Cement and Mixes

Standing in the aisle staring at bags that all look the same? Here’s the short version. Cement is the binder. Cement is the key ingredient in concrete that, when mixed with water, acts as the glue that binds the coarse and fine aggregate together to create concrete. On its own, a bag of Portland cement isn’t what most jobs need. What most buyers actually want is a mix — cement already blended with the right aggregate for the work in front of them.

For broader project planning, explore WBC’s building materials resources.

Three mixes cover the bulk of what shows up at a Montana jobsite, and getting them straight saves failed repairs and weak slabs.

Concrete mix — cement, sand, and gravel together. This is the structural workhorse: slabs, footings, setting posts. The gravel is what gives it the muscle to carry load.

Sand mix — same idea, minus the gravel. Built for thin repair work under two inches. Pour it where a full concrete mix would be too coarse to finish clean.

Mortar mix — cement and sand, formulated to bond. This is your brick, block, and stone glue. Spec Mix Type S is the masonry bonding mix on the shelf.

Pick wrong and the job tells on you. A sand-mix patch poured thick will crack. A mortar mix won’t carry a footing. Match the mix to the work first, then worry about ratios and water.

Portland Cement Types and What Each One Does

When you do need straight cement — mixing your own from sand and aggregate, or a job that calls for a specific type — Portland cement comes in graded types under ASTM C150. The bag on the shelf here is Portland Type I/II, the dual-rated general workhorse.

Ordinary Portland cement is the most widely used type of cement manufactured and used worldwide. “Portland” is a generic name derived from a type of building stone quarried on the Isle of Portland in Dorset, England. OPC is suitable for most general concrete jobs and mortar or stucco construction projects.

The type numbers matter when soil or schedule pushes you off general-purpose.

Type	What it’s for
Type I	General use — residential foundations, commercial builds, or large-scale infrastructure
Type II	Moderate sulfate resistance — structures near groundwater or marine environments
Type III	High early strength — precast, repairs, and time-sensitive construction
Type V	High sulfate resistance — aggressive sulfate conditions in certain soils or groundwater

Sulfate resistance isn’t an academic concern around here. Where Montana soils or groundwater carry sulfates, a footing or foundation pour wants Type II or Type V rather than plain Type I. Sulfate-resisting cement helps reduce the risk of sulfate side effects on concrete. Its most common use is for constructing foundations in soil with high sulfate content. This concrete type can also be beneficial for projects like canal linings, culverts and retaining walls.

Two more types earn mention for cold-country work. Extra-rapid-hardening cement may set and become durable even faster than OPC and rapid-hardening cement. Construction professionals achieve this by adding calcium chloride to rapid-hardening cement. This cement type may be useful for cold-weather concrete projects due to its fast setting rate. And for freeze-thaw country specifically — exterior slabs, driveways, steps that ride through forty cycles a winter — air-entraining cement is more workable with a smaller water-cement ratio than OPC and other types of cement. Manufacturers add air-entraining agents like glues, sodium salts and resins to the clinker during the grinding process to create this cement. A common use for this type of cement is for frost resistance in concrete. On a Montana exterior pour, frost resistance is the whole ballgame.

Mix Ratios and the Water That Makes or Breaks the Pour

If you’re batching your own concrete, the ratio is simple: a 1:2:3 cement-to-sand-to-gravel ratio for structural strength. One part cement, two sand, three gravel. Where most people go wrong isn’t the dry mix — it’s the water.

The key to achieving a strong, durable concrete lies in the careful proportioning and mixing of the ingredients. The quality of the paste determines the character of the concrete. The strength of the paste, in turn, depends on the ratio of water to cement. Pour in too much water because it mixes easier and you’ve traded away strength. Excess water creates voids, weakening the slab.

The target: keep the water-to-cement ratio between 0.45 and 0.50 for optimal durability. That’s the weight of water divided by the weight of cement — you produce high-quality concrete by lowering the water-cement ratio as much as possible without sacrificing the workability of fresh concrete, allowing it to be properly placed, consolidated, and cured.

Proportion-wise, a finished mix runs roughly 7% to 15% cement, 60% to 75% aggregate, and 14% to 21% water. Entrained air in many concrete mixes may also be up to an additional 8%. That air content is the freeze-thaw insurance for anything poured outside.

