calcium hydroxide and calcium sulfate admixtures showed lesssatisfactory results. Therefore, mineral admixtures are a suitable alternative to increasecarbonation resistance of AAC although their efficiency is highly dependent on theirphysical and chemical features and AAC composition.