Alkaline aggregate reaction refers to the chemical reaction between the alkaline oxide in cement (Na2O or K2O) and the active silica in the aggregate, which generates a complex alkali-silicate gel on the surface of the aggregate, which produces a large volume expansion after water absorption (the volume can increase by more than 3 times), thus leading to concrete expansion cracking and damage. The alkali aggregate reaction causes significant changes in the strength and other properties of the concrete, with the tensile strength reduced by about 30% to 50%. Due to the reduction in tensile strength, the concrete produces cracks, which may lead to the loss of tensile capacity of the structure for concrete containing little reinforcement, causing a reduction in the shear and bond strength of the reinforcement, which leads to the destruction of the structure.
Alkali aggregate reaction began to be discovered in the 1930s when concrete structures such as levees, roads, and bridges in the western United States experienced abnormal expansion and cracking. After entering the 1970s, reports of structural damage caused by alkali aggregate reaction continued to come from Europe, South Africa, and other places, and alkali aggregate reaction was brought up as a common problem worldwide. In the Sea of Japan coast, many harbor buildings, bridges, etc., less than 1 year after completion, the concrete surface cracking, spalling, corrosion of reinforcing steel exposed, the main reason is alkali aggregate reaction. Beijing’s Sanyuan overpass piers, less than two years after completion, individual places occur “herringbone” cracks, is also the result of alkali aggregate reaction. It is difficult to repair the structure damaged by alkali aggregate reaction, and the effect is also poor, so the best way is to actively take preventive measures.
The addition of micro-silica fume powder in the concrete, the alkali aggregate reaction has a significant preventive control effect. Micro-silica fume powder added to 5% to 10% of the expansion of concrete can be reduced by 10% to 20%, and its control effect varies according to the type of reactive aggregates and micro-silica fume powder. Experimental studies have shown that the control effect of micro-silica fume powder on the reaction of alkali aggregate is mainly in the following aspects.
(1) After the addition of micro-silica fume powder, due to the micro-silica fume powder particles are very fine, mixing concrete need to increase the amount of water, the result of changing the void ratio in the concrete moderated the local pressure.
(2) Micro-silica fume powder has a large specific surface area and can adsorb alkali ions, which reduces the concentration of hydroxide ions in the fine pore solution of concrete and generates a non-expansive gel with high calcium content. No alkali aggregate reaction occurs when the concentration of hydroxide ions is reduced below a certain limit.
(3) After adding micro-silica fume powder, it is the first to react with hydrated hydroxide due to its small particle size, and the resulting unhardened alkali-silicate gel fills the pore structure, and the reaction has to consume a certain amount of calcium hydroxide needed for alkali aggregate reaction, so that the calcium hydroxide is evenly distributed, thus controlling the harmful expansion.
The preventive control effect of micro-silica fume powder on alkali aggregate reaction effectively improves the durability of concrete and prolongs the service life of structures. It is also an important way to reduce the production of concrete and is an important way to achieve environmental load reduction.