Commercial mortar is different from concrete as a structural material. In addition to the necessary strength (except for ground mortar, the general requirements for its strength are not high), the main conditions are to meet water retention, bonding, waterproofing, crack resistance, impact resistance, and anti-corrosion. In terms of freeze-thaw, high-temperature resistance, thermal insulation, and other functions, the composition of commercial mortar is relatively complicated.
Commercial mortar development and production are inseparable from standard and exceptional raw materials, typical raw materials such as various commonly used cement, gypsum, lime, and other cementitious materials, fine aggregates and fine fillers with different particle gradations, etc. Raw materials to improve mortar’s plastic properties and meet mortar’s special performance requirements after hardening include polymer emulsion and Redispersible Polymer powder, water retaining agent and thickener, various admixtures for concrete and mortar, fibers, and pigments.
The primary raw materials of commonly used dry powder mortar are:
1. Polymer emulsions and Redispersible Polymer powder
2. water-retaining agent and thickener
3. Admixtures for cement concrete and mortar
4. Fiber
5. Pigment
2. Basic composition of mortar
A certain proportion of cementitious material, granular fine aggregate, mineral admixture, water, chemical admixture, fiber, pigment, and other components prepares commercial mortar. Cementitious material, fine aggregate, and water constitute the essential components.
1. cementitious materials
In construction engineering, loose granular materials such as stones and sand or massive materials such as masonry can be bonded into a whole through a series of physical and chemical actions, collectively called cementitious materials. According to its chemical composition, cementitious materials can generally be divided into two categories: inorganic and organic.
The inorganic cementitious materials commonly used in commercial mortar can be divided into air-hardening cementitious materials and hydraulic cementitious materials according to their hardening conditions. Air-setting cementitious materials can only be hardened in the air, which means, can only maintain or continue to develop their strength in air, and under dry condition, such as gypsum and lime. Generally speaking,these materials only apply to the ground or dry environment. It is not suitable for use in a humid environment, let alone in water. Hydraulic cementitious materials can not only harden in the air but also better in water, maintain and continue to develop their strength, such as various series of cement suitable for ground and underground or underwater projects.
According to their sources, the organic cementitous materials commonly used in commercial mortar can be divided into natural organic(pitch) and organic synthetic (synthetic polymers). Asphalt cementitious material is mainly used to prepare asphalt floor mortar, which has moisture resistance, water resistance, corrosion resistance, elasticity, toughness, and high strength. Synthetic polymers like epoxy and unsaturated polyester resins can be formulated into resin cement and mortar for anti-corrosion floor engineering.
Cement
Cement is a powdery hydraulic cementitious material that can be mixed with water to form a plastic slurry suitable to mix with sand and stone and can harden in the air and in a humid environment, as well as in water, to maintain and increase strength. Cement occupies a prominent and vital position in cementitious materials and is one of the most important materials in construction engineering.
According to its performance and use, cement can be divided into general cement and special cement.
Basic properties of cement
| Item | Basic properties |
| relative density and apparent density | The relative density of ordinary Portland cement is 3.0-3.15, usually 3.1; the apparent density is 1000kg/m³~1600kg/m³, usually 1300kg/m³ |
| fineness | Fineness related to the thickness of cement particles. The finer the particles, the faster the hardening of the cement and the higher the early strength, but the volume will shrink significantly when hardened in a dry atmosphere. |
| Setting time | The initial setting time is it from adding water to cement paste loses plasticity; and the final setting time is it from adding water to cement paste loses plasticity and begins to produce strength. The initial setting of cement should be early enough to facilitate construction operations, but the final setting should be brief so that the concrete can harden as soon as possible and reach a particular strength to encourage the following process. The setting time of cement is related to the type of cement and the amount of mixed materials. |
| strength | The strength of cement is one of the primary quality indicators which is also the basis for determining the strength grade of cement. It measures the pressure per square centimeter of the cement mortar specimen after curing for 28 days under standard conditions. |
| stability | Stability is related to whether the volume change of cement slurry with standard consistency is uniform during the setting and hardening process. The structure of the cement will be unevenly changed, even destroyed, if the cement contains highly free lime, magnesia, or sulfur trioxide. Cement with unqualified stability shall not be used in engineering. |
| heat of hydration | The heat of hydration is when cement is in contact with water for a hydration reaction to generate heat. It is expressed by the heat (J) induced by 1kg of cement. The heat of hydration of cement is unfavorable for mass concrete since the heat of hydration accumulates inside and is not easy to dissipate, resulting in a significant temperature difference between inside and outside, causing internal stress and cracks in concrete. Therefore, mass concrete projects should use low-heat cement and take necessary cooling measures simultaneously. |
