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Starting from expansion measurements on concrete prisms, produced with two different alkali reactive aggregates, the influence of two aluminium containing SCMs on the expansion of concrete prisms and on the solubility of the aggregates in an alkaline solution were investigated. The solubility of aggregates in an alkaline solution reflects the alkali reactivity of the corresponding aggregate.
Initially, the solubility experiments with the aggregates and SCMs were made separately and then in mixtures of each aggregate and SCM. In a further step, the SCMs were stored in KOH solutions
containing Ca(OH)2. The concentrations of SiO2 and Al2O3 were determined to investigate the effect of calcium on the solving processes of SCMs. The combination of concrete expansion test and
dissolution experiments of two alkali reactive aggregates and two aluminium containing SCMs allows first conclusions about the influence of such SCMs on the reactivity of aggregates.
Changes of pore solution composition under accelerated mortar bar and concrete prism test conditions
(2012)
Accelerated tests of mortar bars and concrete prism were carried out at temperatures ≥ 40°C in order to reduce the test duration. Because the OH-/SO42- equilibrium is shifted to lower values with increasing
temperature and alkali content of the cement, there exists the possibility of side reactions between sulphate ions from the cement and aluminate ions which can come from mineral admixtures and aggregates. Solubility measurements of two types of aggregates in sulphate free KOH solutions indicate that the temperature effect on the silica releasing rate is bigger than the inhibiting effect of the decreasing OH- concentration. To avoid side reactions during aggregate test procedure, expansion measurements should be performed at 40°C in combination with adapted solubility measurements.
Supplementary cementing materials (SCMs) are an important part of modern concrete structures. Their use has many advantages (reduction of the cement amount, improvement of the durability and therefore CO2 reduction). For example certain SCMs added to concrete mixtures can avoid the alkali-silica reaction (ASR). But such materials have a wide range of composition and therefore the mechanisms can be very differently. The knowledge of dissolution processes of aggregates and SCMs separate and together in alkaline solutions can help to understand the damage of concretes caused by ASR and the effects to avoid ASR in more details. Many dissolution experiments were performed in highly alkaline solutions (KOH) under CPT conditions (pH 14, 40°C) using aggregates in original grain sizes and Si and Al containing SCMs in different ratios. Also Calcium hydroxide was added to investigate the influence of calcium ions as a main part of the pore solution. The concentrations of soluble silica and alumina were determined by ICP-OES. Such dissolution experiments, with which an interaction was settled between the aggregate and the SCM via the alkaline solution, arose that under specific conditions the dissolution of aggregates was strongly influenced. Obviously both main constituents of concretes interact with each other. Additionally effects of the dissolved calcium ions must be considered. It was found that the efficiency depends on the amount and chemical composition of the SCMs used and on the reactivity of aggregates.
The use of supplementary cementing materials (SCMs) added to concrete mixtures can avoid the alkali-silica reaction. Such materials have a wide range of composition and therefore the inhibition mechanisms can be very differently.
The effectiveness of SCMs which provide silica and alumina into the alkaline solution cannot only be explained by reducing the OH--concentration of the pore solution. From dissolution experiments in potassium hydroxide solutions, an interaction was noted between the aggregate and five SCMs via the alkaline solution. Under specific conditions, no silica is released from the aggregate grains.
Mineralogical investigations (XRD, SEM+EDX) of the grain surfaces confirm that quartz is the main source of silica. In the presence of alumina providing SCMs, the quartz dissolution is strongly reduced or even sometimes stopped. On surfaces of grains a very thin layer can be observed which is probably responsible for reduction or stopping of the silica dissolution and therefore for the inhibition of ASR.