The choice of plastering gypsum raw materials and proper aggregate gradation are the main reasons that directly affect the quality of the product. Sand is one of the main raw materials for the production of plastering gypsum mortar. The demand and use of construction sand in plastering gypsum mortar are very large. The demand for natural sand accounts for the vast majority, but their distribution is extremely uneven, and in recent years in the large-scale exploitation in the past, natural sand resources have been quite scarce. In addition, as the requirements for environmental protection are getting higher and higher, many areas have banned the excavation of natural sand, which makes the contradiction between supply and demand of sand increasingly prominent. The situation is forced to find resources to replace natural sand. Taking materials locally and making full use of local resources or some industrial solid waste such as tailings sand, slag, coal gangue, steel slag to replace natural sand. This is not only environmentally friendly, but also canachieve better economic and social benefits. Therefore, we must understand and treat separately in the application of plastering gypsum mortar according to different regions, different sand source conditions, and different aggregates.
1. The role of natural sand in plastering gypsum mortar and its influence on performance
Sand is an inert material that does not participate in chemical reactions in plastering gypsum mortar. It acts as a skeleton or filler in plastering gypsum mortar. The density of plastering gypsum mortar can be adjusted by sand gradation to control the crack resistance of plastering gypsum mortar. The adjustment of sand grading should take into account the fineness and gradation of sand. In the configuration of plastering gypsum mortar, it has poor water retention, good crack resistance, high strength and low fluidity with coarse sand. And it has good water retention, excellent adhesion, but the surface with poor crack resistance, easy to produce micro-cracks on the surface with fine sand.
The thickness of the sand directly affects the required amount of gypsum mortar, which affects the mix design of the plastering gypsum mortar, and also affects the performance of the mortar and the performance of the mortar hardened body. When the particle size of the sand is large, the specific surface area is relatively small, and the amount of gypsum material covering the sand is relatively small. As the particle size of the sand becomes finer and the specific surface area increases, the demand for gypsum mortar coating the sand increases significantly. As the fineness of the sand decreases, the water consumption of the plastering gypsum mortar increases, and the density of the mixedplastering gypsum mortar decreases, resulting in a decrease in its strength. The thickness of the sand has a decisive influence on the strength of the plastering gypsum mortar.
Natural sand has a smooth surface and is mostly round. In the process of configuring plastering gypsum mortar, lower gypsum dosage and mixing water can meet the needs. Natural sand with reasonable gradation is undoubtedly ideal aggregatefor producing plastering gypsum mortar.
Generally, the particles of natural sand are more uniform, and the gradation is also good. The dry density of the plastering gypsum mortar hardened body using medium sand is higher than that using fine sand (in the case of the same strength), but per cubic meter of the amount of hydrated gypsum powder, admixtures and additives in lime gypsum are all reduced. If the fineness of the sand becomes finer, the amount of water used increases, the density of the mortar decreases, the stability of the mortar also deteriorates, and the bleeding phenomenon is also likely to occur, which increases the amount of additives such as retarder and water retaining agent in the formulation. At the same time, the compressive strength of the plastering gypsum mortar hardened body will also be reduced, which will increase the cost of plastering gypsum mortar materials.
In construction sand, the sand with a fineness modulus in the range of 3.7–3.0 is coarse sand, the sand in the range of 3.0–2.0 is medium sand, the sand in the range of 2.0–1.0 is fine sand, and the sand in the range of 1.0–0.1 is extra fine sand. Plastering gypsum mortar generally uses the gradation of medium sand with fine sand and extra fine sand. A large amount of medium sand forms the skeleton, a small amount of fine sand fills the larger voids, and the extra fine sand fills the smaller voids. The gypsum mortar is evenly wrapped and adheres to the sand grains to form a relatively dense system. The mud in natural sand is harmful to plastering gypsum mortar, and its content must be strictly controlled. Because the mud is an inactive substance with very fine particles, it will absorb a lot of water, which will make the interface between the gypsum mortar and the aggregates poor. In addition, the water it adsorbs is free water, which will cause poor water retention and easy cracking of plastering gypsum mortar.
2. Effect of extra fine sand on plastering gypsum mortar configuration
At present, there is less and less medium-fine sand resources for construction. In some areas, due to the causal relationship between geographical location and climatic conditions, a large amount of extra-fine sand is generated. The use of extra-fine sand in the configuration of plastering gypsum mortar should obviously affect the workability of plastering gypsum mortar and the strength of hardened body. The ultra-fine sand particles are fine and have a large specific surface area. When using it to prepare plastering gypsum mortar, the sand content is lower. The reason is that the ultra-fine sand particles are poorly graded, the mud content exceeds the standard, and the amount of water used increases and it is easy to bleed. The amount of gelling materials and additives should be increased. When the proportion of extra-fine sand is increased, the adhesion performance of plastered gypsum mud is increased, the fluidity and workability are deteriorated, and the risk of shrinkage and cracking is increased. When used in gradation with medium sand, extra-fine sand accounts for 20%-30% is better.
In the application of extra fine sand, adding appropriate amount of secondary fly ash can effectively improve the workability of plastering gypsum mortar, make up for its inherent deficiencies, the role is to replace part of the fine aggregate to improve the strength and performance of plastering gypsummortar. In short, the larger the cement-to-sand ratio of ultrafine sand, the lower its strength and the worse its adhesion.