In order to ensure the construction progress, the construction unit sometimes needs to carry out concrete pouring at a lower temperature.Production is often solved by heating the mixing water.Not only can it ensure the mold temperature of concrete, but also help to reduce the hysteresis effect of concrete slump with increasing time at low temperature.It is generally believed that the higher the temperature, the more the cement particles adsorb to the polycarboxylate superplasticizer, and the higher the temperature, the more obvious the consumption of the polycarboxylate superplasticizer by the cement hydration product.
Under the combined effect of the two effects, the fluidity of the concrete deteriorates with increasing temperature.
This conclusion is a good explanation for the increase of concrete fluidity when sudden cooling occurs and the increase of concrete slump loss when temperature rises.However, it was found that the concrete had poor fluidity at low temperature, and the fluidity of the concrete increased after the temperature of the mixing water was increased.To find out the reason for the contradiction, give the suitable temperature range of concrete.
According to the analysis, when the machine exit time is short, the cement paste expansion increases first and then decreases with the increase of temperature.
The reason for this phenomenon is that the temperature affects both the cement hydration rate and the rate of adsorption of the water reducing agent.The faster the adsorption rate of the water reducing agent molecules when the temperature is raised, the better the early dispersion effect.At the same time, the hydration rate of the cement is accelerated, and the consumption of the water reducing agent by the hydration product is increased, so that the fluidity is lowered.
The initial expansion of cement paste is a combination of these two factors. When the temperature of the mixing water is ≤10 °C, the adsorption rate of the water reducing agent and the hydration rate of the cement are both small.
The adsorption of the water reducing agent on the cement particles is a controlling factor. When the temperature is low, the water reducing agent adsorbs slowly on the cement particles, and the initial water reduction rate is low, which shows that the initial fluidity of the cement paste is small.When the temperature of the mixing water is between 20 and 30 °C, the adsorption rate of the water reducing agent and the cement hydration rate increase simultaneously, and the molecular adsorption rate of the water reducing agent increases more obviously, which is manifested by the increase of the initial fluidity of the cement paste.
When the temperature of the mixing water is ≥40 °C, the hydration rate of the cement increases significantly and becomes a controlling factor.The net adsorption rate (adsorption rate minus consumption rate) of the water reducing agent molecule is reduced, and the cement paste also exhibits insufficient water reduction.Therefore, it is considered that the initial dispersion effect of the water reducing agent is best when the mixing water is at 20 to 30 ° C and the cement paste temperature is between 18 and 22 ° C.When the machine is out for a long time, the cement paste expansion is consistent with the commonly concluded conclusion.When the time is sufficient, the polycarboxylate water reducing agent can adsorb to the cement particles at various temperatures to saturation, while the cement hydration consumes less water reducing agent at low temperature.Therefore, as time goes by, the cement paste expansion decreases with increasing temperature.The data shows that for every 10 °C increase in water temperature, the concrete temperature is increased by about 3 °C, so the temperature of the mixing water should not exceed 60 °C.
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