Watch the water you mix with, too. Excessive impurities in mixing water can affect setting time and concrete strength, and can also cause efflorescence, staining, corrosion of reinforcement, volume instability, and reduced durability. Drinkable water with no strong taste or odor works fine.

Bag or Ready-Mix, and What Goes Under the Pour

There’s a point where dragging bags stops making sense. The line: use bags for projects under 1.5 cubic yards; otherwise, order ready-mix. Below that, bagged mix is the practical call. Above it, a truck saves your back and gives you a more consistent batch. Each batch of ready mixed concrete is tailor-made according to the contractor’s specifics and delivered to the site in a plastic condition.

Bagged options on the shelf cover the range — standard premix concrete in 80-pound bags running about two-thirds of a cubic foot, Sakrete fast-set at 4000 PSI for quick-turn work, and a dedicated Sakrete cold-weather concrete mix for pours when the temperature won’t cooperate. For thin repair and resurfacing, Sakrete Top & Bond handles the work a sand mix is built for.

Whatever you pour, the base under it matters as much as the mix in it. Always install over 4 to 6 inches of compacted gravel base to prevent settling. In Montana that compacted base does double duty — it limits frost heave under slabs and posts, not just settling. Skip it and the freeze-thaw cycle will find the weakness.

Once it’s down, the pour isn’t done. Hydration continues during the curing process: The longer it is kept moist, the stronger and more durable concrete will become. The rate of hardening depends on the composition and fineness of the cement, the mix proportions, and the moisture and temperature conditions. Most of the hydration and strength gain take place within the first month of concrete’s life cycle, but hydration continues at a slower rate for many years. Keep it wet. That’s the cheapest strength you’ll ever buy.

Storing Cement Before You Ever Open It

Cement is moisture-reactive by design, which means it goes bad if you let it. Store cement in dry, weatherproof areas to prevent moisture contamination. Proper storage is essential to maintain the quality and usability of the cement. Moisture can cause the cement to harden prematurely, rendering it unusable. A bag that’s gone lumpy in a damp garage is scrap.

Keep it simple. Use sealed containers or bags and keep them off the ground to avoid contact with damp surfaces. Regularly check storage conditions to ensure the cement remains dry and in good condition. Off the slab, sealed, and under cover.

Frequently Asked Questions

What’s the difference between cement and concrete? Cement is the binder, not the finished material. Cement is the key ingredient in concrete that, when mixed with water, acts as the glue that binds the coarse and fine aggregate together to create concrete. The reaction with water is what makes cement a hydraulic material and gives concrete the essential versatility to be placed in a plastic state and then harden into a strong, durable rock-like mass. Concrete is cement plus sand plus gravel plus water.

Which mix do I use to set fence posts or pour a footing? A concrete mix — the kind with gravel in it. Concrete mix is built for structural slabs, footings, and posts. The gravel carries the load. A sand mix or mortar mix won’t do that work.

What mix bonds brick and block? Mortar mix — cement and sand formulated to bond masonry. Type S is the common spec for brick, block, and stone bonding.

Why does my concrete keep cracking? The two usual culprits are too much water and no proper base. A mixture that has too little paste to fill all the voids between the aggregates is difficult to place and produces rough surfaces and porous concrete. Keep the water-to-cement ratio between 0.45 and 0.50, pour over four to six inches of compacted gravel, and keep it moist while it cures.

Which cement should I use for a foundation in sulfate soil? Type II for moderate sulfate exposure near groundwater, Type V for aggressive sulfate conditions. Both are formulated to resist the sulfate attack that breaks down standard Type I in those soils.

Can I pour concrete in cold weather? There are mixes built for it. A cold-weather concrete mix and faster-setting cement types are formulated for low temperatures — useful for cold-weather concrete projects due to its fast setting rate. For anything poured outside in freeze-thaw country, an air-entrained mix adds the frost resistance an exterior slab needs.

When should I switch from bags to a ready-mix truck? At about 1.5 cubic yards. Under that, bags are practical. Over it, order ready-mix — you’ll get a more consistent batch and save the labor of mixing bag after bag by hand.