Types of Cement Bags and How to Choose

Open-mouth, valve and block-bottom cement bags compared: which packer each needs, how each lets air out, breakage in handling, and what to specify.

A cement plant's packing machine decides which bag it can use. Cement leaves the silo fluffed up with air, travels fast, is often still warm, and has to be in a closed bag at full weight within a few seconds. The bag is the part of that system that has to let the air out without letting the cement out. Every type of cement bag is a different answer to that problem, and the right one depends on the packer first, the distribution chain second and the price third.

The five types in use

Open mouth or valve: the packer decides

An open-mouth bag is hung under a filling spout, filled from the top, and then carried to a sewing head that stitches it shut. It suits small plants, grinding units and packing by hand, and it works with almost any product, which is why the same format is used for fertilizer and feed: our PP laminated fertilizer bag and PP laminated animal feed bag are open-mouth sacks of this kind. A valve bag has no open top. Both ends are closed at the bag factory and a short sleeve in one corner fits over the packer's spout. Cement blown in through the sleeve presses it flat from inside when the bag is full, so the bag seals itself and nobody sews anything. Rotary packers that fill thousands of bags an hour need valve bags; they cannot run open-mouth ones. Ask the plant which machine it runs before asking for a price.

Our PP woven cement bag is made in both formats, open mouth and valve, with a sewn or heat-sealed bottom, in 45 x 75 cm and 50 x 80 cm for a 50 kg fill, or to a custom size. Valve dimensions have to match the spout, so a valve bag order starts with the packer model and a sample of the bag it runs today.

Letting the air out

Aerated cement can be a third air by volume as it enters the bag. If that air cannot escape in the second or two the fill takes, the bag stays soft, reads underweight, and blows dust out of the valve when it is dropped onto the belt. Unlaminated woven fabric vents through the gaps between tapes, quickly and evenly, which is one reason it became the default for cement. The cost is some sifting of the finest particles through the same gaps. A coated fabric stops the sifting and the moisture but also stops the air, so coated and block-bottom sacks are micro-perforated: thousands of pinholes sized to pass air and hold powder. Plants and converters that sew their own sacks buy the two fabrics as rolls, the standard PP woven fabric roll for breathing sacks and the PP laminated woven fabric roll for coated ones. Fill speed, bag firmness and dust all come down to that balance, and it is tuned by trial on the actual packer.

Sewn bottom or block bottom

A sewn sack is a tube with the end folded and stitched. Filled, it forms a pillow: stable enough in a truck, but pillows roll off each other in a tall stack and waste space at the corners. A block-bottom sack has both ends folded into flat rectangles, like a brick, and in the welded version made from coated polypropylene the folds are joined by hot air rather than thread or glue, a construction sold under the Starlinger trade name AD*STAR. The brick shape stacks squarely on a pallet and prints on six faces. It needs coated fabric, its own converting line, and a packer set up for it. The sewn valve sack is simpler and cheaper and remains the working standard where bags are loaded by hand.

Woven polypropylene or paper

The choice follows the route. Where bags are thrown onto trucks, carried on shoulders and stored in open sheds, breakage decides it and woven sacks win. Where bags move on pallets from packer to shop floor, paper's grip and print surface count for more. Woven sacks can be given grip: our anti-slip PP woven fabric carries a coating with a bag-on-bag friction coefficient of 0.55 or more, so stacks hold on a pallet or a truck bed.

What to put in a cement bag specification

Packer

Make and model, spout diameter, and whether bags are placed by hand or by an applicator.

Format

Open mouth or valve; valve position and sleeve length for valve bags.

Size and fill weight

Width x length and the fill weight sold in the market, 50 kg or 25 kg.

Fabric

GSM and mesh; our cement bag runs 90-115 GSM with a warp breaking strength above 3,500 N.

Seams

Stitch type and fold; ours is a double lock stitch.

Sunlight

UV stabilization for yard and site storage; standard on our cement bag.

Print

Number of colours and the statutory markings the destination requires; up to four colours on ours.

Standard

The national standard the plant must meet, such as IS 11652 for woven cement sacks in India.

In India the sack standard is tied to a compulsory certification order, which changes who may supply a cement plant and what paperwork travels with the bags. Our guide to the BIS quality control order sets out what the standard covers and what the licence involves; read it before treating a standard number on a quote as proof of anything.