Building Materials Basics: Cement Grades, Aggregates and Bricks
Almost every structure you will design or supervise in India is built from three basic materials: cement, aggregates and bricks (or blocks). Before you study concrete mix design or estimation, you must know what each material is, how it is graded, and how it is checked on site. This article covers the common cement types and grades, the difference between fine and coarse aggregate with a worked fineness modulus calculation, and the classification and simple tests for burnt clay bricks.
Cement Types and Grades
Ordinary Portland Cement (OPC) is made by burning limestone and clay to form clinker, then grinding the clinker with a small amount of gypsum. Gypsum controls the setting time. OPC is sold in India in three grades, now covered together by IS 269. The grade number is the minimum 28-day compressive strength in N/mm² (MPa) of a standard cement-sand mortar cube, not of concrete.
| Grade | 3-day strength (MPa, min) | 7-day strength (MPa, min) | 28-day strength (MPa, min) | Typical use |
|---|---|---|---|---|
| OPC 33 | 16 | 22 | 33 | Plastering, masonry mortar, low-grade PCC |
| OPC 43 | 23 | 33 | 43 | General RCC, residential buildings |
| OPC 53 | 27 | 37 | 53 | High-grade RCC, precast, early de-shuttering |
Blended cements replace part of the clinker with industrial by-products. Portland Pozzolana Cement (PPC), covered by IS 1489 (Part 1), uses fly ash. Portland Slag Cement (PSC), covered by IS 455, uses ground granulated blast furnace slag. Both gain strength more slowly than OPC but generate less heat and give better long-term durability, which is why PPC is now the most common bagged cement in Indian housing.
Every cement must satisfy a few physical requirements. Initial setting time must be not less than 30 minutes, so the concrete can be mixed, transported and placed. Final setting time must be not more than 600 minutes. Setting time and standard consistency are found with the Vicat apparatus, following the IS 4031 series of test methods.
Simple site checks for cement
- Check the manufacturing week and year printed on the bag; cement older than about three months loses strength and should be tested before use in structural work.
- Put your hand into the bag: it should feel cool and there should be no hard lumps.
- A pinch rubbed between the fingers should feel smooth, not gritty.
- A small quantity thrown into a bucket of water should float briefly before sinking.
Fine and Coarse Aggregates
Aggregates make up roughly 60 to 75 percent of the volume of concrete, so their quality controls strength, workability and cost. IS 383 covers aggregates for concrete. Material passing the 4.75 mm IS sieve is fine aggregate (river sand or manufactured sand, M-sand). Material retained on 4.75 mm is coarse aggregate, usually crushed stone of 10 mm or 20 mm nominal size for building work.
Fine aggregate is further classified into grading Zones I to IV based on its sieve analysis. Zone I is the coarsest and Zone IV the finest. Zone II sand is most commonly assumed in mix design. A quick single number used to describe grading is the fineness modulus (FM).
Fineness modulus = (Sum of cumulative % retained on standard sieves) / 100 Standard sieves for sand: 4.75 mm, 2.36 mm, 1.18 mm, 600 µm, 300 µm, 150 µm
Worked example: fineness modulus of sand
A 1000 g sand sample is sieved and the masses retained are recorded. The table shows the calculation.
| Sieve | Mass retained (g) | % retained | Cumulative % retained |
|---|---|---|---|
| 4.75 mm | 20 | 2.0 | 2.0 |
| 2.36 mm | 80 | 8.0 | 10.0 |
| 1.18 mm | 150 | 15.0 | 25.0 |
| 600 µm | 220 | 22.0 | 47.0 |
| 300 µm | 310 | 31.0 | 78.0 |
| 150 µm | 170 | 17.0 | 95.0 |
| Pan | 50 | 5.0 | - |
Sum of cumulative % retained = 2 + 10 + 25 + 47 + 78 + 95 = 257 FM = 257 / 100 = 2.57
An FM of 2.57 indicates a medium sand. As a general guide, fine sand has an FM around 2.2 to 2.6, medium sand 2.6 to 2.9 and coarse sand 2.9 to 3.2. A higher FM means coarser sand, which needs less water for the same workability. For a proper zone classification, compare the percent passing on each sieve with the limits given in IS 383.
Burnt Clay Bricks
Common burnt clay bricks are covered by IS 1077. The standard modular brick size is 190 x 90 x 90 mm. With a 10 mm mortar joint, the nominal size becomes 200 x 100 x 100 mm, which makes quantity calculation easy: 500 bricks per cubic metre of masonry. Many local kilns still make non-modular bricks around 230 x 110 x 75 mm, so always measure the bricks actually supplied.
Bricks are classified by their average compressive strength. IS 1077 lists classes starting at 3.5 N/mm², and the class designation is that strength. First class bricks used in load-bearing walls should be well burnt, of uniform copper-red colour, with sharp edges and no cracks.
Field and laboratory tests for bricks
| Test | How it is done | What a good brick shows |
|---|---|---|
| Visual and dimension | Measure 20 bricks laid end to end | Uniform size and shape, straight edges |
| Sound test | Strike two bricks together | Clear metallic ringing sound |
| Hardness | Scratch with a fingernail | No mark left on the surface |
| Water absorption | Dry, weigh, soak 24 h, weigh again | Low absorption, generally under 20% by mass |
| Efflorescence | Stand brick in shallow water, then dry | Nil or slight white salt deposit |
| Compressive strength | Crush in testing machine with frog filled | Meets the class designation |
Water absorption (%) = (W2 - W1) / W1 x 100 Example: dry mass W1 = 3.10 kg, wet mass W2 = 3.62 kg Absorption = (3.62 - 3.10) / 3.10 x 100 = 16.8 %
Common Mistakes on Site
- Assuming OPC 53 is always better. It gains strength fast but releases more heat; for plastering and mass concrete, PPC or OPC 43 is often the better choice.
- Stacking cement bags directly on the floor or against walls. Keep them on a raised wooden platform, away from walls, and no more than about ten bags high.
- Using sand with silt and clay. Do a simple silt test in a measuring cylinder with salt water; heavy silt content weakens concrete and mortar.
- Ignoring bulking of sand. Damp sand occupies more volume than dry sand, so volume batching with wet sand gives less sand than intended.
- Not soaking bricks before laying. Dry bricks absorb water from the mortar and weaken the joint.
Summary
OPC grades 33, 43 and 53 refer to minimum 28-day mortar strength in MPa, while PPC and PSC are blended cements with better durability. Aggregates are split at 4.75 mm into fine and coarse, and sand grading is summarised by the fineness modulus, which you can compute from a simple sieve analysis. Bricks are graded by compressive strength and checked by size, sound, absorption and efflorescence tests. With these basics clear, you are ready to look at concrete grades and mix proportions.