(A) the impact of cement
The mineral composition and fineness of cement have an important effect on the strength of concrete. The cement with high content of C3S and C3A grows faster in the early stage and has high strength in the early stage, but the strength grows slowly in the late stage. The cement with high content of C2S grows continuously in the late stage and has high strength in the late stage. The size of cement fineness directly affects the speed of hydration rate, generally speaking, with the increase of cement fineness, cement hydration rate also becomes faster, and the strength of concrete increases. The rapid growth of strength in the early stage is easy to cause the slow growth of strength in the later stage, and even the phenomenon of strength shrinkage.
(2) The influence of mineral admixtures
Mineral admixtures have become indispensable components of concrete, and the purpose of their use has changed from simply pursuing economic benefits at the beginning to improving the performance of concrete. The physical "filling effect" and chemical "volcanic ash effect" of mineral admixtures, especially secondary hydration, change the microstructure of concrete, improve the density of concrete, and promote the improvement of the strength in the later stage.
However, the properties and characteristics of different mineral admixtures are different, so we should pay attention to and understand their properties when using mineral admixtures, and give full play to their respective performance. For example, the chemical composition and fineness of fly ash have a great impact on the strength of concrete, and the finer the fineness of fly ash, the higher the activity and the better the filling efficiency. However, with the increase of fly ash content, the early strength of concrete decreases obviously, and the later strength catches up with or exceeds the strength of concrete without fly ash. Slag powder is a potentially active admixture, its activity and fineness have a great relationship, with the increase of fineness, the contribution to the 28d strength of concrete is also increased, generally required slag powder specific surface area is greater than 400m2/kg, but too high specific surface area will increase energy consumption. The specific surface area of silica powder is large and the particle size is very small, which can cause volcanic ash reaction in concrete in a short time. The contribution of silica powder to the strength of concrete is mainly before 28d. In terms of long-term strength growth, silica powder concrete is inferior to pure cement concrete or fly ash concrete. The compound use of mineral admixtures with different properties can produce supersuperposition effect, which contributes greatly to the early and late strength of concrete.
(3) The influence of aggregate
The mud content is the main factor affecting the strength of concrete, the mud content is large, the strength is low. Mud content also affects the growth rate of concrete strength at the same age. Generally speaking, the strength growth rate of concrete mixed with sand with low mud content is greater than that of concrete mixed with sand with high mud content. Machine-made sand generally has many angles, rough surface, contains a lot of stone powder, and the interface is fresh, which improves the bond strength between sand and cement stone interface. However, the grain shape characteristics of machine-made sand will also cause the water consumption of concrete is higher than that of natural river sand mixed concrete, which can be controlled by high efficiency water reducing agent. The stone powder in the machine-made sand can supplement the amount of cementing material in the low-strength grade concrete, play the role of filling compaction, and improve the strength of concrete. However, in the process of use, attention should be paid to distinguish whether the machine sand contains stone powder or mud powder.
